A Versatile Tuner for SOTA Activations by KX0R

Numerous tuner designs are available, and endless possibilities exist for combining tuners and antennas to use for SOTA. My tuners have evolved through experimentation and modification. I want to share my current tuner design, so other SOTA activators may enjoy the benefits of similar tuners. I’ve had consistently good results using these tuners. This is intended to be a manual to share what I have learned.

This is a detailed, technical piece, intended mostly for “makers” and more creative activators, who enjoy building and using their own tools on the summits. My drawings are crude, but the content is powerful.

We recognize these key points:

  1. Most portable radios require an impedance near 50 ohms at the antenna port.
  2. Most real antennas don’t present an impedance of 50 ohms at frequencies we use
  3. A tuner is a device to change the impedance of our antenna to about 50 ohms
  4. A tuner performs complex functions to transform the antenna’s impedance
  5. We don’t need to understand the mathematics of impedance matching to use a tuner
  6. We do need to understand the basic concepts of what a tuner does to use it effectively
  7. Mathematical theory, involving complex variables, is required to understand impedance-matching devices
  8. Tuners can be modeled with available software
  9. Some key intuitive concepts can take us far enough that we can make successful tuners
  10. Bench experiments and field tests can yield acceptable results
  11. An antenna does not have to be resonant in order to radiate effectively at various frequencies

The impedance of any antenna can be expressed as two variables: resistance R and reactance X.

Z = R + jX

Z is a complex number, with j = SQRT of -1.

In order to change the actual Z of the antenna to a simple resistance of 50 ohms, our tuner must do two things:

  1. Change the antenna’s actual R to 50 ohms
  2. Supply an opposite reactance to cancel out X

To match an antenna’s complex impedance, ideally, we could do this procedure:

  1. Measure the complex impedance with an analyzer
  2. Design a network of L and C components to transform the impedance to 50 ohms
  3. Build the network and test it - verify that the impedance has been transformed to 50 ohms

For our antenna to work on several frequency bands, we would analyze the antenna at various frequencies of interest, design and test matching networks, and then configure and package the matching device(s). We might decide to change the antenna as well, leading to more work.

With so many choices in the process, we might spend a lot of time and effort trying to come up with a practical antenna and impedance-matching device that would work well in the field.

Faced with this tricky puzzle, many ham operators take shortcuts in order to get on the air more quickly:

  1. Purchase a ready-made “solution”, often an antenna with a broad-band matching network
  2. Purchase a limited manual tuner with some attractive features
  3. Purchase an auto-tuner
  4. Use an antenna they think is 50 ohms - but usually isn’t!
  5. Try some other clever tricks that others suggest

The results vary greatly, but if radio contacts happen, many operators continue to use their compromised equipment, unaware of what they’re missing.

A different approach is presented here:

  1. We accept that antennas are mathematical functions, in which R and X vary with frequency
  2. Voltage and Current are not usually in phase at the feedpoint, i.e., the load is a complex number
  3. Voltage and Current may vary over a great range of values at the feedpoint
  4. We don’t know what Z is, but we won’t analyze our system in detail, only very generally
  5. We create versatile networks of components that can transform a wide range of complex impedances
  6. We include enough variables in these networks that we can achieve a desired match in a few seconds
  7. We include a device to indicate visually when we have a match, i.e., the feed impedance is 50 ohms resistive
  8. We learn to use these devices in the field and make improvements over time
  9. We accept that our tools include compromises, so we need to be aware of limitations

Impedance-matching devices consist mostly of inductors and capacitors combined in various configurations. Many popular circuits have been developed, all with various trade-offs in performance. We must decide how much complexity we can tolerate in order to get the performance we want.

My original design goals:

  1. Multiple bands – 40-30-20-17M
  2. 50 ohm input
  3. 10W RF power
  4. Match 25-5000 ohms resistive
  5. Match wide range of reactance
  6. Optimize for unbalanced loads, end-fed wires, etc.
  7. Also use with balanced loads, dipole, loop, etc.
  8. Minimal size – weigh a few ounces
  9. Low cost
  10. Durable enough for SOTA activations
  11. Accurate visual indication of 50-ohm match
  12. Use with counterpoise or not
  13. Good efficiency
  14. Use for SOTA activations in the Rocky Mountains of Colorado

NOT Goals:

  1. Cover the entire Smith Chart
  2. Simple and easy to use by others
  3. Foolproof
  4. Minimize cost
  5. Design for production
  6. Tune automatically
  7. Digital or remote control, etc.

Next, I show a series of concepts progressing to a practical tuner topology. The end result is relatively complex, but it provides plenty of performance in a tiny package. Most of the lower-cost alternatives are simpler, with fewer features, but they may perform OK, if you stay within the constraints of their designs.


A conventional transformer is wound on a powdered-iron toroid core. This increases the impedance from 50 ohms to some higher value, depending on the turns-ratio of the windings L1 and L2. This is the basic concept of all the tuners included here.


A variable capacitor across L2 is added . This allows parallel tuning the secondary, so that it can correct some reactance presented by the high-Z load. The impedance step-up can be adjusted by changing the turns ratio of the windings L1 and L2. The turns shown are just examples, concept only.


A tap is added to the secondary winding, so that a lower voltage can be selected for a second output connection. This is for a much lower-impedance load. The impedance output of the new tap is controlled by its position on the winding. Position shown is just an example.

More taps are added to the secondary, so that a greater range of impedances can be matched. These outputs may be connected to separate terminals, or a rotary switch may be used to select the active tap.


In all the previous examples, we can tune the secondary to resonance with CT. We also have some control of the coupling and impedance transformation by choosing the primary and secondary turns, when we wind the transformer.

In this example, a variable capacitor is placed in series with the low-impedance primary winding. This “input capacitor” is chosen to create a series resonance with the primary winding, more or less.

This series capacitor has a very powerful set of effects on the transformer:

  1. When L1 and CC are tuned near series-resonance, the coupling between the windings is increased, because more current flows in the primary winding L1.
  2. As we tune the primary circuit off-resonance, we both change the coupling and add or subtract reactance from the input network.
  3. A new voltage transformation ratio and complex impedance is passed through the transformer to the secondary circuit consisting of L2 and CT.
  4. The secondary parallel resonant circuit also may be adjusted, to help correct the complex load impedance with the combined impedance available in the tuner circuit.
  5. Whatever changes we make in the secondary with CT are coupled to the primary, so that we may need to adjust CC again to correct any remaining reactance or resistance error.
  6. Even though the two capacitor adjustments interact considerably, each changing resonant frequency, mutual coupling, and net reactance, we can often obtain an acceptable match, with only a few quick adjustments of the two knobs.
  7. The range of impedances that can be matched is greatly increased by having two variable capacitors, because the series primary resonant circuit is closely coupled and transformed to the parallel resonant secondary circuit, and the reverse is true as well.

This is much easier to do than to explain in detail. This basic circuit has been in use for many decades, both in transmitters and tuners, and it has numerous variations.

The key idea is that with just two variable capacitors we can control at least four parameters of our tuner:

  1. Frequency of operation – resonance of our system
  2. Impedance ratio
  3. Coupling (related to Impedance Ratio)
  4. Reactance correction

In order to cover a really wide range of impedances, we still need multiple taps on the secondary, or some similar technique.


The turns on primary L1 are increased, and a tap is added, with a switch to select a shorter or a longer primary. This provides a higher or lower voltage step-up ratio. Selecting different primary turns makes a major change in the tuner; with the larger primary, inductance is added, so the series-resonant circuit of L1 and CC changes. This interacts with the secondary side, so that a different set of allowable impedance values is available.

Generally, using more primary turns helps us match:

  1. A lower frequency
  2. A lower output impedance
  3. Certain reactance values

Several primary taps could be included, along with a rotary switch, allowing numerous combinations of allowable impedance ranges for the total network. In practice this may be overkill, but we see this kind of control, using multiple taps on both windings, in the Fuchskreis tuners, etc.

If we try to match a single wire to operate over several amateur frequency bands, a wide range of impedances will be presented to our tuner. In practice a single primary winding is not always enough to cover all the impedance combinations we may need. Adding one switch to increase the primary is very powerful, when combined with the continuous adjustment of the variable input capacitor CC.


This is a simple change to CT. A polyvaricon capacitor with two sections is used. A small section, about 60 pF, is used for the higher frequencies. The larger section of the polyvaricon, about 140 pF, is switched in parallel by switch SC, for the lower frequencies. Using the small section has two advantages for the high frequencies:

  1. The tuning rate is less critical – it is easier to adjust the null
  2. The minimum capacitance is a few pF less than with both sections, extending the upper frequency limit.

The switch SC and wiring must have very low capacitance, to avoid losing the highest frequencies.


  1. A switch SA is included to add fixed capacitors to the larger polyvaricon section, often ~140 pF.
  2. A SPDT switch with a center OFF position is used for SA
  3. Either 200 pF or 400 pF may be added.
  4. Fixed capacitors are mica or C0G/NP0 ceramic – 200V or more.
  5. 60M and 80M bands may be tuned with the added capacitance.
  6. Note that the new circuitry is added to the 140 pF side of the polyvaricon, so that the 60 pF section is not increased and “pulled down” by the added capacitance of SA


A switch C ADD is included to add capacitance to the input variable capacitor CC. This is a SPDT switch with center off. Either 200 or 400 pF may be added.

  1. The added capacitance increases the coupling of L1
  2. The adjustment range of CC is increased
  3. Either 200 pF or 400 pF may be added.
  4. Fixed capacitors are mica or C0G/NP0 ceramic – 200V or more.
  5. The improvement is mostly needed on the lower frequencies


A Dan Tayloe-type LED Bridge is included, so that the tuner may be easily adjusted. These LED bridges are included in popular tuners like the BLT, etc., and they make matching a small tuner much easier. They also provide a reasonable load for the transmitter while tuning, avoiding possible damage to the final transistors.

The tuning bridge is improved by using metal oxide (MO) resistors – not carbon film or carbon composition resistors. 1W and 2W MO resistors are very small, almost non-inductive, and they tolerate some abuse, not changing their value very much when overloaded slightly. If you have a 10W radio, the bridge resistors should be able to withstanding the full 10W for a few seconds without damage. I use a parallel pair of 100 ohm MO resistors for each 50 ohm bridge resistor. Use the best you can find – some small ones are rated for 2W.

Tuning should be done at low power, usually around 1-2W input, to avoid stressing the bridge resistors. If you forget and start calling CQ with the bridge switch in the TUNE position, so you have ¾ of your 10W going into the bridge resistors – this is 2.5W for each 50-ohm arm, or 1.25W per 100-ohm resistor, if they’re in parallel pairs. You may make several contacts before you discover the error – I have! That’s when you need the better MO resistors!

Choose a non-diffused, ultra-bright or ultra-super-bright LED, rated at 2000 mcd or more. These are worth their cost, and they will make your SOTA life better! The LED should be painfully bright if viewed indoors. You must be able to null the bridge outdoors, even in full sun.

If the bridge is built small and symmetric, it will look like 50 ohms when the tuner is matched, because the tuner itself will be one of the four 50-ohm legs of the bridge. The null of the LED also should occur when all four legs are 50 ohms. Unfortunately errors may exist in these bridges, depending on how they are built, due to stray capacitance and inductance.

I was able to improve the accuracy of my bridges by adding small compensating capacitances, usually in the range of 10-30 pF, across one or more of the bridge resistors. Using a well-calibrated MFJ-259B analyzer, and an accurate 50 ohm load, I was able to improve the accuracy of my nulls above 14 MHz. Compensation is a fine point, not really necessary, but it’s nice to know that when the LED is out, the impedance of the tuner, as well as the bridge, is very close to 50 ohms.

When the LED bridge is bypassed, Operate Mode, a little RF leaks across the switch, and the LED lights slightly when the radio is keyed. This little defect is useful for seeing that the RF is going out.


Here are two KX0R tuners. One is larger than the other. Hopefully these will be useful to build a tuner using the ideas presented here. This is not a how-to-make-it article – experience and improvisation are required.

I plan to continue this article if possible - the second part contains:

  1. Detailed schematics
  2. Selection of components
  3. Actual operation

Please hold questions about these matters until Part 2 is uploaded.




Well done George !
Nice and complete work and description.


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Great work George!
Thanks for this complete report! Very useful!

Thanks George, I think I will use some of your ideas to modify my Fuchskreis tuner from QRP Shop. I’m sure many hams will find this information invaluable.
73 de OE6FEG

Or you can dispose of the ‘tuner’ and use resonant antennas. One less item to fail. :slight_smile:

73 Andrew VK1AD


George, thank you for sharing your ideas and results.
Great job, indeed.

Hi George
Thank you very much for all nice work, waiting for 2nd part too :slight_smile:
73 Eric F5JKK

Unless you are on a summit with BC and commercial interference, then you might find your receiver overloaded; something a tuned circuit avoids. Then you have the weight of your coax and associated losses to consider.

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Great information George I am in the process of learning & building a complete SOTA kit. Just finished a Pacific Antenna DC30B kit plan on taking it out for its 1st activation today. The next item I was looking at to build is a tuner, and I was looking around on their web site and I found your article, Building Tip Section, on tuners. Looking forward to Part ll. Also you can put me down for some boards :slight_smile: Thanks Geo. Kris

Well done, George; a worthwhile contribution.
Best, Ken

Had a question about the size of the wire you used for L1/L2
I can see enlamed copper et another in different color !
73 Eric F5JKK

George - I need to put this in my archives for later reference. Thanks for sharing.


Thanks George - great article.

Shame we don’t have a “SOTA Magazine” this would be a great article to have in it!

73 Ed.

It would be a great article, I dare say that Sprat would like to publish it?

It’s just so very nicely presented. I find the hand drawn circuits relaxing to look at.




Thank you.

Here’s PART 2, with the actual circuits I use for SOTA activations. This part is for those who are interested mostly in building a tuner to use.


Here are two KX0R tuners showing the actual circuits. This is not a how-to-make-it article – experience and improvisation are required.

For best performance, these tuners should be compact, and small wire should be used to reduce capacitance. I suggest wire-wrap wire, or small single-conductor silver-plated Teflon wire. While a PCB could be used, it likely would add capacitance, which adversely affects these circuits at higher frequencies and high impedances. The output impedance may be several thousand ohms, where every pF is a problem – this is the opposite of working with 50 ohms.



This is just the combination of the concepts from the drawings up above, posted earlier. Most of the clutter comes from all the extra switches and connections required to extend the frequency and impedance ranges.


The polyvaricons I have are marked “TT” and “TTWM”. These have a small section of about 2-60 pf, and a larger section about 4-140 pF. These work well generally, and the Q is good compared to other capacitors I use. Fixed caps should be silver mica or C0G/NP0 ceramic, 200V or better.

The sub-miniature toggle switches are made by Mountain Switch – these are available from Mouser and elsewhere – the E-switch brand is similar – they are great for this application! A good part number to start with is 108-0044-EVX – this is the SPDT-Center Off switch in my tuners. Other versions are available – DPDT, SPST, etc. The colored output jacks are mini-banana jacks from Pomona (Mouser) – sample P/N Pomona 2142-5, Mouser 565-2142-5. The mating plugs are Cinch (Johnson) – small and simple – sample P/N 108-1002-001, Mouser 530-108-1002-1.

These mini-banana connectors are also nice for making links in antennas.

RCA phono connectors are good for the 50 ohm connections. They’re lighter and simpler than BNC, and they pull loose if something bad happens in the field. A sample P/N for the receptacle is Mouser 161-1052. The little boxes came from Radio Shack….I had to remove some extra plastic inside to make room for some added parts.

The larger tuner includes a big T106-6 toroid core – the intent is to minimize transformer loss. A T68-6 would work fine, with different turns counts. The total primary has 6 turns, with a tap at 3 turns. The total secondary is 16 turns total, with taps as shown on the drawing. The secondary is wound around just over half of the toroid, and it measures about 4.1 uH. If you use a smaller core, 4.1 uH is a good value to use for a starting point. The primary of the current version is wound over (among) the low-Z end of the secondary. The toroid is mounted on a cushion of foam insulation, with a single screw and insulated washers to hold it to the panel. The mounting scheme used for the toroid MUST NOT create a shorted turn (closed loop).

This tuner will match many useful loads from about 3.5 to 22 MHz. Performance is optimum in the 40-30-20-17M bands. High Z loads like the end-fed half wave and the end-fed full wave wires are easy to match, as well as low-Z loads like a ¼-wave vertical wire with a ¼-wave counterpoise. This tuner includes a switch to isolate the secondary circuit, useful when feeding balanced lines for a dipole, etc.


I regularly use this tuner with an end-fed 66-foot wire. The tuner matches this wire at 7, 10.1, 14, and 21 MHz, with no counterpoise. This antenna is approximately high-Z resonant at 7 and 14 MHz, but it’s reactive on 10.1 MHz. The 30M match is as easy as for 7 and 14 MHz, but it’s more sensitive to conditions at the site, stray capacitance, etc. No traps or links are needed, but they can be used too. I also employ a 52-foot wire with a 12-foot counterpoise – this has low impedance on 20M, somewhat low Z on 40M, and very high Z on 30M and 17M. All the matches are easy. Occasionally in haste I’ve omitted the counterpoise and “forced” the low-Z matches using the tuner, and performance is only down slightly.

High-Z loads are just as easy to use as low-Z loads. Various tests on the bench have confirmed that this tuner is reasonably efficient, even when feeding loads near 5000 ohms, as well as below 50 ohms. Power should be limited to 10W, because the polyvaricon capacitors are not rated for high voltage. Likewise, power should not be applied without a load connected. The system should be tuned to band noise before tuning with any power. Using low power and the tuning bridge, a match should be found – only then should full power be used.

When tuning an unknown load, it should first be tried on the highest-Z tap jack. If necessary, it can be moved to a lower-impedance tap; then the tuner is re-adjusted for a better match - etc.

Adjusting these little tuners is somewhat of an art. With repeated use the common settings are learned. Then just little tweaks are needed to get on the air or to QSY. Just adjusting the band noise to peak will result in a pretty good match, especially if the right taps and switch settings are known. Some loads produce sharp tuning settings, but good signals may result anyhow. Broad matches occur mostly with low-Z resonant loads, but low-Z resistors will tune up the same way! Broad matches are not always a sign of perfection in the system, nor are they usually a problem.

In some cases, more than one match may be obtained, with different settings for the input capacitor. This is usually OK. The match with the best null should be used. Resistors can be used to get a feel for the tuner on various bands, at various impedances. Once you “know” a tuner, weird conditions are more obvious. Most tuners have spurious nulls caused by unwanted resonances, but in these tuners, spurious resonances occur at very high frequencies. Spurious nulls can be found by adjusting the tuner at very low power, with the load connections open and shorted. An MFJ-259B analyzer works well for such tests. As long as the construction is compact, with short leads, unwanted resonances won’t be a problem.

Balanced lines may be fed by connecting to a pair of output taps, determined by experiment. Balance may not be perfect, but a match will be obtained. A switch S6 is provided for special configurations. Three configurations are available:

  1. GND – a tap on the secondary is “grounded”
  2. Center OFF – FLOAT – this is for a floating balanced line, with no “ground” connection to the secondary
  3. U – Unbalanced - the bottom of the secondary is “grounded” – this is the normal unbalanced configuration for use with end-fed wires, etc.

A 22 megohm resistor is connected between “ground” and the low-Z side of the secondary – this is for use in the “FLOAT” mode, with a balanced line, to drain off static electricity – the tuner would need an actual minimal connection to the ground for this to work - a nail and a wire is enough.

Any impedance-matching RF circuit has loss; there’s also loss in traps, broadband transformers, un-uns, baluns, and in small coaxial cable. I use only two feet of RG-316 cable in my system – from the radio to the tuner - my antennas are usually end-fed.

Performance has been fun! I’ve used this tuner for several hundred activations, with decent results.

On March 8, 2019, I activated a local peak, W0C/FR-109. I set up on the ground, which was damp from melting snow. The site is pretty average - nothing special. My 66-foot end-fed wire was about 15 feet high at the top of the bent-over fishing pole, and the far end of the wire was only 2-3 feet off the ground. I used a 12-foot counterpoise, but only for 60M. Running my KX2 at 10W out, I made over 50 CW contacts on 20, 30, 40, 17, and 60M. Included was a 20M DX contact with G4OBK, Phil, in England; another 20M DX contact with EA8/HB9FIH in the Canary Islands, and a 17M DX contact with ZL1BYZ, John, in NZ. I also made S2S contacts with W5ODS in Arkansas and AC1Z in New Hampshire. My RBN spots were good, compared to other activators. This was a normal activation, and not set up at an ideal spot on the mountain. There’s no magic - only a tuner that works well - and really good chasers!


This tuner weighs less than 4 ounces (about 110 g).

The Tiny Tuner 2 preceded the larger tuner above; it was re-built from a different, earlier tuner - the “Tiny Tuner”. That tuner was derived partly from the BLT.



Concept, construction and components are similar to the larger version above. A T68-6 toroid is used, and the detailed drawing shows the turn counts. Note that the turns are different from those on the Concept drawing.

The secondary has 28 total turns, and the entire winding measures about 4.2 uH. The primary has 11 turns total, with a tap 5T from the top end. The primary is wound on the low-Z end of the secondary - see photo.

Operation is very similar to the larger tuner. 60M and 80M are not available, and the circuit is simpler and smaller. As far as I can tell, this tuner is just as good as the larger one, for the 5 bands it covers. It handles the 10W from the KX2 just fine!

Part of the intent of this article is to make SOTA activators aware that traditional RF rules are meant to be broken, to some degree. Antennas don’t need to be resonant, wires may be fed at the end, an actual physical counterpoise is not always needed, a ground is seldom required for SOTA, an effective antenna may be quite low, coax cable is not desirable for connecting to the antenna, and above all – compact, light equipment is better for SOTA!!

Hopefully some of you will enjoy building and using similar tuners as much as I do!



Thanks for the detailed and practically-based write-up George, always interesting to see how other people do it!

In getting on for 500 activations the only problem I’ve had with an ATU has been when I accidentally knocked the “bypass” switch whilst unpacking it. Took me a few minutes to work out why it wasn’t tuning up but now checking the position of the switch is a regular part of my set-up procedure! And on many occasions I’ve been very glad of a band-change without having to crawl out of my snow hole or from behind what meagre shelter I’ve found or created into the blizzard/horizontal rain/teeth of the gale to swap links etc :wink:

73 de Paul G4MD

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This site has some ideas for the experimenter.



"I find hand drawn circuits relaxing to look" a_t

:open_mouth: Wow! (no offense meant). I look at circuit diagrams hand drawn or computer designed and my eyes glaze over. You may as well talk to me in Klingon !!

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I plan to build the tuner by KX0R, the larger one. Which lengths of wire and lengths of counterpoise work good for which frequencies?

From the reading, I already got:

  • 66 ft, no counterpoise => 7, 10, 14 and 21 MHz
  • 52 ft, 12 ft counterpoise => 7, 10, 18 MHz

73 Axel DF1ET