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From The Help DeskChris Prioli AD2CS[email protected] |
About From The Help Desk
[email protected]
From The Help Desk column is a monthly feature, penned by Chris Prioli AD2CS. The purpose of this column is to provide public answers to questions that have been asked, frequently at the Clubhouse where a one-on-one answer was most likely already given. However, due to the probable widespread interest in the topic, it may seem to be an appropriate topic for this feature.
If you have a question to which you would like a thorough answer to be provided and shared with the wider group, feel free to submit the question to [email protected]. If selected for an answer, the question and answer will be published in an upcoming issue of CrossTalk, this website, on the Club's Facebook page, as well as showing up on www.ad2cs.com.
[email protected]
From The Help Desk column is a monthly feature, penned by Chris Prioli AD2CS. The purpose of this column is to provide public answers to questions that have been asked, frequently at the Clubhouse where a one-on-one answer was most likely already given. However, due to the probable widespread interest in the topic, it may seem to be an appropriate topic for this feature.
If you have a question to which you would like a thorough answer to be provided and shared with the wider group, feel free to submit the question to [email protected]. If selected for an answer, the question and answer will be published in an upcoming issue of CrossTalk, this website, on the Club's Facebook page, as well as showing up on www.ad2cs.com.
July 2026
Q : I have a couple of hundred linear feet of rain gutter on my house, and I would like to use some of the gutter length to make “stealth” antennas. How can I go about doing this?
A : The main thing that you will need to do in order to “carve” your rain gutter length up into proper antenna-length sections is to come up with a means of isolating each cut section from its neighboring section of gutter. Let’s suppose that you want to make a gutter antenna for the 40-meter band. The job will involve cutting the overall gutter length into segments of the proper length, and then isolating those segments, sealing them to maintain gutter functionality, and then attaching the feedline to the segments. Finally, you will most likely end up needing a matching transformer as well. Let’s look at the various steps required to do this job.
Before we begin, we must gather the requisite supplies and tools. For supplies, you will need your feedline, some RTV sealant, some flexible H-channel, some thin (1/64” to 1/16” thick) plastic sheets roughly 2” x 12” in size, #8 or #10 ring terminals, machine screws with KEPS nuts, some sandpaper, and some liquid electrical tape. For tools, you will need a tape measure, a saw (either a hacksaw or a reciprocating saw will do, though a Dremel rotary tool with a saw blade or cutoff wheel is actually the best option), a flexible putty knife, a portable drill motor, and a set of twist bits. Of course, you will need your ladder and a source of electrical power if you are using a corded saw and/or drill motor. Ultimately, you will also want an antenna analyzer and at some point, a matching transformer, which you may end up constructing later on.
Q : I have a couple of hundred linear feet of rain gutter on my house, and I would like to use some of the gutter length to make “stealth” antennas. How can I go about doing this?
A : The main thing that you will need to do in order to “carve” your rain gutter length up into proper antenna-length sections is to come up with a means of isolating each cut section from its neighboring section of gutter. Let’s suppose that you want to make a gutter antenna for the 40-meter band. The job will involve cutting the overall gutter length into segments of the proper length, and then isolating those segments, sealing them to maintain gutter functionality, and then attaching the feedline to the segments. Finally, you will most likely end up needing a matching transformer as well. Let’s look at the various steps required to do this job.
Before we begin, we must gather the requisite supplies and tools. For supplies, you will need your feedline, some RTV sealant, some flexible H-channel, some thin (1/64” to 1/16” thick) plastic sheets roughly 2” x 12” in size, #8 or #10 ring terminals, machine screws with KEPS nuts, some sandpaper, and some liquid electrical tape. For tools, you will need a tape measure, a saw (either a hacksaw or a reciprocating saw will do, though a Dremel rotary tool with a saw blade or cutoff wheel is actually the best option), a flexible putty knife, a portable drill motor, and a set of twist bits. Of course, you will need your ladder and a source of electrical power if you are using a corded saw and/or drill motor. Ultimately, you will also want an antenna analyzer and at some point, a matching transformer, which you may end up constructing later on.
Let’s look more closely at the two “strange” bits of supply material that I itemized above - the H-channel and the plastic sheets. These only seem strange because you don’t know where I am going with them yet. Both items are available online from Amazon. The flexible plastic sheets are 8” x 12” x 0.040” thick, and will need to be cut to size. These sheets, found on the Amazon website at : https://www.amazon.com/Plastic-Flexible-Materials-Handicrafts-Decoration/dp/B0CCMNPWGW, are used to insulate the gutter from the drip-edge flashing that is often located above the gutter. The sheets will be slipped in between the existing drip-edge flashing and the upper edge of the gutter rear member, so as to prevent contact between the antenna segments of the gutter and the drip edge. Note that if the drip edge is vinyl, it is not necessary to insulate it from the rain gutter. The H-channel, which is not truly an H shape (Figure 1), is meant to be cut to the proper lengths, as three pieces, so as to be slipped into the cuts between the gutter segments. The channel is found at : https://www.amazon.com/Outwater-Plastic-Material-Moulding-Adhesive/dp/B07N4B15M5. Both of these plastic items are black in color, but they can be painted to match the gutter afterwards if so desired, though care must be taken to avoid paints with any metallic compounds in their formulation. Now, on to the build details.
Suppose we are shooting for the center of the overall 40-meter band, or 7.150 MHz. The math, taken in feet, looks like this :
Suppose we are shooting for the center of the overall 40-meter band, or 7.150 MHz. The math, taken in feet, looks like this :
Thus, a half-wave (λ/2) antenna would need to be 68.811 feet, making each segment 34.4 feet long. This equates to 34 feet, 4-3/4 inches in length. This, in turn, should be derated by about 5% to account for the greater density of the metal than that of a vacuum or even of air. Ninety-five percent of the calculated segment length above brings us to a final adjusted segment length of 32 feet, 8-1/4 inches. OK - identify the center point, where the feedline will attach, and make sure that you have adequate gutter length on each side of that point for the antenna that you want to construct. If all is good, use your saw to make a cut completely through the gutter, from the front all the way through the back face and edge of the gutter. If there is drip-edge flashing overlapping the top of the gutter, it is best to avoid cutting that, but we will need to insulate the drip-edge flashing from the gutter later on, as discussed above regarding the plastic sheets. Now, measure and mark the gutter at the proper distance from the first cut to lay out the antenna segments, and then cut them in the same manner as the first cut. Next, we will need to isolate the segments from each other and from the rest of the gutter.
This is where the H-channel comes in. Measure as best you can the length of channel that is required to fill in between the gutter segments, at the back of the gutter, across the bottom of the gutter, and up the front of the gutter, cutting three sets of these pieces (one for each cut in the gutter). Slip the channel sections in between the gutter segments with the wide face to the outside. These strips will support the gutter segments as well as maintaining separation between them electrically. Use the sandpaper to clean the inside surfaces adjacent to the H-channel strips, scrubbing down to bright metal. This is necessary to make it more likely that the RTV sealant will bond to the metal. Clean the sanding dust out of the gutter, and apply sealant to the joints, spreading it evenly with the flexible putty knife. This will allow the gutter to continue to do its primary job of handling the rainwater runoff from the roof of the house without leaking at the cut locations.
Now it is time to isolate the rain gutter from any metallic drip-edge flashing that may be present. Do this by cutting the 8” x 12” plastic sheets into 2 x 12” strips. Then, slip a strip in between the top of the rain gutter and the bottom of the drip edge, push the strips in as far as you can. Ideally, the strips will fit under the drip-edge and be trapped in place behind the vertical lip of the drip edge flashing. It may be necessary to cut a slot in the strip at the location of each nail that secures the drip-edge flashing to the roof in order to slide the plastic strip into place. Do this for the entire length of the antenna segments defined in your antenna design, thus ensuring that any metallic drip-edge flashing cannot come into contact with the metallic rain gutter within the length of the antenna.
OK - now comes the attachment of the feedline to the gutter. Prepare the end of the feedline by stripping the insulation from the wire as needed. If the feedline is coaxial cable, be sure to strip enough of the outer jacket to permit separation of the shield braid and the center conductor far enough to connect them to the two gutter segments. Using the sandpaper again, remove some paint from the gutter front or lower face in the locations where the feedline will be connected.
Install a ring terminal having a #8 or #10 ring onto each of the feedline wires. Then drill a hole in the sanded area of the gutter at each feedline attachment point. Finally, using a machine screw and KEPS nut for each terminal, secure the ring terminals to the gutter segments. Apply some liquid electrical tape compound to each of the screw attachment points and the feed line wires.
That brings us to the matching transformer. To accomplish this task, you will need to start out by measuring the impedance of the rain gutter antenna. Using that number and a targeted 50-ohm value, do some basic mathematics to determine the impedance ratio that is needed. For example, if the gutter antenna measures out to be 768 ohms of impedance, a 15:1 impedance matching transformer is required. Remember that the impedance and the turns ratio are not the same! Consider that a transformer with a 3:1 turns ratio provides a 9:1 impedance ratio. The impedance ratio is the square of the turns ratio, or, coming at it from the opposite direction, the turns ratio is the square root of the impedance ratio. Thus, the 15:1 impedance ratio of our example above ends up at a 3.87:1 turns ratio. Such a transformer would most likely need to be home-brewed, but that it not an insurmountable task.
I hope that this article helps to improve your understanding of rain gutter antenna construction. Hopefully, it will also encourage you to experiment a little bit with new and different ways of designing and building custom home-brewed antennas. Overall, it should help to remove some of the mystery, hesitation, and fear of the unknown when it comes to this type of stealth antenna.
This is where the H-channel comes in. Measure as best you can the length of channel that is required to fill in between the gutter segments, at the back of the gutter, across the bottom of the gutter, and up the front of the gutter, cutting three sets of these pieces (one for each cut in the gutter). Slip the channel sections in between the gutter segments with the wide face to the outside. These strips will support the gutter segments as well as maintaining separation between them electrically. Use the sandpaper to clean the inside surfaces adjacent to the H-channel strips, scrubbing down to bright metal. This is necessary to make it more likely that the RTV sealant will bond to the metal. Clean the sanding dust out of the gutter, and apply sealant to the joints, spreading it evenly with the flexible putty knife. This will allow the gutter to continue to do its primary job of handling the rainwater runoff from the roof of the house without leaking at the cut locations.
Now it is time to isolate the rain gutter from any metallic drip-edge flashing that may be present. Do this by cutting the 8” x 12” plastic sheets into 2 x 12” strips. Then, slip a strip in between the top of the rain gutter and the bottom of the drip edge, push the strips in as far as you can. Ideally, the strips will fit under the drip-edge and be trapped in place behind the vertical lip of the drip edge flashing. It may be necessary to cut a slot in the strip at the location of each nail that secures the drip-edge flashing to the roof in order to slide the plastic strip into place. Do this for the entire length of the antenna segments defined in your antenna design, thus ensuring that any metallic drip-edge flashing cannot come into contact with the metallic rain gutter within the length of the antenna.
OK - now comes the attachment of the feedline to the gutter. Prepare the end of the feedline by stripping the insulation from the wire as needed. If the feedline is coaxial cable, be sure to strip enough of the outer jacket to permit separation of the shield braid and the center conductor far enough to connect them to the two gutter segments. Using the sandpaper again, remove some paint from the gutter front or lower face in the locations where the feedline will be connected.
Install a ring terminal having a #8 or #10 ring onto each of the feedline wires. Then drill a hole in the sanded area of the gutter at each feedline attachment point. Finally, using a machine screw and KEPS nut for each terminal, secure the ring terminals to the gutter segments. Apply some liquid electrical tape compound to each of the screw attachment points and the feed line wires.
That brings us to the matching transformer. To accomplish this task, you will need to start out by measuring the impedance of the rain gutter antenna. Using that number and a targeted 50-ohm value, do some basic mathematics to determine the impedance ratio that is needed. For example, if the gutter antenna measures out to be 768 ohms of impedance, a 15:1 impedance matching transformer is required. Remember that the impedance and the turns ratio are not the same! Consider that a transformer with a 3:1 turns ratio provides a 9:1 impedance ratio. The impedance ratio is the square of the turns ratio, or, coming at it from the opposite direction, the turns ratio is the square root of the impedance ratio. Thus, the 15:1 impedance ratio of our example above ends up at a 3.87:1 turns ratio. Such a transformer would most likely need to be home-brewed, but that it not an insurmountable task.
I hope that this article helps to improve your understanding of rain gutter antenna construction. Hopefully, it will also encourage you to experiment a little bit with new and different ways of designing and building custom home-brewed antennas. Overall, it should help to remove some of the mystery, hesitation, and fear of the unknown when it comes to this type of stealth antenna.
June 2026
Q : How can the two leads of a typical through-hole LED be identified as to which is the anode and which is the cathode, and why does it matter?
Q : How can the two leads of a typical through-hole LED be identified as to which is the anode and which is the cathode, and why does it matter?
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A : Let’s start by answering the last question first, and then move on to the more difficult one. All LED’s are polarized devices. At its heart, an LED is, after all, just another diode, and as such it must be connected into the circuit with the correct polarity in order for it to do what we want it to do… which is usually to produce a light output.
In order to produce its light output, an LED must be biased in the forward direction, which means that its anode must be at a voltage that is sufficiently more positive than its cathode to meet that LED’s forward bias requirements. Notice that I did not say that one terminal must be positive and the other negative. That is because they can, in reality, both be either positive or negative, so long as the difference in voltage between the two terminals is great enough to satisfy the forward voltage bias requirement for operation of that LED. For example, if the forward voltage of a given LED is 2.1VDC, that LED will illuminate just as well with the anode at +5.6VDC and the cathode at +3.5VDC as it will with the anode at +1.5VDC while the cathode is at -0.6VDC, or with the anode held at -1.0VDC while the cathode is at -3.1VDC. It is the voltage difference that is important here, not the actual voltages. So… if it is the difference in voltage that matters, and if it is the polarity of that voltage that matters, then it is equally important that we know which lead, or in other words which terminal, is which. That |
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brings us to the first question, which deals with how to identify which terminal is which. There are three common methods of identifying the terminal assignments, two of which may be inconclusive, leaving one method that should work every time… but which may also be the most difficult to discern.
Refer to the LED cutaway view shown in Figure 1. Look carefully and it will become apparent that one of the two structures inside the LED is somewhat larger than is the other. The manner in which many LED’s are assembled has the semiconductor die mounted to a supporting structure to which the body of the N-type material is attached. A wire “whisker” is then bridged from the P-type material, which is deposited on top of the N-type body, over to the smaller internal structure. These internal structures are integral to the LED terminal leads. Thus, when viewed through the body or envelope of the LED, the lead which is common to the larger internal structure will be the negative or cathode lead. This leaves the lead that is common to the smaller internal structure as the positive or anode lead. This method of terminal lead identification is always reliable… when a) the envelope of the LED is transparent or is at least translucent enough to permit the internal structure to be seen, and b) when there is a difference in size between the internal structures, which is not always the case. That brings us to method number two.
In this method, we attempt to determine the terminal lead identity by the length of the individual leads. More often than not, the anode is the longer of the two leads, while the cathode is the shorter lead. As I said, however, this is usually, but by no means always a reliable method of identification. In order to know for sure, it would be necessary to either combine this method with another one of the three methods, or to have a datasheet available for the LED at hand. The problem is that LED’s are available with so-called “long cathodes”. In some cases, that is the standard lead scheme used by a given manufacturer. In other situations, the “long cathode” is an optional or even a by-request configuration. Occasionally, LED’s will be encountered with both terminal leads having the same length. So much for method two; now it is time to move on to method three.
This method is, to the best of my knowledge and experience, universal and is therefore always reliable. However, it is also often very difficult to see and identify, especially in smaller LED’s like the 3mm or T1 size devices. The key to using this method is to identify the flatted side of the LED envelope. Refer again to the Figure 1 drawing, and this time notice the flat on one side of the base of the LED envelope. This flat will always be positioned in alignment with the cathode terminal lead of the LED. When a given LED has a flanged lower edge, the flat can be more easily identified than what is possible when the LED has no flange. A magnifying glass can help, as can running a fingernail around the base of the LED envelope to feel the “points” where the shape of the LED changes from round to flattened and back again. The good part is that when the flat can be found, it is absolutely diagnostic and reliable.
I hope that this explanation helps to improve your understanding of LED terminal lead identification. Hopefully, it will also broaden your overall knowledge of the design and structure of discrete LED’s. Taken in its entirety, this information should help to make your electronics project and kit assembly tasks just a bit easier.
Refer to the LED cutaway view shown in Figure 1. Look carefully and it will become apparent that one of the two structures inside the LED is somewhat larger than is the other. The manner in which many LED’s are assembled has the semiconductor die mounted to a supporting structure to which the body of the N-type material is attached. A wire “whisker” is then bridged from the P-type material, which is deposited on top of the N-type body, over to the smaller internal structure. These internal structures are integral to the LED terminal leads. Thus, when viewed through the body or envelope of the LED, the lead which is common to the larger internal structure will be the negative or cathode lead. This leaves the lead that is common to the smaller internal structure as the positive or anode lead. This method of terminal lead identification is always reliable… when a) the envelope of the LED is transparent or is at least translucent enough to permit the internal structure to be seen, and b) when there is a difference in size between the internal structures, which is not always the case. That brings us to method number two.
In this method, we attempt to determine the terminal lead identity by the length of the individual leads. More often than not, the anode is the longer of the two leads, while the cathode is the shorter lead. As I said, however, this is usually, but by no means always a reliable method of identification. In order to know for sure, it would be necessary to either combine this method with another one of the three methods, or to have a datasheet available for the LED at hand. The problem is that LED’s are available with so-called “long cathodes”. In some cases, that is the standard lead scheme used by a given manufacturer. In other situations, the “long cathode” is an optional or even a by-request configuration. Occasionally, LED’s will be encountered with both terminal leads having the same length. So much for method two; now it is time to move on to method three.
This method is, to the best of my knowledge and experience, universal and is therefore always reliable. However, it is also often very difficult to see and identify, especially in smaller LED’s like the 3mm or T1 size devices. The key to using this method is to identify the flatted side of the LED envelope. Refer again to the Figure 1 drawing, and this time notice the flat on one side of the base of the LED envelope. This flat will always be positioned in alignment with the cathode terminal lead of the LED. When a given LED has a flanged lower edge, the flat can be more easily identified than what is possible when the LED has no flange. A magnifying glass can help, as can running a fingernail around the base of the LED envelope to feel the “points” where the shape of the LED changes from round to flattened and back again. The good part is that when the flat can be found, it is absolutely diagnostic and reliable.
I hope that this explanation helps to improve your understanding of LED terminal lead identification. Hopefully, it will also broaden your overall knowledge of the design and structure of discrete LED’s. Taken in its entirety, this information should help to make your electronics project and kit assembly tasks just a bit easier.
May 2026
Q : “I am confused about exactly how to use a manual antenna tuner with my HF radio setup. I have heard many conflicting procedures, and seen different methods on different online videos. Can you explain the proper procedure for adjusting a manual antenna tuner?”
Q : “I am confused about exactly how to use a manual antenna tuner with my HF radio setup. I have heard many conflicting procedures, and seen different methods on different online videos. Can you explain the proper procedure for adjusting a manual antenna tuner?”
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A : Understanding how to properly adjust an antenna tuner requires that the user first understand just what the tuner is meant to accomplish, and then further understand that while there are many different approaches to the process, there is really no clear-cut right or wrong procedure to adjusting the tuner. The end goal is the thing that really matters. The basic intent of an antenna tuner is to make an antenna that is resonant at one (and only one) specific frequency appear to be as close to resonance at the various other frequencies at which the radio system is operated as is possible. While it is a truth that any antenna is resonant at only one fixed frequency, how well that antenna will perform at other frequencies above or below the resonant frequency will usually depend upon the SWR bandwidth of the antenna system. An antenna system with a very narrow antenna bandwidth will fall off rapidly in performance on either side of its resonant frequency. Conversely, an antenna with a wide SWR bandwidth will maintain a much more acceptable level of performance through a greater variation from the resonant frequency. An antenna tuner simply artificially extends that SWR bandwidth to include the frequency to which the radio is tuned at the time of adjustment of the antenna tuner. |
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Another point that must be considered is just how much mismatch you are willing to accept in an antenna system, As we all know by now, a perfect match between the radio and the antenna system would show a standing-wave ratio or SWR of 1:1. What this means is that for a radio with a fifty-ohm output impedance, the input impedance of the antenna system is also right at fifty ohms. This is almost never the case - and really should not be most of the time, based on the physical installation conditions of the antenna. However, any reported SWR greater than a one-to-one match indicates a certain amount of mismatch loss in the antenna system.
Note that this loss is not the only loss in the antenna system, as the feedline itself also presents a certain predictable amount of loss. Feedline loss is dependent upon three basic factors : 1) the feedline type, 2) the feedline length, and 3) the operating frequency. Feedline loss is given by the cable manufacturers as a certain value in decibels per one-hundred feet of feedline length at a given frequency. The point here is that the feedline losses are not related to nor are they changed by changes in reported SWR. Instead, the feedline losses are cumulative to the SWR losses.
So… how much loss is actually imparted by a given SWR value? Look at it this way. A 1.0:1 match results in 100% of the radio’s output RF power being transferred into the antenna system. However, a 1.1:1 match results in a 0.2366% loss, a 1.2:1 match gives us a loss of 0.826% of the output RF power, and with a 1.5:1 match, we get a 4% power loss. All of those values are generally considered to be acceptable, though the lower the numeric match is, the better. On the other hand, a 2.0:1 match yields a whopping 11.1% power loss. Of course, it goes up from there.
What exactly does SWR measure or indicate then? Simply put, the standing wave ratio is the ratio of transmitted output power in watts to the power level in watts reflected from the antenna system back to the transmitter.
Note too that there is another peculiarity to SWR measurement. For example, there are two scenarios that both result in a 2:1 SWR. If the transmitter output impedance is fifty ohms, an antenna system input impedance of either twenty-five ohms or one-hundred ohms will result in a 2:1 SWR match. This is because SWR, when it is not a perfect 1:1 ratio, is always reported with the antecedent as a value greater than one, and the consequent as a unity value, or a value of one. Thus, either way, we get a 2:1 ratio from the impedance pairs of fifty ohms and twenty-five ohms, which is naturally a 2:1 ratio, or fifty ohms and one-hundred ohms, which is naturally a 1:2 ratio, but as an SWR value is expressed as a 2:1 value. Again, the antecedent, or “first” number is always greater than the consequent, or “second” number, when it is not a 1:1 match.
The next thing that must be understood is what happens to the reflected power once it enters the feedline after being reflected by the mis-matched antenna. Remember what I said about feedline loss? Well, it works both ways. The reflected power will make the reverse trip up the feedline towards the transmitter, where it will again be reflected back down the feedline towards the antenna. This will occur repeatedly until the reflected power has all been dissipated by the ohmic resistance of the feedline - or in other words by the feedline loss. Each trip through the feedline consumes a certain percentage of the power, until it is all gone, having been converted to heat in the feedline.
Note that this loss is not the only loss in the antenna system, as the feedline itself also presents a certain predictable amount of loss. Feedline loss is dependent upon three basic factors : 1) the feedline type, 2) the feedline length, and 3) the operating frequency. Feedline loss is given by the cable manufacturers as a certain value in decibels per one-hundred feet of feedline length at a given frequency. The point here is that the feedline losses are not related to nor are they changed by changes in reported SWR. Instead, the feedline losses are cumulative to the SWR losses.
So… how much loss is actually imparted by a given SWR value? Look at it this way. A 1.0:1 match results in 100% of the radio’s output RF power being transferred into the antenna system. However, a 1.1:1 match results in a 0.2366% loss, a 1.2:1 match gives us a loss of 0.826% of the output RF power, and with a 1.5:1 match, we get a 4% power loss. All of those values are generally considered to be acceptable, though the lower the numeric match is, the better. On the other hand, a 2.0:1 match yields a whopping 11.1% power loss. Of course, it goes up from there.
What exactly does SWR measure or indicate then? Simply put, the standing wave ratio is the ratio of transmitted output power in watts to the power level in watts reflected from the antenna system back to the transmitter.
Note too that there is another peculiarity to SWR measurement. For example, there are two scenarios that both result in a 2:1 SWR. If the transmitter output impedance is fifty ohms, an antenna system input impedance of either twenty-five ohms or one-hundred ohms will result in a 2:1 SWR match. This is because SWR, when it is not a perfect 1:1 ratio, is always reported with the antecedent as a value greater than one, and the consequent as a unity value, or a value of one. Thus, either way, we get a 2:1 ratio from the impedance pairs of fifty ohms and twenty-five ohms, which is naturally a 2:1 ratio, or fifty ohms and one-hundred ohms, which is naturally a 1:2 ratio, but as an SWR value is expressed as a 2:1 value. Again, the antecedent, or “first” number is always greater than the consequent, or “second” number, when it is not a 1:1 match.
The next thing that must be understood is what happens to the reflected power once it enters the feedline after being reflected by the mis-matched antenna. Remember what I said about feedline loss? Well, it works both ways. The reflected power will make the reverse trip up the feedline towards the transmitter, where it will again be reflected back down the feedline towards the antenna. This will occur repeatedly until the reflected power has all been dissipated by the ohmic resistance of the feedline - or in other words by the feedline loss. Each trip through the feedline consumes a certain percentage of the power, until it is all gone, having been converted to heat in the feedline.
OK - let’s move on to the antenna tuner itself. Antenna tuners are made with various architectures, which will directly affect the number of controls to be manipulated on the front panel of the tuner unit. For example, the Emtech ZM-2 ATU tuner (Figure 3) uses a single toroid with a complex winding system as its inductor, and it uses two air-gap variable dual capacitors as the tuning controls. Its architecture is basically a CL/C “L” filter, having a series capacitor and inductor and a second shunt capacitor. This is a QRP tuner, meant strictly for low-power systems.
Another example is the MFJ-941 Versa-Tuner, whose schematic is shown in Figure 4. This tuner uses a slightly different arrangement of two air-gap variable capacitors and a step-selectable coil as the inductor. In this case, the capacitors are both in series, and the inductor is in shunt, forming a CLC “T” filter.
I could go on and show other approaches, especially as related to the auto-tuner types, but my point is sufficiently made already. With multiple architectures and circuit designs, and with different control arrangements, how can there possibly be one single procedure for tuning these devices?
The first step in adjusting your manual antenna tuner is to determine what the indicated forward power is like when you have as close to a 1:1 SWR as is possible, and this is best done at a low power setting - as low as your particular radio will adjust is fine. In some cases that will be five watts, but in others, like some Alinco models, it is ten watts. Whatever your lowest power setting is, set the radio to that output RF power level, and set the radio to a carrier-type of output. In most cases, this may well be the CW mode. Tune the set to the desired output frequency. Finally, place the SWR meter directly in line after the transmitter, and connect the feedline to a fifty-ohm resistive dummy load. Perform any calibration steps that your SWR meter calls for. Then, trigger the RF output into the dummy load and note both the forward and the reflected power levels while also noting the indicated SWR. When the SWR and power meters are integrated into the antenna tuner, often as a single meter, this is a bit easier as it is often just a matter of setting the proper output selection and then setting the meter function to SWR to read the standing-wave ratio, or to POWER to read the indicated power levels. Remember that in the case of a dual-needle (or cross-needle) meter, all three levels are read at once. One needle indicates the forward power, the opposite needle indicates the reflected power, and the point where the two needles cross each other indicates the SWR.
OK - you now have a baseline reading to start with. You know that your radio had RF output, and you know what power level to expect as you tune further. Now, select (or connect to) the antenna output for the antenna that you want to tune. Then, if your antenna tuner includes an inductor control, select the lowest inductance level to start with. Next, alternating between the two capacitor controls, make small adjustments in the capacitor mesh and note the indicated SWR when you trigger the RF output. Do this quickly so as not to be tuning excessively on the air. If you cannot adjust the SWR to an acceptable low value, change the inductance selector position by one step and repeat the capacitor manipulation, again alternating back and forth between the two capacitors, making small adjustments each time. Small adjustment increments are important, as with a large capacitor change, you can tune right through the optimal position and never know that you did so.
Repeat this entire process until you achieve the lowest SWR possible. Then, once you have reached an acceptable SWR match for that frequency, do yourself a favor and record the specific inductor and capacitor control positions that resulted in that match. Over time, if you do this each time you tune to a new frequency, you will develop a table of control positions that will allow you to very rapidly tune the system for a given frequency. It is for this reason that the inductor positions and the capacitor dial ranges have incremental markings. They really do not mean anything other than to give the user a means of quickly repeating a tuned position that was determined earlier. With time and experience, you will develop the skill to quickly and accurately tune the system for any frequency that you choose to use.
If your particular antenna tuner does not, for example, have an inductor control like the Emtech ZM2 ATU mentioned earlier, it is simply a matter of alternating capacitor adjustments until you reach the desired SWR value. That particular tuner actually incorporates a “tuned” indicator in the form of an LED that illuminates when a low SWR value is reached.
OK - so what do you do when (not “if”) you cannot achieve a sufficiently low SWR with which you are comfortable? At some point this is very likely to occur, and with frequent occurrences for certain types of antennas. For example, what happens when that ice storm hits and it pulls down one side of your wire dipole? Or, what happens when that pesky squirrel or chipmunk chews right through your feedline coaxial cable? Or, how about when the ice load and/or repetitive wind action on your dipole stretches the wire to some unusually long length? All of these events will show up as variations in the SWR attainable with your system. The fact of this matter is that when you cannot achieve the expected results, look for something about your antenna system to have changed. Similarly, when the control positions that once worked for a given frequency no longer do so, once again look for a physical change in the antenna system.
I hope that this has given you some insight into the workings and operation of common antenna tuners as used with typical HF radios. The bottom line is that there is no real “one size fits all” method to antenna tuner operation - but it really doesn’t matter - so long as you ultimately attain the lowest SWR value that you can get.
April 2026
Q : I am thinking about buying a used HF plus 50MHz transceiver, but I want to be sure that it operates properly before I complete the deal. I have given the seller a refundable deposit, and he is allowing me to take the radio to a shop for evaluation before I buy it. Is it really necessary to have the radio checked out? I was thinking that if the seller is willing to allow me to take it to be tested, it must be OK. Should the radio be tested anyway, and how is that done?
A : The fact that the seller is willing to allow you to take the radio for testing could be a good sign, indicating that the seller at least believes that the radio is in good condition. However, that belief could just as easily be a false belief based in trust and expectation on the part of the seller. For example, consider the situation where the seller was not the user of the radio and actually knows very little about amateur radio in general nor about that radio in specific. Maybe he is selling items from his father’s or grandfather’s estate and is hoping to simply get the most that he can for the equipment. A somewhat similar situation came up recently, wherein one of our Club members actually purchased a radio on Facebook Marketplace (FBMP) where the radio had been advertised as a “police scanner”.
FBMP is one arena where I seldom if ever recommend the purchase of radio equipment, as there are far too many scammers out there selling junk radios on FBMP and leaving the buyer with little or no recourse. I am aware of multiple Club members who have been taken by “good deals” on FBMP and were then out an aggregate of almost $3500 for equipment that would never work again but where the sellers refused to make good on the sales. For that reason, I was skeptical when one radio, a nicely-kept Icom IC-756 PRO III was brought in for testing and evaluation.
Let’s talk about how I go about testing one of these radios. With almost any antenna connected to the antenna jack, I listen to the audio output while tuning across the various bands, listening for distortion in the received audio, while simultaneously verifying the operation of the various controls that have direct and indirect impacts on the received signal. If the receiver checks out OK, I move on to the T/R switch and the transmitter. For this testing, I prefer to use my Siglent SVA1015X combination spectrum analyzer and vector network analyzer, in conjunction with one of my Bird 43 directional wattmeters or maybe my Telepost LP-500 Digital Station Monitor. Also necessary are some cables and adapters, and an appropriate attenuator used to protect the SA from high-level RF signals entering the analyzer front end. Finally, the last required item is a suitable dummy load, but more about that later.
The radio is configured with both a mic and a Morse key. RF output from the radio is routed to the Bird meter inlet or to the LP-500 adapter inlet. The Bird meter or adapter output is passed directly to a 100W dummy load for some basic power testing. The Bird meter should be equipped with an appropriate element for measuring the entire HF band as well as its upper-end and lower-end neighboring bands, meaning that I can test from 1MHz through to 60MHz.
In this case, basic power output testing in CW mode, AM mode, and FM mode showed that the full advertised power for the IC-756 PRO III was being developed and sent to the antenna. We saw 40 watts in the AM mode, and 100 watts in both CW and FM modes. With the assurance that when set to maximum RF power, the full advertised power would be developed, we moved on to frequency testing.
To accomplish the frequency testing, I modified the output cabling scheme slightly. The Bird meter outlet was now provided with a “TEE” fitting that allows the signal to travel two ways. One signal path from the tee is directly to and into the dummy load. The second output signal path is to the SA, where it is passed into the RF IN port via an inline attenuator. In this situation, I used my 10W 40dB inline attenuator, installed directly at the RF IN port of the SA. The SVA1015X was set to SA mode, with a stimulus range of between 500kHz and 60MHz, theoretically allowing us to see the entire HF band. I was now ready for frequency testing.
I began by setting the radio up for CW mode, at minimum output power of five watts, and with the VFO set for 1.8MHz. The result was a single peak on the SA screen, but offset slightly high from the VFO value. I moved to 3.6MHz and repeated the test, obtaining similar results. This was the case all the way up the band list to 10-meters, where I decided to stop and make some changes. The one thing that we never saw at all were any peaks at locations that would represent harmonics of the fundamental frequencies at play here. From that, it was safe to draw the conclusion that the harmonics were properly attenuated and should therefore cause no problems nor any reason to reject the radio.
Because of certain patterns that became evident as the testing progressed, I became convinced that the difference between the frequency set in the IC-756 VFO and that reported by the SA was a function of the granularity of the SA, based upon its sampling rate and the width of the frequency span that we were using as a test basis. I decided to focus directly on each transmit frequency, band-by-band, looking for the highest level of accuracy that I could attain for each chosen frequency.
This time, when I set the VFO to 1.8MHz, I also set the SA stimulus to a frequency range of 1MHz (START) and 2MHz (STOP). This time, when making the transmit test, the SA reported the output peak to be exactly at the frequency set in the radio VFO window. Using this process, I tested a couple of frequencies in each of the bands supported by the IC-756 PRO III, with the same excellent results. This radio was as closely aligned as any radio ever could be. Now it was time to go back to some more detailed power output testing.
I removed the tee fitting and the cable connection to the SA, leaving just the Bird meter and the dummy load in circuit. However, when I began the power output test, I discovered that there was only a very minimal power output from the radio in any mode, on any frequency, and at any power setting. A quick check of the feedline circuit cables and connectors showed everything to be correctly installed and connected. Setting the front panel “S” meter to its SWR mode, a quick keying of the mike showed that the SWR would fully deflect the needle to the right - indicating infinite SWR.
Stripping down the test circuit to just a dummy load and a single length of coaxial cable made no difference. By this time, some of the other Club members who were at the Clubhouse got involved, and we quickly tested both lengths of coaxial cable that had been used, checking them for shorts as well as for opens. The cables tested out OK. We then measured the resistance through the dummy load, and found it to be as close to fifty ohms as was possible. The only test left to make was an SWR test of the dummy load.
I switched the SA1015X to its VNA mode and connected the dummy load up to the S11 port, after configuring the VNA for SWR measurement with a stimulus range of 1MHz to 60MHz. A very surprising result was obtained. The SWR of this dummy load had suddenly and without any warning changed from a 1:1 ratio up to a high of 33:1 at the 1MHz end of the sweep, a low of 9:1 at about 14MHz, and then back up to about 17:1 at the 60MHz end of the sweep.
I swapped out that dummy load for another one from the test bench and resumed the testing of the radio. The output power testing showed that the power level tracked the power level control setting quite accurately on all bands and in all modes. In CW and RTTY as well as in FM mode, the maximum measured output power was right at about one hundred watts, as advertised. Furthermore, the maximum output power in the AM mode was right at forty watts, also as advertised.
The end results were quite interesting. First was the fact that all frequencies generated by this radio are right exactly where they are supposed to be, which is a great thing. Next is the fact that the output powers are all up to snuff and are right where the specifications sheet says that they should be. No scientific testing was done either as to the modulation levels on transmit nor as regards any of the receive functions. However, the very non-scientific but more real-world method of human-ear testing showed this radio to be in good shape in those areas as well. Lastly, we got to see first-hand that the protective power-roll-back system does in fact work and it works quite well. When the SWR went off the chart, the radio did its job and cut back on the output power so as not to damage the output transistors in the radio.
In all ways that matter, this radio seemed to test out in great shape, and I would have no hesitation about buying it, which just goes to show that there’s an exception to every rule. My “don’t buy radio equipment from a FBMP seller” policy was shown to be invalid in this case. I am very happy that Josh Boylan KE2FSC, the new owner of this great radio, got such a tremendous deal on a really nice radio.
This is a good illustration of the type of testing that can and most likely should be made to any used radio that a ham may tentatively decide to purchase. If the radio checks out OK, then by all means, go ahead and buy it. But… if it falls short, either re-negotiate the purchase price based on those findings, or just walk away from the deal completely. I hope that this illustrative case history explains the type of testing that can and probably should be done when time and conditions permit, before a radio is purchased.
See you next month!
Q : I am thinking about buying a used HF plus 50MHz transceiver, but I want to be sure that it operates properly before I complete the deal. I have given the seller a refundable deposit, and he is allowing me to take the radio to a shop for evaluation before I buy it. Is it really necessary to have the radio checked out? I was thinking that if the seller is willing to allow me to take it to be tested, it must be OK. Should the radio be tested anyway, and how is that done?
A : The fact that the seller is willing to allow you to take the radio for testing could be a good sign, indicating that the seller at least believes that the radio is in good condition. However, that belief could just as easily be a false belief based in trust and expectation on the part of the seller. For example, consider the situation where the seller was not the user of the radio and actually knows very little about amateur radio in general nor about that radio in specific. Maybe he is selling items from his father’s or grandfather’s estate and is hoping to simply get the most that he can for the equipment. A somewhat similar situation came up recently, wherein one of our Club members actually purchased a radio on Facebook Marketplace (FBMP) where the radio had been advertised as a “police scanner”.
FBMP is one arena where I seldom if ever recommend the purchase of radio equipment, as there are far too many scammers out there selling junk radios on FBMP and leaving the buyer with little or no recourse. I am aware of multiple Club members who have been taken by “good deals” on FBMP and were then out an aggregate of almost $3500 for equipment that would never work again but where the sellers refused to make good on the sales. For that reason, I was skeptical when one radio, a nicely-kept Icom IC-756 PRO III was brought in for testing and evaluation.
Let’s talk about how I go about testing one of these radios. With almost any antenna connected to the antenna jack, I listen to the audio output while tuning across the various bands, listening for distortion in the received audio, while simultaneously verifying the operation of the various controls that have direct and indirect impacts on the received signal. If the receiver checks out OK, I move on to the T/R switch and the transmitter. For this testing, I prefer to use my Siglent SVA1015X combination spectrum analyzer and vector network analyzer, in conjunction with one of my Bird 43 directional wattmeters or maybe my Telepost LP-500 Digital Station Monitor. Also necessary are some cables and adapters, and an appropriate attenuator used to protect the SA from high-level RF signals entering the analyzer front end. Finally, the last required item is a suitable dummy load, but more about that later.
The radio is configured with both a mic and a Morse key. RF output from the radio is routed to the Bird meter inlet or to the LP-500 adapter inlet. The Bird meter or adapter output is passed directly to a 100W dummy load for some basic power testing. The Bird meter should be equipped with an appropriate element for measuring the entire HF band as well as its upper-end and lower-end neighboring bands, meaning that I can test from 1MHz through to 60MHz.
In this case, basic power output testing in CW mode, AM mode, and FM mode showed that the full advertised power for the IC-756 PRO III was being developed and sent to the antenna. We saw 40 watts in the AM mode, and 100 watts in both CW and FM modes. With the assurance that when set to maximum RF power, the full advertised power would be developed, we moved on to frequency testing.
To accomplish the frequency testing, I modified the output cabling scheme slightly. The Bird meter outlet was now provided with a “TEE” fitting that allows the signal to travel two ways. One signal path from the tee is directly to and into the dummy load. The second output signal path is to the SA, where it is passed into the RF IN port via an inline attenuator. In this situation, I used my 10W 40dB inline attenuator, installed directly at the RF IN port of the SA. The SVA1015X was set to SA mode, with a stimulus range of between 500kHz and 60MHz, theoretically allowing us to see the entire HF band. I was now ready for frequency testing.
I began by setting the radio up for CW mode, at minimum output power of five watts, and with the VFO set for 1.8MHz. The result was a single peak on the SA screen, but offset slightly high from the VFO value. I moved to 3.6MHz and repeated the test, obtaining similar results. This was the case all the way up the band list to 10-meters, where I decided to stop and make some changes. The one thing that we never saw at all were any peaks at locations that would represent harmonics of the fundamental frequencies at play here. From that, it was safe to draw the conclusion that the harmonics were properly attenuated and should therefore cause no problems nor any reason to reject the radio.
Because of certain patterns that became evident as the testing progressed, I became convinced that the difference between the frequency set in the IC-756 VFO and that reported by the SA was a function of the granularity of the SA, based upon its sampling rate and the width of the frequency span that we were using as a test basis. I decided to focus directly on each transmit frequency, band-by-band, looking for the highest level of accuracy that I could attain for each chosen frequency.
This time, when I set the VFO to 1.8MHz, I also set the SA stimulus to a frequency range of 1MHz (START) and 2MHz (STOP). This time, when making the transmit test, the SA reported the output peak to be exactly at the frequency set in the radio VFO window. Using this process, I tested a couple of frequencies in each of the bands supported by the IC-756 PRO III, with the same excellent results. This radio was as closely aligned as any radio ever could be. Now it was time to go back to some more detailed power output testing.
I removed the tee fitting and the cable connection to the SA, leaving just the Bird meter and the dummy load in circuit. However, when I began the power output test, I discovered that there was only a very minimal power output from the radio in any mode, on any frequency, and at any power setting. A quick check of the feedline circuit cables and connectors showed everything to be correctly installed and connected. Setting the front panel “S” meter to its SWR mode, a quick keying of the mike showed that the SWR would fully deflect the needle to the right - indicating infinite SWR.
Stripping down the test circuit to just a dummy load and a single length of coaxial cable made no difference. By this time, some of the other Club members who were at the Clubhouse got involved, and we quickly tested both lengths of coaxial cable that had been used, checking them for shorts as well as for opens. The cables tested out OK. We then measured the resistance through the dummy load, and found it to be as close to fifty ohms as was possible. The only test left to make was an SWR test of the dummy load.
I switched the SA1015X to its VNA mode and connected the dummy load up to the S11 port, after configuring the VNA for SWR measurement with a stimulus range of 1MHz to 60MHz. A very surprising result was obtained. The SWR of this dummy load had suddenly and without any warning changed from a 1:1 ratio up to a high of 33:1 at the 1MHz end of the sweep, a low of 9:1 at about 14MHz, and then back up to about 17:1 at the 60MHz end of the sweep.
I swapped out that dummy load for another one from the test bench and resumed the testing of the radio. The output power testing showed that the power level tracked the power level control setting quite accurately on all bands and in all modes. In CW and RTTY as well as in FM mode, the maximum measured output power was right at about one hundred watts, as advertised. Furthermore, the maximum output power in the AM mode was right at forty watts, also as advertised.
The end results were quite interesting. First was the fact that all frequencies generated by this radio are right exactly where they are supposed to be, which is a great thing. Next is the fact that the output powers are all up to snuff and are right where the specifications sheet says that they should be. No scientific testing was done either as to the modulation levels on transmit nor as regards any of the receive functions. However, the very non-scientific but more real-world method of human-ear testing showed this radio to be in good shape in those areas as well. Lastly, we got to see first-hand that the protective power-roll-back system does in fact work and it works quite well. When the SWR went off the chart, the radio did its job and cut back on the output power so as not to damage the output transistors in the radio.
In all ways that matter, this radio seemed to test out in great shape, and I would have no hesitation about buying it, which just goes to show that there’s an exception to every rule. My “don’t buy radio equipment from a FBMP seller” policy was shown to be invalid in this case. I am very happy that Josh Boylan KE2FSC, the new owner of this great radio, got such a tremendous deal on a really nice radio.
This is a good illustration of the type of testing that can and most likely should be made to any used radio that a ham may tentatively decide to purchase. If the radio checks out OK, then by all means, go ahead and buy it. But… if it falls short, either re-negotiate the purchase price based on those findings, or just walk away from the deal completely. I hope that this illustrative case history explains the type of testing that can and probably should be done when time and conditions permit, before a radio is purchased.
See you next month!
March 2026
Q : “I recently opened up an old balun that I was given, so that I could look at what was inside the housing and see how it was made. What I found confused me, which brings me to the question that I need to have answered. I was told that this was a 9:1 balun, but there are only eight turns wound on the toroid. What is the type and turns ratio of a toroidal transformer wound with three parallel (not twisted) wires making eight turns on the toroid, with each wire labeled as A1 at one end and A2 at the other end, and so forth through B1/B2 and C1/C2, and where A1 goes to the antenna terminal, B1 connects to A2, C1 connects to the transmitter and to B2, and C2 connects to ground? Also, how much power can this balun handle?”
Q : “I recently opened up an old balun that I was given, so that I could look at what was inside the housing and see how it was made. What I found confused me, which brings me to the question that I need to have answered. I was told that this was a 9:1 balun, but there are only eight turns wound on the toroid. What is the type and turns ratio of a toroidal transformer wound with three parallel (not twisted) wires making eight turns on the toroid, with each wire labeled as A1 at one end and A2 at the other end, and so forth through B1/B2 and C1/C2, and where A1 goes to the antenna terminal, B1 connects to A2, C1 connects to the transmitter and to B2, and C2 connects to ground? Also, how much power can this balun handle?”
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A : To start out, the device is actually not a balun at all, but is actually an unun. Refer to the drawing at Figure 1 as you read the response that follows. In a nutshell, what you have described, and what is shown in the drawing, is a 9:1 unun, which is an unbalanced device to unbalanced device transformer (or autotransformer) used for antenna impedance matching. |
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Type and Function
This configuration creates an autotransformer or, more specifically in amateur radio applications, an unun. An unun matches an unbalanced line (coaxial cable from the transmitter) to another unbalanced impedance (like a random wire antenna).
The purpose of this specific wiring is to match a 50-ohm transmitter output to a much higher impedance antenna, typically in the range of 450 ohms (9 x 50 ohms).
Turns Ratio
The turns ratio is determined by the total number of turns used for the primary (input) winding and the secondary (output) winding.
Referring to the wire end designations shown in the Figure 1 drawing, the input (primary winding) is across C1 (connected to the transmitter center pin) and C2 (connected to ground). C1 connects to B2, B1 connects to A2, and A1 goes to the antenna terminal. The input uses the turns of winding C and winding B in series, with C2 grounded. So, the number of turns for the primary is the eight turns of wire C, with B and A windings acting as part of the total coupled system.
The output (secondary winding) is across A1 (antenna terminal) and C2 (ground). The output uses the series combination of windings A, B, and C. So, the number of turns for the secondary is the eight turns of A plus the eight turns of B plus the eight turns of C, totaling twenty-four turns.
As to the turns ratio of the primary to the secondary (primary : secondary) windings, what we have is eight turns of the primary to twenty-four turns of the secondary (8 : 24), which simplifies to a 1 : 3 turns ratio.
Impedance Ratio
The impedance ratio is a bit different. The impedance ratio can be calculated from the turns ratio, as the square of the related turns ratio. Therefore, with a 1:3 primary to secondary turns ratio, we would have 12 : 32, or 1 : 9 as the primary to secondary impedance ratio. This results in a 9:1 impedance transformation, which is a common ratio for matching a 50-ohm transmitter to a high-impedance random wire antenna.
This unun would be appropriate for connection of a random-length wire antenna or an end-fed antenna of either band-cut (tuned) length or random length. Ideal antenna impedances would be on the order of 450 ohms, though impedances as low as 200 ohms will operate satisfactorily.
Power Handling
Power handling is primarily a function of the size and material of the transformer core upon which the transformer is wound. To a much lesser extent, it is also a function of the wire gauge and type used in the construction of the transformer. As you did not provide any core or wire data, the power-handling capability of this unun cannot readily be determined. However, some information can be provided, based upon common design practices for this type of device.
It is common practice to wind the transformers of such devices using solid copper enamel-coated magnet wire of a diameter ranging from 14 AWG to, under certain circumstances, 22 AWG. Typically, for medium - to high-power applications, 14 AWG to 16 AWG magnet wire is used. For low - to medium-power applications, 18 AWG to 22 AWG magnet wire is common, and for the highest power applications, 14 AWG to 10 AWG magnet wire is used.
One common design uses an FT240-43 toroid and 15 AWG wire, with Teflon™ tube sleeving on the wire. Such an arrangement will produce an unun capable of handling 200 to 400 watts in continuous wave operation, and from 500 to 700 watts in single-sideband operations. This design, using the same 15 AWG wire, can be boosted to 1500W full legal power capability by stacking two or three of the FT240-43 toroidal cores. Stacking the cores helps to prevent core saturation and also serves to provide for better heat management.
I hope that this helps you to understand a little bit about this impedance-matching device and how its ratios can be determined. Sometimes, a diagram helps to visualize the circuit a bit better than words alone can do.
See you next month!
This configuration creates an autotransformer or, more specifically in amateur radio applications, an unun. An unun matches an unbalanced line (coaxial cable from the transmitter) to another unbalanced impedance (like a random wire antenna).
The purpose of this specific wiring is to match a 50-ohm transmitter output to a much higher impedance antenna, typically in the range of 450 ohms (9 x 50 ohms).
Turns Ratio
The turns ratio is determined by the total number of turns used for the primary (input) winding and the secondary (output) winding.
Referring to the wire end designations shown in the Figure 1 drawing, the input (primary winding) is across C1 (connected to the transmitter center pin) and C2 (connected to ground). C1 connects to B2, B1 connects to A2, and A1 goes to the antenna terminal. The input uses the turns of winding C and winding B in series, with C2 grounded. So, the number of turns for the primary is the eight turns of wire C, with B and A windings acting as part of the total coupled system.
The output (secondary winding) is across A1 (antenna terminal) and C2 (ground). The output uses the series combination of windings A, B, and C. So, the number of turns for the secondary is the eight turns of A plus the eight turns of B plus the eight turns of C, totaling twenty-four turns.
As to the turns ratio of the primary to the secondary (primary : secondary) windings, what we have is eight turns of the primary to twenty-four turns of the secondary (8 : 24), which simplifies to a 1 : 3 turns ratio.
Impedance Ratio
The impedance ratio is a bit different. The impedance ratio can be calculated from the turns ratio, as the square of the related turns ratio. Therefore, with a 1:3 primary to secondary turns ratio, we would have 12 : 32, or 1 : 9 as the primary to secondary impedance ratio. This results in a 9:1 impedance transformation, which is a common ratio for matching a 50-ohm transmitter to a high-impedance random wire antenna.
This unun would be appropriate for connection of a random-length wire antenna or an end-fed antenna of either band-cut (tuned) length or random length. Ideal antenna impedances would be on the order of 450 ohms, though impedances as low as 200 ohms will operate satisfactorily.
Power Handling
Power handling is primarily a function of the size and material of the transformer core upon which the transformer is wound. To a much lesser extent, it is also a function of the wire gauge and type used in the construction of the transformer. As you did not provide any core or wire data, the power-handling capability of this unun cannot readily be determined. However, some information can be provided, based upon common design practices for this type of device.
It is common practice to wind the transformers of such devices using solid copper enamel-coated magnet wire of a diameter ranging from 14 AWG to, under certain circumstances, 22 AWG. Typically, for medium - to high-power applications, 14 AWG to 16 AWG magnet wire is used. For low - to medium-power applications, 18 AWG to 22 AWG magnet wire is common, and for the highest power applications, 14 AWG to 10 AWG magnet wire is used.
One common design uses an FT240-43 toroid and 15 AWG wire, with Teflon™ tube sleeving on the wire. Such an arrangement will produce an unun capable of handling 200 to 400 watts in continuous wave operation, and from 500 to 700 watts in single-sideband operations. This design, using the same 15 AWG wire, can be boosted to 1500W full legal power capability by stacking two or three of the FT240-43 toroidal cores. Stacking the cores helps to prevent core saturation and also serves to provide for better heat management.
I hope that this helps you to understand a little bit about this impedance-matching device and how its ratios can be determined. Sometimes, a diagram helps to visualize the circuit a bit better than words alone can do.
See you next month!
February 2026
Q : If I want to program a mobile radio with the repeaters that would be available along a given route that I plan to use for a road trip, how would I go about doing this?
A : There are several sources available that provide information about amateur radio repeaters located within certain areas, including both print and online sources. However, depending upon the geographic area to be traveled and also depending upon the specific route(s) to be travelled, some extensive detective work may be required.
By far, the best resource for this information is RepeaterBook and by extension, www.repeaterbook.com. RepeaterBook is a print-format reference that lists radio repeaters in various geographic areas. The volume lists both amateur radio repeaters and GMRS repeaters, which makes it a simple matter to identify the location, frequency, call sign, offset, and access codes for a huge number of repeaters, worldwide. While there are many countries without repeaters listed, the vast majority of the so-called “civilized world” is covered and will have repeaters listed in this reference. The listed repeaters are arranged geographically, and then by frequency.
A very user-friendly adjunct to the print version of RepeaterBook is the online version of the reference. The primary segregation of listings is that of North America and then the “rest of the world”. Within the North American repeater listings, specific location listings are broken down in several different ways. For example, it is possible, on a state-by-state basis, to search for repeaters based upon the bands on which they operate, the modes under which the repeaters operate, features supported by the repeaters, systems to which the repeaters may be linked, emergency services supported by the repeaters, highways nearby to the repeaters, the towns in which they are located or which are nearby the repeater’s location, and the county in the repeater is located. The illustration at Figure 1 depicts an online listing for repeaters in the Atlantic City, NJ area, while Figure 2 shows a listing of repeaters along US-322 in New Jersey. The online version of Repeater Book is found at https://www.repeaterbook.com.
Q : If I want to program a mobile radio with the repeaters that would be available along a given route that I plan to use for a road trip, how would I go about doing this?
A : There are several sources available that provide information about amateur radio repeaters located within certain areas, including both print and online sources. However, depending upon the geographic area to be traveled and also depending upon the specific route(s) to be travelled, some extensive detective work may be required.
By far, the best resource for this information is RepeaterBook and by extension, www.repeaterbook.com. RepeaterBook is a print-format reference that lists radio repeaters in various geographic areas. The volume lists both amateur radio repeaters and GMRS repeaters, which makes it a simple matter to identify the location, frequency, call sign, offset, and access codes for a huge number of repeaters, worldwide. While there are many countries without repeaters listed, the vast majority of the so-called “civilized world” is covered and will have repeaters listed in this reference. The listed repeaters are arranged geographically, and then by frequency.
A very user-friendly adjunct to the print version of RepeaterBook is the online version of the reference. The primary segregation of listings is that of North America and then the “rest of the world”. Within the North American repeater listings, specific location listings are broken down in several different ways. For example, it is possible, on a state-by-state basis, to search for repeaters based upon the bands on which they operate, the modes under which the repeaters operate, features supported by the repeaters, systems to which the repeaters may be linked, emergency services supported by the repeaters, highways nearby to the repeaters, the towns in which they are located or which are nearby the repeater’s location, and the county in the repeater is located. The illustration at Figure 1 depicts an online listing for repeaters in the Atlantic City, NJ area, while Figure 2 shows a listing of repeaters along US-322 in New Jersey. The online version of Repeater Book is found at https://www.repeaterbook.com.
The www.repeaterbook.com website offers the user the ability to set up a user account, and it also offers subscriptions to the latest news about the publication and the website. However, of much more value than a simple user account is the RepeaterBook Plus feature. This service is available at a subscription rate of $3.99 per month or for a flat $14.99 per year. Being a RepeaterBook Plus user gains you certain access, advertisement-free, to exportable and customizable searches that can be saved for future use.
The www.repeaterbook.com site offers extensive search and export capabilities, including exports in various formats for popular radios and programming software. For example, the user can perform a search along Interstate 80 on a state-by-state basis clear across the USA from one coast to the other, building a file that can be exported directly into CHIRP or the RT Systems programming application for upload directly to the radio. Suffice it to say that if I were to be planning a road trip, I would certainly use RepeaterBook as a part of the planning process. The online availability also makes the reference very portable, making it easy to rework a route if the plans should change somewhere along the way.
I should also point out that the RepeaterBook database is also accessible and searchable from within the CHIRP software, as are a few other databases of use for this purpose. Refer to Figure 3, where the Radio > Query Source command is shown. Note that six different search sources are available, all of which will bring the retrieved data into a CHIRP tab for easy copy-and-paste operations. I mention the copy-and-paste routine because that is what would need to be done to move the retrieved data into the tab for the radio being programmed - the retrieved data is brought into CHIRP in its own new tab.
The www.repeaterbook.com site offers extensive search and export capabilities, including exports in various formats for popular radios and programming software. For example, the user can perform a search along Interstate 80 on a state-by-state basis clear across the USA from one coast to the other, building a file that can be exported directly into CHIRP or the RT Systems programming application for upload directly to the radio. Suffice it to say that if I were to be planning a road trip, I would certainly use RepeaterBook as a part of the planning process. The online availability also makes the reference very portable, making it easy to rework a route if the plans should change somewhere along the way.
I should also point out that the RepeaterBook database is also accessible and searchable from within the CHIRP software, as are a few other databases of use for this purpose. Refer to Figure 3, where the Radio > Query Source command is shown. Note that six different search sources are available, all of which will bring the retrieved data into a CHIRP tab for easy copy-and-paste operations. I mention the copy-and-paste routine because that is what would need to be done to move the retrieved data into the tab for the radio being programmed - the retrieved data is brought into CHIRP in its own new tab.
I realize that this answer may barely scratch the surface in describing the available options for planning a route, but the RepeaterBook route is, by far, the most common method used. I hope that this response helps the reader to understand a little bit about what is out there as far as route planning options.
See you next month!
January 2026
Q : Why are different numbers sometimes given as the dividend when calculating wire lengths for dipole antenna construction? For example, sometimes we are told to divide the frequency in megahertz into 300, or sometimes into 984, or other times we are told to use 285, or maybe 934. Other times we see values used like 150 or 492, or else we see 142, while other times we might see 467 used. None of this makes any sense. Is there an explanation for all of the different numbers we are given?
A : While it may seem somewhat confusing at first, there really is a reason for it all. We must remember that while antenna length is based upon wavelength, it most often is not a full wavelength or even a full fractional wavelength. The basic equation for calculating wavelength, abbreviated with the Greek letter lambda (λ), is λ = c/f_MHz where c is the speed of light in free space and fMHz is the frequency of the pertinent signal, given in megahertz.
When it comes to antenna design, however, we must derate c by a certain percentage due to the higher density of the material of which the antenna driven elements are constructed as compared to the “free space” density at which c is normally stated. Bear in mind that “free space” implies that the signal is propagated through a vacuum. Even transmitting a signal through air will slow it down a little bit, but not enough to worry about. However, that is not the case when we send the radio signal through a metal element. The derating percentage most often used is 5%.
The two metals most frequently used in amateur radio antenna construction are aluminum and copper. Copper has a density and therefore a volume per unit mass of about three times that of aluminum, making aluminum the material of choice for most large antenna elements. On the other hand, copper or copper-clad steel wire is most often used in wire antennas. However, when it comes to radio wave propagation, copper and aluminum have propagation characteristics that are fairly similar to each other. Further variations in the dividend value are derived from the fractional wavelength dimension at which the specific antenna element is sized. Table 1 below provides some common antenna element length equations and their method of derivation. When the term “metal” is used, the reference is to either copper or aluminum, interchangeably.
Q : Why are different numbers sometimes given as the dividend when calculating wire lengths for dipole antenna construction? For example, sometimes we are told to divide the frequency in megahertz into 300, or sometimes into 984, or other times we are told to use 285, or maybe 934. Other times we see values used like 150 or 492, or else we see 142, while other times we might see 467 used. None of this makes any sense. Is there an explanation for all of the different numbers we are given?
A : While it may seem somewhat confusing at first, there really is a reason for it all. We must remember that while antenna length is based upon wavelength, it most often is not a full wavelength or even a full fractional wavelength. The basic equation for calculating wavelength, abbreviated with the Greek letter lambda (λ), is λ = c/f_MHz where c is the speed of light in free space and fMHz is the frequency of the pertinent signal, given in megahertz.
When it comes to antenna design, however, we must derate c by a certain percentage due to the higher density of the material of which the antenna driven elements are constructed as compared to the “free space” density at which c is normally stated. Bear in mind that “free space” implies that the signal is propagated through a vacuum. Even transmitting a signal through air will slow it down a little bit, but not enough to worry about. However, that is not the case when we send the radio signal through a metal element. The derating percentage most often used is 5%.
The two metals most frequently used in amateur radio antenna construction are aluminum and copper. Copper has a density and therefore a volume per unit mass of about three times that of aluminum, making aluminum the material of choice for most large antenna elements. On the other hand, copper or copper-clad steel wire is most often used in wire antennas. However, when it comes to radio wave propagation, copper and aluminum have propagation characteristics that are fairly similar to each other. Further variations in the dividend value are derived from the fractional wavelength dimension at which the specific antenna element is sized. Table 1 below provides some common antenna element length equations and their method of derivation. When the term “metal” is used, the reference is to either copper or aluminum, interchangeably.
The decimal values in the dividends of the various table entries may be encountered as rounded whole-number values, and depending upon exactly what derate percentage is used, the values may appear as other values that are fairly close to the base value. For example, if a 5.5% derating factor is used instead of an even 5%, the dividend of 285 for the full-wave λ in free space would become 283.5, while a 4.5% derate factor would yield 286.5 as the new dividend value.
As it can now be readily seen, these equations can use a wide variety of values, but in any event, these values are intended only to produce a starting point for the final cutting and tuning of the antenna.
I hope that this has helped to shed some light on the confusion that results when we look at the various cut lengths recommended for the design and construction of your newest homebrew antenna.
As it can now be readily seen, these equations can use a wide variety of values, but in any event, these values are intended only to produce a starting point for the final cutting and tuning of the antenna.
I hope that this has helped to shed some light on the confusion that results when we look at the various cut lengths recommended for the design and construction of your newest homebrew antenna.
December 2025
Q : I keep hearing about the need to calibrate my NanoVNA before using it. What is this all about, and how does one calibrate a NanoVNA?
A : The NanoVNA is an incredibly powerful tool for the radio amateur, and even more so when the price of the tool is considered. However, any tool is only as good as its sharpest edge, and the sharpest edge of the NanoVNA is found in that condition where it has been properly calibrated for the task at hand.
What’s it all about? Simply put, while the unit has been factory calibrated and is electronically “adjusted” to give the best results possible, those results can be skewed by certain operating conditions. The calibration process is designed to “zero out” the specific circumstances that would otherwise skew the measurement results displayed on the NanoVNA. Any time that we change the NanoVNA connection plane or the stimulus frequency to be used, we must perform a calibration in order to garner proper accuracy under the current test conditions.
To better understand the need for calibration, let’s explore the operational conditions of the NanoVNA. Suppose that you are about to use the NanoVNA to determine a complex impedance of an antenna system, as displayed on a Smith chart. Radio theory tells us that making a change to the length of an antenna feedline will not affect the VSWR of that antenna system, but it will affect the impedance of the system. Remember that the impedance inverts every λ/4 of feedline length, and that the impedance repeats every λ/2 along a feedline. Thus, if we add a segment of feedline equal to anything less than λ/2 in length, we will end up with a different impedance. This is pretty clear, and is black-letter law as related to radio theory. It does not stop there, however.
If we want to measure the complex impedance of an antenna system via the NanoVNA, we must, as a part of that measurement, connect the antenna system to the NanoVNA. If, in doing so, we add a length of coaxial cable between the antenna system feed point and the NanoVNA, we will have the same effect as if we were simply adding feedline length to the antenna system - it will change the overall impedance of the antenna system.
So… in order to “see” just the actual impedance of the antenna system and not to have that impedance padded by the connecting cable, we must have a means of “zeroing” out the effect of the connecting cable. That method is what we call “NanoVNA calibration.”
It is important to understand that the calibration is to be made to the connection plane at which the antenna system under test will be connected. In this manner, the “zero” point is at the far end of the connecting cable, and that cable length will therefore not be taken as a part of the antenna system - it will effectively be invisible to the NanoVNA.
Calibration is done by use of a set of so-called “calibration standards”, a set that consists of three different external resistances in a physical form that will allow the standards to be installed to the connecting cable at the connection plane to be used. The three resistances used most often are infinite resistance (open), zero resistance (short), and the characteristic impedance for the antenna system under test (load). More often than not, for amateur radio, that load value is fifty ohms. In addition to the three calibration standards, a calibration kit will also include a double-ended female adapter that will permit connection of two cables with male connectors to each other, and a pair of short connecting cables. Ideally, the calibration kit will also contain a second “load” standard, though this is not often the case. (More about that later.)
Anyone who works with multiple different cable types in conjunction with the NanoVNA will have a set of calibration standards for each connector type frequently used, and in both genders. My calibration kit includes standards in male BNC, female BNC, female UHF, male N, and female N. The NanoVNA normally ships with a set of male SMA standards. My plan is to expand my set further to include female SMA and male UHF standards, though I seldom have a need for those types.
The intent in having all of these different standards is that they will enable calibration right at the connection plane without the use of adapters during the calibration process. Even the length of cable adapters and gender changers will add effective feedline length to the overall net, and will therefore affect the final complex impedance measurement. That small length will make a difference if the intent is to design a matching network that will bring the antenna system impedance to a true and clean fifty ohms.
Let’s now look at the calibration procedure :
1. Install any needed or desired connecting cables for attaching the antenna system to the NanoVNA
2. Determine what type of calibration will be made
a. If only Port S11 will be used, a single port calibration is acceptable; but
b. If a through (Port S21) connection will be used, then a through calibration must be done
3. Set up the NanoVNA for the test to be made, including setting up the…
a. Traces to be displayed
b. Markers to be used
c. Port and format to be used
d. Any other specific settings necessary; and
e. Stimulus frequencies, either START and STOP or CENTER and SPAN
4. Go to CALIBRATE > RESET to reset the calibration in preparation for the new calibration
5. Go to CALIBRATE > CALIBRATE
6. Install the open standard at the S11 connection plane
7. Tap OPEN and wait for the check mark to appear
8. Remove the open standard
9. Install the short standard at the S11 connection plane
10. Tap SHORT and wait for the check mark to appear
11. Remove the short standard
12. Install the load standard at the S11 connection plane
13. Tap LOAD and wait for the check mark to appear
14. If a single-port test is to be made, proceed to Step 15 below; if a two-port test is to be made, jump ahead to Step 18 below
15. Remove the load standard
16. Tap DONE and then select a suitable storage slot to save the calibration if so desired (recommended)
17. Proceed to test(s) to be made
18. Install a load standard to the second port, at the S21 connection plane. Note that this step requires the use of a second load standard, which may not always be on hand
19. Tap ISOLN and wait for the check mark to appear
20. Remove the load standards from the two ports
21. Connect the two ports to each other at the connection plane, using the shortest connecting adapter possible
22. Tap THRU and wait for the check mark to appear.
23. Disconnect the port jumper.
24. Tap DONE and then select a suitable storage slot to save the calibration if so desired (recommended).
25. Proceed to the test(s) to be made.
The NanoVNA is now calibrated and ready for use. On-screen, along the left edge, will be a legend that indicates which stored calibration is in use, and if it is in use directly as stored, or if there is a change from the stored settings.
The legend starts out with the letter “C”. If the “C” is in upper case text, the calibration is in use exactly as it was saved. However, if the “c” is in lower case text, it indicates that the stimulus frequency has been changed from that which was stored. The numeral after the “C” indicates which specific memory storage slot is in use. If the calibration has not been saved to a slot, the numeral will be replaced with an asterisk (*).
In addition, there are some letters that appear in a column under the calibration indicator, as follows :
Other power levels that can be selected are 2mA, 4mA, 6mA and 8mA. These are selected under the MENU > POWER menu item. If the text in the calibration status indicator is red in color, the “no calibration” calibration status is indicated. A properly calibrated NanoVNA will show its calibration status in white text, but during the calibration process, the text will show in a red color until the calibration is complete.
I hope that this article makes your NanoVNA just a bit more friendly and useful. Proper calibration is absolutely necessary for any degree of accuracy to be achieved. Knowing how and when to perform the calibration makes you more adept at using the NanoVNA.
Q : I keep hearing about the need to calibrate my NanoVNA before using it. What is this all about, and how does one calibrate a NanoVNA?
A : The NanoVNA is an incredibly powerful tool for the radio amateur, and even more so when the price of the tool is considered. However, any tool is only as good as its sharpest edge, and the sharpest edge of the NanoVNA is found in that condition where it has been properly calibrated for the task at hand.
What’s it all about? Simply put, while the unit has been factory calibrated and is electronically “adjusted” to give the best results possible, those results can be skewed by certain operating conditions. The calibration process is designed to “zero out” the specific circumstances that would otherwise skew the measurement results displayed on the NanoVNA. Any time that we change the NanoVNA connection plane or the stimulus frequency to be used, we must perform a calibration in order to garner proper accuracy under the current test conditions.
To better understand the need for calibration, let’s explore the operational conditions of the NanoVNA. Suppose that you are about to use the NanoVNA to determine a complex impedance of an antenna system, as displayed on a Smith chart. Radio theory tells us that making a change to the length of an antenna feedline will not affect the VSWR of that antenna system, but it will affect the impedance of the system. Remember that the impedance inverts every λ/4 of feedline length, and that the impedance repeats every λ/2 along a feedline. Thus, if we add a segment of feedline equal to anything less than λ/2 in length, we will end up with a different impedance. This is pretty clear, and is black-letter law as related to radio theory. It does not stop there, however.
If we want to measure the complex impedance of an antenna system via the NanoVNA, we must, as a part of that measurement, connect the antenna system to the NanoVNA. If, in doing so, we add a length of coaxial cable between the antenna system feed point and the NanoVNA, we will have the same effect as if we were simply adding feedline length to the antenna system - it will change the overall impedance of the antenna system.
So… in order to “see” just the actual impedance of the antenna system and not to have that impedance padded by the connecting cable, we must have a means of “zeroing” out the effect of the connecting cable. That method is what we call “NanoVNA calibration.”
It is important to understand that the calibration is to be made to the connection plane at which the antenna system under test will be connected. In this manner, the “zero” point is at the far end of the connecting cable, and that cable length will therefore not be taken as a part of the antenna system - it will effectively be invisible to the NanoVNA.
Calibration is done by use of a set of so-called “calibration standards”, a set that consists of three different external resistances in a physical form that will allow the standards to be installed to the connecting cable at the connection plane to be used. The three resistances used most often are infinite resistance (open), zero resistance (short), and the characteristic impedance for the antenna system under test (load). More often than not, for amateur radio, that load value is fifty ohms. In addition to the three calibration standards, a calibration kit will also include a double-ended female adapter that will permit connection of two cables with male connectors to each other, and a pair of short connecting cables. Ideally, the calibration kit will also contain a second “load” standard, though this is not often the case. (More about that later.)
Anyone who works with multiple different cable types in conjunction with the NanoVNA will have a set of calibration standards for each connector type frequently used, and in both genders. My calibration kit includes standards in male BNC, female BNC, female UHF, male N, and female N. The NanoVNA normally ships with a set of male SMA standards. My plan is to expand my set further to include female SMA and male UHF standards, though I seldom have a need for those types.
The intent in having all of these different standards is that they will enable calibration right at the connection plane without the use of adapters during the calibration process. Even the length of cable adapters and gender changers will add effective feedline length to the overall net, and will therefore affect the final complex impedance measurement. That small length will make a difference if the intent is to design a matching network that will bring the antenna system impedance to a true and clean fifty ohms.
Let’s now look at the calibration procedure :
1. Install any needed or desired connecting cables for attaching the antenna system to the NanoVNA
2. Determine what type of calibration will be made
a. If only Port S11 will be used, a single port calibration is acceptable; but
b. If a through (Port S21) connection will be used, then a through calibration must be done
3. Set up the NanoVNA for the test to be made, including setting up the…
a. Traces to be displayed
b. Markers to be used
c. Port and format to be used
d. Any other specific settings necessary; and
e. Stimulus frequencies, either START and STOP or CENTER and SPAN
4. Go to CALIBRATE > RESET to reset the calibration in preparation for the new calibration
5. Go to CALIBRATE > CALIBRATE
6. Install the open standard at the S11 connection plane
7. Tap OPEN and wait for the check mark to appear
8. Remove the open standard
9. Install the short standard at the S11 connection plane
10. Tap SHORT and wait for the check mark to appear
11. Remove the short standard
12. Install the load standard at the S11 connection plane
13. Tap LOAD and wait for the check mark to appear
14. If a single-port test is to be made, proceed to Step 15 below; if a two-port test is to be made, jump ahead to Step 18 below
15. Remove the load standard
16. Tap DONE and then select a suitable storage slot to save the calibration if so desired (recommended)
17. Proceed to test(s) to be made
18. Install a load standard to the second port, at the S21 connection plane. Note that this step requires the use of a second load standard, which may not always be on hand
19. Tap ISOLN and wait for the check mark to appear
20. Remove the load standards from the two ports
21. Connect the two ports to each other at the connection plane, using the shortest connecting adapter possible
22. Tap THRU and wait for the check mark to appear.
23. Disconnect the port jumper.
24. Tap DONE and then select a suitable storage slot to save the calibration if so desired (recommended).
25. Proceed to the test(s) to be made.
The NanoVNA is now calibrated and ready for use. On-screen, along the left edge, will be a legend that indicates which stored calibration is in use, and if it is in use directly as stored, or if there is a change from the stored settings.
The legend starts out with the letter “C”. If the “C” is in upper case text, the calibration is in use exactly as it was saved. However, if the “c” is in lower case text, it indicates that the stimulus frequency has been changed from that which was stored. The numeral after the “C” indicates which specific memory storage slot is in use. If the calibration has not been saved to a slot, the numeral will be replaced with an asterisk (*).
In addition, there are some letters that appear in a column under the calibration indicator, as follows :
- D : Directivity - indicates that directivity error correction is applied
- R : Reflection tracking - indicates that error correction is applied
- S : Source match - indicates that error correction is applied
- T : Transmission tracking - indicates that error correction is applied
- X : Crosstalk - a indicates that isolation (crosstalk) error correction is applied; and
- P : Power - indicates power level set at time of calibration (a - automatic)
Other power levels that can be selected are 2mA, 4mA, 6mA and 8mA. These are selected under the MENU > POWER menu item. If the text in the calibration status indicator is red in color, the “no calibration” calibration status is indicated. A properly calibrated NanoVNA will show its calibration status in white text, but during the calibration process, the text will show in a red color until the calibration is complete.
I hope that this article makes your NanoVNA just a bit more friendly and useful. Proper calibration is absolutely necessary for any degree of accuracy to be achieved. Knowing how and when to perform the calibration makes you more adept at using the NanoVNA.
November 2025
Q: Can the impedance of a section of unknown coaxial cable be determined using a NanoVNA? If so, how is it done?
A: The short answer is “YES” - coax line impedance can be determined using a NanoVNA. For the second part of the question, some preparatory information becomes necessary. The information provided herein is specific to the NanoVNA H4 with firmware version 1.2.40, but it should be very similar for other NanoVNA models and firmware versions.
Q: Can the impedance of a section of unknown coaxial cable be determined using a NanoVNA? If so, how is it done?
A: The short answer is “YES” - coax line impedance can be determined using a NanoVNA. For the second part of the question, some preparatory information becomes necessary. The information provided herein is specific to the NanoVNA H4 with firmware version 1.2.40, but it should be very similar for other NanoVNA models and firmware versions.
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This process will take advantage of the fact that the impedance in a feedline inverts every quarter-wavelength (λ/4) along its length, called the λ/4 inversion property. In a feedline, the impedance looking into the end of the feedline divided by the line impedance is equal to the line impedance divided by the load impedance. There is a simple equation that describes this relationship, as follows : |
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which can be rearranged or simplified as : |
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and then simplified again as : |
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These equations show how the line impedance is mathematically derived. The NanoVNA will provide us with the ZIN value for use in the equation. However, we must also do a little bit more very simple math before we are ready to go.
The math that we need to work now deals with the STOP frequency of the RF sweep that we will use to measure the cable. The intent is to determine a sweep STOP frequency that is greater than the λ/4 frequency of the cable under test. This can be determined easily by dividing estimated length of the cable under test into one-quarter of the speed of light. When done as explained below, this calculation will produce the free space λ/4 frequency in megahertz, which will always be somewhat greater than the actual cable λ/4 frequency. If estimating the cable length in feet, divide that length into 246, but if estimating the cable length in meters, divide that length into 75. You should by now understand the significance of these numbers. Once you have an estimated STOP frequency, use the procedure below to determine the cable impedance.
Procedure
Example
The complex impedance value displayed on the NanoVNA at the prime axis crossing point is 52.23-j29.5mΩ. If the real resistance value of 52.23Ω is then multiplied by the load impedance of 50Ω, we get a product of 2611.5. If we then determine the square root of that product, we find a line impedance value of 51.10Ω, which we can then assume indicates a 50-ohm cable.
I hope that this article will help to make your NanoVNA more useful to you as well as helping to improve your NanoVNA operational skills. The NanoVNA is an incredibly useful and powerful tool for the radio amateur to have in his/her tool arsenal. Knowing how to use it makes it just that much more valuable.
These equations show how the line impedance is mathematically derived. The NanoVNA will provide us with the ZIN value for use in the equation. However, we must also do a little bit more very simple math before we are ready to go.
The math that we need to work now deals with the STOP frequency of the RF sweep that we will use to measure the cable. The intent is to determine a sweep STOP frequency that is greater than the λ/4 frequency of the cable under test. This can be determined easily by dividing estimated length of the cable under test into one-quarter of the speed of light. When done as explained below, this calculation will produce the free space λ/4 frequency in megahertz, which will always be somewhat greater than the actual cable λ/4 frequency. If estimating the cable length in feet, divide that length into 246, but if estimating the cable length in meters, divide that length into 75. You should by now understand the significance of these numbers. Once you have an estimated STOP frequency, use the procedure below to determine the cable impedance.
Procedure
- Go to MENU > DISPLAY > TRACE and turn off all traces except TRACE 0.
- Go to MENU > DISPLAY > FORMAT S11 (REFL) > and select SMITH, and then be sure that the SMITH form is set to R + jX.
- Go to MENU > DISPLAY > MARKER and make sure that only MARKER 1 is selected and active.
- Go to MENU > STIMULUS > START and set the START frequency to 50kHz.
- Go to MENU > STIMULUS > STOP and set the STOP frequency to the value determined using the simple equation discussed above.
- Connect any adapters needed to connect the cable to be tested to the NanoVNA.
- Perform a calibration of the NanoVNA at the connection plane established in the previous step.
- Save the calibration to a suitable memory slot in the NanoVNA.
- Attach a 50Ω load (terminator) to the cable to be tested.
- Connect the cable to be tested to the NanoVNA at the connection plane established in Step 6 above.
- Use the jog wheel to move MARKER 1 along the displayed trace just until the first time that it crosses the prime axis.
- Read the real (resistive) portion of the displayed complex impedance at the upper left corner of the display screen.
- Use that resistance value in the impedance equation explained earlier, multiplying the resistance value by the load value of 50Ω, and then determine the square root of that product. This result will be the approximate line impedance of the cable under test.
Example
The complex impedance value displayed on the NanoVNA at the prime axis crossing point is 52.23-j29.5mΩ. If the real resistance value of 52.23Ω is then multiplied by the load impedance of 50Ω, we get a product of 2611.5. If we then determine the square root of that product, we find a line impedance value of 51.10Ω, which we can then assume indicates a 50-ohm cable.
I hope that this article will help to make your NanoVNA more useful to you as well as helping to improve your NanoVNA operational skills. The NanoVNA is an incredibly useful and powerful tool for the radio amateur to have in his/her tool arsenal. Knowing how to use it makes it just that much more valuable.
October 2025
Q : How can the length of a coil of wire be measured using the NanoVNA?
A : Not just any coil or length of cable can be measured in this fashion. The cable under test must have at least two conductors, such as a length of coaxial cable, or window line, or even Cat-5 LAN cable with its four twisted pairs inside the jacket. The measurement test methodology utilizes a scheme known as time domain reflectometry or TDR. In this test, a relatively low-frequency signal is placed onto the cable under test, where it travels down the cable to the opposite end and is then reflected back toward the source. The time that it takes to make the round trip to the opposite end of the cable is then used, together with the cable’s velocity factor (VF), to calculate the length of the cable. The TDR function utilizes the NanoVNA’s TRANSFORM function to support its calculations.
Velocity factor is a rating, expressed as a percentage of the speed of light, at which an RF signal will travel within a cable of a given type. The increased molecular density of the cable causes the RF signal to be slowed down in the cable as compared to the speed of light in free space. However, it is actually the material of the dielectric used in the cable construction that has the greatest effect upon the velocity factor of that cable type. Table 1 provides VF’s for many typical dielectric materials used in cable construction while Table 2 list the VF’s for several common cable types.
Q : How can the length of a coil of wire be measured using the NanoVNA?
A : Not just any coil or length of cable can be measured in this fashion. The cable under test must have at least two conductors, such as a length of coaxial cable, or window line, or even Cat-5 LAN cable with its four twisted pairs inside the jacket. The measurement test methodology utilizes a scheme known as time domain reflectometry or TDR. In this test, a relatively low-frequency signal is placed onto the cable under test, where it travels down the cable to the opposite end and is then reflected back toward the source. The time that it takes to make the round trip to the opposite end of the cable is then used, together with the cable’s velocity factor (VF), to calculate the length of the cable. The TDR function utilizes the NanoVNA’s TRANSFORM function to support its calculations.
Velocity factor is a rating, expressed as a percentage of the speed of light, at which an RF signal will travel within a cable of a given type. The increased molecular density of the cable causes the RF signal to be slowed down in the cable as compared to the speed of light in free space. However, it is actually the material of the dielectric used in the cable construction that has the greatest effect upon the velocity factor of that cable type. Table 1 provides VF’s for many typical dielectric materials used in cable construction while Table 2 list the VF’s for several common cable types.
The specific velocity factor of the cable to be tested must be known in order to properly calculate the length of that cable via NanoVNA TDR measurement. If you cannot identify the specific cable or cable type and do not have a datasheet for the cable, find the most similar cable in Table 2 and use that VF value. If that is not possible, then just use the Table 1 VF value for the dielectric material used in the cable to be tested.
Before we can make the measurement, we have to do some basic mathematics. The math is used to determine a suitable STOP frequency for the RF sweep that will be used for the test; the sweep START frequency is generally set at 50kHz.
The STOP frequency chosen will determine the maximum distance range and resolution for the test being made. The higher the STOP frequency is, the shorter the total distance capability will be, while a lower STOP frequency will provide for a longer total distance measurement capability.
Although it is not the best choice for a calculation constant, the value 6000 is an easy value to remember and is also an easy value for working out arithmetic results. We will use that value to determine our STOP frequency, dividing 6000 by the estimated maximum length to be measured in meters. The equation to be used is as follows :
The STOP frequency chosen will determine the maximum distance range and resolution for the test being made. The higher the STOP frequency is, the shorter the total distance capability will be, while a lower STOP frequency will provide for a longer total distance measurement capability.
Although it is not the best choice for a calculation constant, the value 6000 is an easy value to remember and is also an easy value for working out arithmetic results. We will use that value to determine our STOP frequency, dividing 6000 by the estimated maximum length to be measured in meters. The equation to be used is as follows :
where we use 6000 as the dividend and the maximum length in meters as the divisor. For example, if we are anticipating a maximum length that is just under 300 feet, we might choose 100 meters as the divisor, giving us a STOP frequency of 60MHz.
Once the NanoVNA is properly calibrated and configured, we will be able to read the calculated length directly from the NanoVNA display. Before we can begin, however, we must also prepare the cable to be tested for the measurement process. If the cable has a connector installed to at least one end of the cable, we can use that connector to attach the cable to the NanoVNA by way of suitable adapter(s). On the other hand, if the cable is unterminated, it must be prepared by exposing and stripping both conductors of the cable to be tested, so that alligator clips can be attached to those conductors. Then, we would use a suitable adapter that will connect to the NanoVNA and has alligator clips at the opposite end. For example, I have a twelve-inch length of RG174 cable that has a male SMA connector at one end and alligator clips at the other end. Similarly, I have another one, this time of RG-58 cable, that has a male BNC connector at one end and has alligator clips at its opposite end. These adapter cables are suitable for this type of test measurement. Note that is it advisable to use the mated wires of a single twisted pair when making TDR measurements on cables like Cat-5 LAN cable, e.g., the blue and blue/white or the orange and orange/white wire pairs. When measurements are being made on double-shielded coaxial cables, use the center conductor and the inner-most shield layer for the TDR measurement.
Follow the procedure below to make the TDR measurement with your NanoVNA. Note that these steps are specific to the NanoVNA H4 with firmware version 1.2.40 installed. The procedure for other NanoVNA models and firmware versions should be very similar.
Procedure
Conclusion
I hope that this article will help to make your NanoVNA more useful to you as well as helping to improve your NanoVNA operational skills. The NanoVNA is an incredibly useful and powerful tool for the radio amateur to have in his/her tool arsenal. Knowing more about how to use the NanoVNA makes it just that much more valuable. This particular use of the NanoVNA will help you to identify the lengths of all of those many assorted cable sections that we all have sitting around our shacks.
Once the NanoVNA is properly calibrated and configured, we will be able to read the calculated length directly from the NanoVNA display. Before we can begin, however, we must also prepare the cable to be tested for the measurement process. If the cable has a connector installed to at least one end of the cable, we can use that connector to attach the cable to the NanoVNA by way of suitable adapter(s). On the other hand, if the cable is unterminated, it must be prepared by exposing and stripping both conductors of the cable to be tested, so that alligator clips can be attached to those conductors. Then, we would use a suitable adapter that will connect to the NanoVNA and has alligator clips at the opposite end. For example, I have a twelve-inch length of RG174 cable that has a male SMA connector at one end and alligator clips at the other end. Similarly, I have another one, this time of RG-58 cable, that has a male BNC connector at one end and has alligator clips at its opposite end. These adapter cables are suitable for this type of test measurement. Note that is it advisable to use the mated wires of a single twisted pair when making TDR measurements on cables like Cat-5 LAN cable, e.g., the blue and blue/white or the orange and orange/white wire pairs. When measurements are being made on double-shielded coaxial cables, use the center conductor and the inner-most shield layer for the TDR measurement.
Follow the procedure below to make the TDR measurement with your NanoVNA. Note that these steps are specific to the NanoVNA H4 with firmware version 1.2.40 installed. The procedure for other NanoVNA models and firmware versions should be very similar.
Procedure
- Go to MENU > DISPLAY > TRACE and turn off all traces except TRACE 0.
- Go to MENU > DISPLAY > FORMAT S11 (REFL) > and select REAL or LINEAR.
- Go to MENU > DISPLAY > MARKER and make sure that only MARKER 1 is selected and active.
- Go to MENU > STIMULUS > START and set the START frequency to 50kHz.
- Go to MENU > STIMULUS > STOP and set the STOP frequency to the value determined using the simple equation discussed above.
- Go to MENU > DISPLAY > TRANSFORM > VELOCITY F and enter the appropriate velocity factor value for the cable under study.
- Go to MENU > DISPLAY > TRANSFORM and select LOW PASS IMPULSE.
- Go to MENU > DISPLAY > TRANSFORM and set the TRANSFORM function to ON.
- Install (to port S11) any adapters needed to connect the cable to be tested to the NanoVNA.
- Perform a proper calibration of the NanoVNA at that connection plane.
- Save the calibration to a suitable slot in the NanoVNA.
- Connect the cable to be tested to the NanoVNA to port S11.
- Go to MENU > DISPLAY > MARKER and tap SEARCH until it shows SEARCH MAXIMUM.
- Read the displayed propagation time and the calculated length in meters from the MARKER 1 (M1:) field at the upper-right corner of the NanoVNA display screen.
Conclusion
I hope that this article will help to make your NanoVNA more useful to you as well as helping to improve your NanoVNA operational skills. The NanoVNA is an incredibly useful and powerful tool for the radio amateur to have in his/her tool arsenal. Knowing more about how to use the NanoVNA makes it just that much more valuable. This particular use of the NanoVNA will help you to identify the lengths of all of those many assorted cable sections that we all have sitting around our shacks.
September 2025
Q: I am getting ready for a road trip to Florida, and I want to put a radio in the car for the trip, and then possibly make it a permanent installation afterwards. What would you suggest in terms of equipment, installation, and the best way to program the radio?
A: I might suggest several different approaches here, depending upon just which bands you may want to have available in the car, whether or not you are bothered by drilling holes in the interior panels, and what is available as to antenna mounting and positioning. Let’s take a look at some options, starting with the radio.
Several very compact radios are available. A good choice for a two-band set might be the TYT TH-8600 2m/70cm transceiver. This is the set that I have in my own POV for VHF/UHF use. The TH-8600 is a 25-watt miniature set capable of two-way comms on the 144-148 MHz and 420-450 MHz band segments. The radio is a mere 4.2” wide, 1.8” high, and 5.4” deep, weighing only about 3-1/2 pounds. While the faceplate is not removable, the radio is compact enough to fit in minimalistic spaces and is light enough that the strong interlocking strip fasteners can hold it to the dash if necessary. This radio has 200 memory channels and a dual VFO display. The microphone has a full numeric keypad, making control via the mic a snap. Programming is via a custom cable that plugs into a rear-panel data port and utilizes a USB connection to the PC. The antenna connection for a dual-band 2m/70cm antenna is via a single SO-239 socket on the radio rear panel. While antenna choices here are almost endless, I personally like and recommend the UAYESOK two-band dual-mast compact magnetic-base antenna, installed near the center of the vehicle roof if possible.
If instead a three-band radio is desired, a good choice might be the BTECH UV-25X4 tri-band transceiver. While I also own one of these units, I have not yet installed it. It too is a 25-watt unit (a 50-watt version, the UV-50X4, is also available, but is somewhat larger), operating on frequencies from 136 to 174 MHz, from 220 to 225 MHz, and from 400 to 520 MHz. Note that some of these ranges extend beyond the legal limits of the amateur bands in the USA. This unit is tiny! It measures only 3.85” wide by 1.83” high by 4.65” deep, and weighs an amazing 0.9 pounds. Its size and weight also lend themselves well to a Velcro®-type of temporary mount. While accessing all three available bands in this radio would require the use of a tri-band antenna, it can, of course, be utilized as a dual-band unit with a standard 2m/70cm antenna, connected via the SO-239 connector on the radio rear panel. A suitable antenna here might be the Nagoya TB-320A tri-band antenna, but other choices are available including the quad-band KT-7900 antenna that I purchased. Programming is via a standard Baofeng-type programming cable, which is then tied into the radio mic port via the specialized “Y” cable that ships with the radio.
Both of the above radios come in at just about $135 plus tax and shipping; the antennas mentioned are about $30 to $40 each. So, let’s talk about the installation of these two units.
Because all else depends upon the location of the radio itself, that location must be selected at the outset. A typical location for a dash-mounted radio might be below and to the left of the steering wheel. Another popular choice is to mount the radio to the side of the center console, which often places the radio at ninety degrees to the viewer. In many cases, most of the radio control is accomplished by way of the buttons on the mic, so all that is really necessary as far as the radio itself goes is to be able to see and read the front panel frequency display. Once the user has become familiar with the radio, actually needing to refer to the front panel is greatly reduced. However, if the radio has a “remote” type of removable face plate (front panel), the options become much broader. For example, the radio body can be installed underneath the driver’s seat with the wire-connected face plate mounted to or on top of the dashboard or even on the steering column shroud.
Wherever (and however) you ultimately decide to install the radio body, it must be secured against free movement. This is important both to protect the radio and to protect the vehicle occupants in the event of a sudden stop or impact. As a driver, I certainly would not want a four-pound projectile accelerating towards my head during an emergency stop.
Protection of the radio revolves around keeping extraneous vibration and bouncing of the radio to a bare minimum. Most if not all modern radios are assembled using lead-free assembly methods, but there is a clear-cut reason why lead-free solder is not permitted in any mission critical equipment such as aircraft avionics, medical or healthcare appliances, and in most military applications. Lead solder joints simply will not remain intact over time, and motion or vibration can accelerate their failure, thus hastening the demise of the equipment. Do your part to help avoid such early failures by properly securing all radios in fixed locations. Your bank account will thank you later.
It is recommended that the radio chosen for vehicle installation be as small as possible while still providing the requisite features. For example, a typical dual-band 2m/70cm unit should provide at least the following features :
Once you have selected a radio and determined how and where to mount it, it is time to move on to the antenna. You must choose an antenna mounting location that will give good performance while permitting the antenna to be securely mounted. I have had the Amazon-purchased UAYESOK dual-band antenna on my POV for almost four years now, with absolutely no problems. The magnetic base is strong, but is well padded to avoid damage to the vehicle. I have never knocked it off the roof despite its having hit low-hanging tree limbs on several occasions. Its installed position at the center of the roof provides the best possible ground plane, which in turn offers the most desirable radiation pattern.
Routing of the antenna cable is an area where some thought must be given. It is important to choose a cable path that will not cause chafing or cutting of the cable, while still maintaining the water-exclusion capability of the door seals through which the cable will probably need to pass. It is often possible to work the cable into the gap under the door seal, dressing it in such a manner that any drip lines formed will release their water load on the outside of the seal. Many modern vehicles have dual-lipped door seals. In such cases, place the cable so that it lies between the lips, routing it so that it follows the door frame down to a point where the cable can be brought inside the vehicle without compromising the seal’s integrity. Inside the vehicle, run the cable along and, if possible, under the carpet at the door sill, ultimately bringing it to the location of the radio.
Be sure to leave sufficient extra coaxial cable length near the radio to permit removal of the radio from its mount while still connected to the antenna. For example, it may at some point become necessary or desirable to set the radio on the seat while programming and testing it. Some radios have data ports that are very difficult to access without being able to actually see the rear panel of the radio. Some require the removal of a screw-retained cover in order to access the data port. The extra cable length can be stowed under the dashboard or under the seat, depending upon the installation location of the radio.
If it becomes necessary to drill any holes in order to accommodate the antenna coaxial cable feedline, be sure to drill the hole(s) large enough for the cable connector to pass through. Then, afterwards, be prepared to seal the hole around the cable using a rubber grommet with a membrane in its center. A grommet such as the Keystone #778 is an ideal choice for this purpose. It has an OD of 1.125” and fits a 1” hole in panels up to 0.062” thick. The center membrane has an expandable 0.25” hole at its center, and fits cables up to 0.3125” in diameter. The seal is necessary to a) prevent chafing of the cable, b) to exclude dust, dirt, and water, and c) to support the cable where it passes through the panel.
Once the antenna cable has been routed and the antenna installed, it is time to move on to the power provision. Despite the fact that some radios are shipped with so-called cigar lighter plugs for the power connection, it is NEVER advisable to use such a connection for the radio power. On most if not all vehicles, the cigar lighter power capability is far beneath that required by the radio. Usually, the wiring to the cigar lighter socket is no more than about a 20AWG wire, certainly not adequate for the eight to ten amperes (or more) required by the radio during TX operation. The general rule of thumb for 20AWG stranded wire is 3.5A continuous duty. Remember that although the radio will receive OK with low current supplied, it will not transmit properly under those conditions, with different results depending upon the radio and the current available. The radio may simply shut down, or it may attempt to transmit to the best of its ability. Such transmissions can be full of spurious signals and distorted audio.
The recommended method of connecting power to the radio is to run the heavy-gauge twin-lead wire from the radio directly to the battery. At the battery, connection is made to each of the battery posts if the vehicle does not have an alternate auxiliary power connection scheme. Many newer vehicles require that any such direct-power connections be made to a dedicated connection point designed and provided specifically for that purpose. Either way, it is necessary to fuse the wire leads as close to the source connection point as is possible. If the harness supplied by the radio manufacturer has a fuse in each lead (the positive and the negative), it is very important to maintain that methodology when making your power connections. On the other hand, if the radio manufacturer fused only the positive lead, it is acceptable to do the same when making your connections. In case it is necessary for some readers, I will mention that the standard color-coding of the power cable is red for positive and black for negative.
The reason that the two fuses are important if the manufacturer shipped the radio with two fuses is because some radios use both the positive and negative leads as incoming power lines, meaning that the black lead does not connect directly to the chassis of the radio, but is instead tied into some of the circuitry as a power supply feed. The purpose of mounting the fuses as close to the power source connection is so that the entire length of the power line(s) will be protected against short circuits, potentially from chafing in the vehicle.
Once again, wherever the power lines feed through from the engine compartment (or other battery location) into the cabin of the vehicle, the power lines must be protected from chafing and the opening must be sealed as with the antenna cable. Be sure to route the power lines in such a manner that there is no probability of burning on hot under-hood parts, no probability of pinching or chafing of the wires, and no interference with any moving parts under the hood. Also as with the antenna cable, allow for some extra wire length at the radio mounting location so that the radio can be removed from its mount while still connected to power.
If running the power lines to the battery is beyond your skill level, don’t feel bad! I was a heavy-duty truck mechanic for many years, but I had a local garage run my power feed to the battery when I first installed my radio in the POV. That shop has a lift and can easily get to the area of the firewall where the wires needed to pass into the cabin. Many independent garages and most car audio shops will run these wires for you for a small fee, and it only takes a few minutes to do the job with the correct equipment available.
Once the power is available at the radio, it is time to make that connection. For this purpose, I fully endorse the use of Anderson Powerpole® connectors. You will need four connector bodies (two red and two black), and then you will need a total of four crimp-on terminals. The crimp-on terminals used for this purpose are available in three different sizes, rated for current and related to wire gauge. The 15A, 30A, and 45A crimp-on connectors all fit into the same connector bodies. Select two pairs of crimp-on connectors appropriate for the wire sizes involved here – the two attached to the radio and the two coming from the battery. Installing the crimp-on terminals to the wires and then into the connector body is a simple task with the proper equipment, but you may want to see the process demonstrated at least once so that you know how to do the job. The W2MMD Clubhouse has the connectors, terminals, and crimp tool all available at the test and repair bench. Don’t forget to contribute to the replenishment kitty if you use Club supplies.
Anderson Powerpole® connectors are “gender-less” connectors, meaning that any body of a given size will mate with another body of that same size. This makes connecting to the radio quite simple. With an Anderson Powerpole® connector pair installed to the power wire coming from the battery and another set installed to the power leads on the radio itself, these two connector sets will simply plug into each other, observing the colors when making the connection.
With all of the connections to the radio made, all that is left is to program and test the radio. In many cases, the programming is easily handled with the CHIRP computer software package. Programming is a topic for another article, so I will not go into it in depth here. However, I will mention a few pertinent pointers about using CHIRP to program a radio :
If planning a road trip, you may want to use Repeater Book or some similar reference to identify all of the repeaters along your proposed route of travel. In that way, you can then program each of the repeaters identified, in route sequence, into an empty segment of the radio’s memory range, making it easy to move from one repeater to another as you travel down the highway. On the return trip, the sequence will still work, except that it will be used in reverse order.
I hope that this information will be useful to anyone planning such an installation. While it is not intended to be a comprehensive, cover all the bases type of explanation, it should nevertheless help to get you started with the job.
Q: I am getting ready for a road trip to Florida, and I want to put a radio in the car for the trip, and then possibly make it a permanent installation afterwards. What would you suggest in terms of equipment, installation, and the best way to program the radio?
A: I might suggest several different approaches here, depending upon just which bands you may want to have available in the car, whether or not you are bothered by drilling holes in the interior panels, and what is available as to antenna mounting and positioning. Let’s take a look at some options, starting with the radio.
Several very compact radios are available. A good choice for a two-band set might be the TYT TH-8600 2m/70cm transceiver. This is the set that I have in my own POV for VHF/UHF use. The TH-8600 is a 25-watt miniature set capable of two-way comms on the 144-148 MHz and 420-450 MHz band segments. The radio is a mere 4.2” wide, 1.8” high, and 5.4” deep, weighing only about 3-1/2 pounds. While the faceplate is not removable, the radio is compact enough to fit in minimalistic spaces and is light enough that the strong interlocking strip fasteners can hold it to the dash if necessary. This radio has 200 memory channels and a dual VFO display. The microphone has a full numeric keypad, making control via the mic a snap. Programming is via a custom cable that plugs into a rear-panel data port and utilizes a USB connection to the PC. The antenna connection for a dual-band 2m/70cm antenna is via a single SO-239 socket on the radio rear panel. While antenna choices here are almost endless, I personally like and recommend the UAYESOK two-band dual-mast compact magnetic-base antenna, installed near the center of the vehicle roof if possible.
If instead a three-band radio is desired, a good choice might be the BTECH UV-25X4 tri-band transceiver. While I also own one of these units, I have not yet installed it. It too is a 25-watt unit (a 50-watt version, the UV-50X4, is also available, but is somewhat larger), operating on frequencies from 136 to 174 MHz, from 220 to 225 MHz, and from 400 to 520 MHz. Note that some of these ranges extend beyond the legal limits of the amateur bands in the USA. This unit is tiny! It measures only 3.85” wide by 1.83” high by 4.65” deep, and weighs an amazing 0.9 pounds. Its size and weight also lend themselves well to a Velcro®-type of temporary mount. While accessing all three available bands in this radio would require the use of a tri-band antenna, it can, of course, be utilized as a dual-band unit with a standard 2m/70cm antenna, connected via the SO-239 connector on the radio rear panel. A suitable antenna here might be the Nagoya TB-320A tri-band antenna, but other choices are available including the quad-band KT-7900 antenna that I purchased. Programming is via a standard Baofeng-type programming cable, which is then tied into the radio mic port via the specialized “Y” cable that ships with the radio.
Both of the above radios come in at just about $135 plus tax and shipping; the antennas mentioned are about $30 to $40 each. So, let’s talk about the installation of these two units.
Because all else depends upon the location of the radio itself, that location must be selected at the outset. A typical location for a dash-mounted radio might be below and to the left of the steering wheel. Another popular choice is to mount the radio to the side of the center console, which often places the radio at ninety degrees to the viewer. In many cases, most of the radio control is accomplished by way of the buttons on the mic, so all that is really necessary as far as the radio itself goes is to be able to see and read the front panel frequency display. Once the user has become familiar with the radio, actually needing to refer to the front panel is greatly reduced. However, if the radio has a “remote” type of removable face plate (front panel), the options become much broader. For example, the radio body can be installed underneath the driver’s seat with the wire-connected face plate mounted to or on top of the dashboard or even on the steering column shroud.
Wherever (and however) you ultimately decide to install the radio body, it must be secured against free movement. This is important both to protect the radio and to protect the vehicle occupants in the event of a sudden stop or impact. As a driver, I certainly would not want a four-pound projectile accelerating towards my head during an emergency stop.
Protection of the radio revolves around keeping extraneous vibration and bouncing of the radio to a bare minimum. Most if not all modern radios are assembled using lead-free assembly methods, but there is a clear-cut reason why lead-free solder is not permitted in any mission critical equipment such as aircraft avionics, medical or healthcare appliances, and in most military applications. Lead solder joints simply will not remain intact over time, and motion or vibration can accelerate their failure, thus hastening the demise of the equipment. Do your part to help avoid such early failures by properly securing all radios in fixed locations. Your bank account will thank you later.
It is recommended that the radio chosen for vehicle installation be as small as possible while still providing the requisite features. For example, a typical dual-band 2m/70cm unit should provide at least the following features :
- Easily readable display with large numerals in a clean typeface
- Adjustable squelch control
- Full CTCSS access (50 discrete frequencies available)
- Simple programming method (non-complex or complicated), preferably via CHIRP
- A number of memory slots for storage of favorite frequencies, the more the better
- Compact size
- Integrated cooling fan
- Simplified switching between VFO and MEM modes
- At least 25 watts RF output on 2m and 20 watts RF output on 70cm bands
- Mic with capability to control the radio via buttons on the mic
Once you have selected a radio and determined how and where to mount it, it is time to move on to the antenna. You must choose an antenna mounting location that will give good performance while permitting the antenna to be securely mounted. I have had the Amazon-purchased UAYESOK dual-band antenna on my POV for almost four years now, with absolutely no problems. The magnetic base is strong, but is well padded to avoid damage to the vehicle. I have never knocked it off the roof despite its having hit low-hanging tree limbs on several occasions. Its installed position at the center of the roof provides the best possible ground plane, which in turn offers the most desirable radiation pattern.
Routing of the antenna cable is an area where some thought must be given. It is important to choose a cable path that will not cause chafing or cutting of the cable, while still maintaining the water-exclusion capability of the door seals through which the cable will probably need to pass. It is often possible to work the cable into the gap under the door seal, dressing it in such a manner that any drip lines formed will release their water load on the outside of the seal. Many modern vehicles have dual-lipped door seals. In such cases, place the cable so that it lies between the lips, routing it so that it follows the door frame down to a point where the cable can be brought inside the vehicle without compromising the seal’s integrity. Inside the vehicle, run the cable along and, if possible, under the carpet at the door sill, ultimately bringing it to the location of the radio.
Be sure to leave sufficient extra coaxial cable length near the radio to permit removal of the radio from its mount while still connected to the antenna. For example, it may at some point become necessary or desirable to set the radio on the seat while programming and testing it. Some radios have data ports that are very difficult to access without being able to actually see the rear panel of the radio. Some require the removal of a screw-retained cover in order to access the data port. The extra cable length can be stowed under the dashboard or under the seat, depending upon the installation location of the radio.
If it becomes necessary to drill any holes in order to accommodate the antenna coaxial cable feedline, be sure to drill the hole(s) large enough for the cable connector to pass through. Then, afterwards, be prepared to seal the hole around the cable using a rubber grommet with a membrane in its center. A grommet such as the Keystone #778 is an ideal choice for this purpose. It has an OD of 1.125” and fits a 1” hole in panels up to 0.062” thick. The center membrane has an expandable 0.25” hole at its center, and fits cables up to 0.3125” in diameter. The seal is necessary to a) prevent chafing of the cable, b) to exclude dust, dirt, and water, and c) to support the cable where it passes through the panel.
Once the antenna cable has been routed and the antenna installed, it is time to move on to the power provision. Despite the fact that some radios are shipped with so-called cigar lighter plugs for the power connection, it is NEVER advisable to use such a connection for the radio power. On most if not all vehicles, the cigar lighter power capability is far beneath that required by the radio. Usually, the wiring to the cigar lighter socket is no more than about a 20AWG wire, certainly not adequate for the eight to ten amperes (or more) required by the radio during TX operation. The general rule of thumb for 20AWG stranded wire is 3.5A continuous duty. Remember that although the radio will receive OK with low current supplied, it will not transmit properly under those conditions, with different results depending upon the radio and the current available. The radio may simply shut down, or it may attempt to transmit to the best of its ability. Such transmissions can be full of spurious signals and distorted audio.
The recommended method of connecting power to the radio is to run the heavy-gauge twin-lead wire from the radio directly to the battery. At the battery, connection is made to each of the battery posts if the vehicle does not have an alternate auxiliary power connection scheme. Many newer vehicles require that any such direct-power connections be made to a dedicated connection point designed and provided specifically for that purpose. Either way, it is necessary to fuse the wire leads as close to the source connection point as is possible. If the harness supplied by the radio manufacturer has a fuse in each lead (the positive and the negative), it is very important to maintain that methodology when making your power connections. On the other hand, if the radio manufacturer fused only the positive lead, it is acceptable to do the same when making your connections. In case it is necessary for some readers, I will mention that the standard color-coding of the power cable is red for positive and black for negative.
The reason that the two fuses are important if the manufacturer shipped the radio with two fuses is because some radios use both the positive and negative leads as incoming power lines, meaning that the black lead does not connect directly to the chassis of the radio, but is instead tied into some of the circuitry as a power supply feed. The purpose of mounting the fuses as close to the power source connection is so that the entire length of the power line(s) will be protected against short circuits, potentially from chafing in the vehicle.
Once again, wherever the power lines feed through from the engine compartment (or other battery location) into the cabin of the vehicle, the power lines must be protected from chafing and the opening must be sealed as with the antenna cable. Be sure to route the power lines in such a manner that there is no probability of burning on hot under-hood parts, no probability of pinching or chafing of the wires, and no interference with any moving parts under the hood. Also as with the antenna cable, allow for some extra wire length at the radio mounting location so that the radio can be removed from its mount while still connected to power.
If running the power lines to the battery is beyond your skill level, don’t feel bad! I was a heavy-duty truck mechanic for many years, but I had a local garage run my power feed to the battery when I first installed my radio in the POV. That shop has a lift and can easily get to the area of the firewall where the wires needed to pass into the cabin. Many independent garages and most car audio shops will run these wires for you for a small fee, and it only takes a few minutes to do the job with the correct equipment available.
Once the power is available at the radio, it is time to make that connection. For this purpose, I fully endorse the use of Anderson Powerpole® connectors. You will need four connector bodies (two red and two black), and then you will need a total of four crimp-on terminals. The crimp-on terminals used for this purpose are available in three different sizes, rated for current and related to wire gauge. The 15A, 30A, and 45A crimp-on connectors all fit into the same connector bodies. Select two pairs of crimp-on connectors appropriate for the wire sizes involved here – the two attached to the radio and the two coming from the battery. Installing the crimp-on terminals to the wires and then into the connector body is a simple task with the proper equipment, but you may want to see the process demonstrated at least once so that you know how to do the job. The W2MMD Clubhouse has the connectors, terminals, and crimp tool all available at the test and repair bench. Don’t forget to contribute to the replenishment kitty if you use Club supplies.
Anderson Powerpole® connectors are “gender-less” connectors, meaning that any body of a given size will mate with another body of that same size. This makes connecting to the radio quite simple. With an Anderson Powerpole® connector pair installed to the power wire coming from the battery and another set installed to the power leads on the radio itself, these two connector sets will simply plug into each other, observing the colors when making the connection.
With all of the connections to the radio made, all that is left is to program and test the radio. In many cases, the programming is easily handled with the CHIRP computer software package. Programming is a topic for another article, so I will not go into it in depth here. However, I will mention a few pertinent pointers about using CHIRP to program a radio :
- Be sure to update CHIRP to the most recent release so as to have the best possible list of radio definitions
- It may be necessary to use a radio definition other than the actual make and model of radio at hand, as the radio that you are programming may be a clone of a different and previously defined make and model
- Be sure to save all data files developed with CHIRP against future needs
If planning a road trip, you may want to use Repeater Book or some similar reference to identify all of the repeaters along your proposed route of travel. In that way, you can then program each of the repeaters identified, in route sequence, into an empty segment of the radio’s memory range, making it easy to move from one repeater to another as you travel down the highway. On the return trip, the sequence will still work, except that it will be used in reverse order.
I hope that this information will be useful to anyone planning such an installation. While it is not intended to be a comprehensive, cover all the bases type of explanation, it should nevertheless help to get you started with the job.