A Happy Tune: Tiny ATUs for the Trail

+ It's very hard to beat a dipole for reliable performance on the trail.

+ No feedline is easier to use and transport than coax.

+ A transmatch is essential gear for field operation, even when using dipoles.

I won't go backpacking without a transmatch. Why? Consider this:

I once constructed a coax fed 40 meter dipole for use on a backpacking trip. I set it up in my back yard just the way I thought it would be used on the trail and spent hours carefully trimming it for 1:1 SWR on 7040 kHz.

I hiked through sandy desert canyons, on lush mountainsides and over rocky passes, hanging my antenna on available supports. I made many QSOs, proving that the antenna "worked," but I was shocked to see how much the SWR varied from site to site.

At home the SWR was near unity, but on the trail it was often greater than 3:1. With mismatch of this magnitude I was fearful for the health of my simple transistor transmitter.

The experience was sobering. I concluded that the operation of a portable antenna can't be taken for granted, even if it is carefully trimmed for a very narrow frequency range. Computer modeling has further convinced me of the dramatic effect that height, soil type and configuration have on antenna impedance.

I have given up trimming portable antennas. Now I simply cut my dipoles to formula and carry a tiny antenna tuner to the field. A tuner allows me to use my antenna in unusual configurations, over varied soil types and at different heights, confident that my transmitter will not be harmed by excessive SWR.

What kind of a tuner is best? There are many commercial antenna tuners on the market. With rare exception even the smallest of these are too bulky for backpacking. I make my own tiny tuners for the trail.

There are a number of tuner circuit designs in amateur publications. T-match designs are easy to build and understand, and they work well for the purpose I've described

My earliest homebrew tuners used air-core coils and variable capacitors from old tube type radios. My tiny tuner for backpacking uses an iron core toroid and tiny variable capacitors salvaged from transistor radios. The tuner was made to use with 20- and 40-meter dipoles, but has found many other applications. It weighs 3 ounces and has been very successful, proving itself on dozens of hiking trips and also for hotel and motel room operating.

The tiny tuner is housed in a 2" x 4" x 1" plastic box (Radio Shack catalog No. 270-1802). It includes a resistive SWR bridge with a LED indicator (see Fisher, Sojourner "A bright idea").

I've also built tuners in "Altoids" boxes and small polystyrene fly fishing boxes from the sporting goods store. Slide switches and RCA connectors could be used to further reduce weight.

This little device quickly tunes SWR to unity whether my 20 or 40 meter dipoles are hung high or low, when configured as inverted vees (PHOTO), inverted Ls, slopers, half slopers, verticals, and a variety of other bizarre shapes on the trail and in hotel rooms. It also works well to load my 40 meter dipole on 15 meters and with a number of experimental portable vertical antennas.

I realize that a tuner probably isn't needed in some instances. But I will not use a portable antenna without measuring its SWR, and since the tuner and its built-in indicator are smaller than any SWR bridge I've been able to find, I simply use the tuner/ indicator combination in every situation.

How did I build my tiny tuner and where did I find the parts for it?

The two variable capacitors used in the T-match circuit were taken from transistor radios purchased for less than a dollar each at thrift stores and garage sales.

Junked radio alarm clocks are especially easy to find and have great capacitors. Tiny tuning capacitors usually have two sections, one 75 pf and the other 150 uF. The center pin connects to the capacitor stator, and the outside pins connect to the rotors. Connecting the outside pins together puts the capacitor sections in parallel, providing a very useful tuning range of roughly 20 to 225 uF. Capacitors in older transistor radios sometimes have three sections, allowing a slightly greater tuning range.

Adjust the tiny trimmers on the capacitors to minimum (fully unmeshed) to reduce the capacitance on the bottom end of the tuning range.

I have no idea how much transmitted power these little capacitors are capable of handling. They may not hold up if used to tune a large mismatch under power. The built in resistive SWR bridge in my tuner is somewhat protective, and under the conditions I've described the capacitors have shown no sign of failure at three to five watts of transmitted power.

Finding appropriate knobs for these reclaimed capacitors is troublesome. I've used thumb wheels salvaged from old radios, but find them large and cumbersome. I prefer to make my own knobs from a variety of materials (including tiddlywinks!)

T68-2, T50-2, and T30-2 toroids (red) and a spool of 24 gauge enameled wire provides inductors for my tiny tuners. T68-6 and the rest of the family of yellow toroids are also appropriate.

The T-match tuner described above uses a coil consisting of only 8 turns of 24 gauge enameled wound on a FT68-2 toroid. The coil is tapped every two turns and switched with a tiny four position switch removed from a dead computer monitor.

The value of the inductor was found experimentally. Vertical antennas with sub optimal ground planes and dipoles at low height exhibit low impedance, and little inductance is needed to tune the dipoles. Very small inductive steps are needed.

Capacitors are higher Q devices than inductors, and so the best match is the one using the smallest value of inductance.

It was difficult to find switches for the tapped inductor until I found that inexpensive 12 position wafer switches from Radio Shack (catalog No. 275-1385) work very well. I've had no problems with this switch at the low power levels I use.

A T68-2 toroid fits neatly into the ring of connectors of the Radio Shack switch for a very neat assembly. Tap the toroid coil by putting a twist of wire at the appropriate place and then solder the taps to the connectors. I've found a very useful coil to be 14 turns of 24 or 28 gauge wire placed on a T68-2 toroid core. Place taps at the second and fourth turns and at each turn thereafter. This provides a coil with a total inductance of roughly 1.1 microhenries and 12 switched inductance steps, almost as good as a roller coil!

Tiny tuners can be built for a variety of other purposes with much greater inductive tuning ranges.

For example, this tuner that was built in a small orange plastic box has a total inductance of about 10 microhenries (uH), enough for a wide variety of 10 through 40 meter applications. It was also built around a Radio Shack 12 position switch, and the same "primary coil" previously described. One or more additional coils wound on T50-2 toroids are switched in series with the primary coil using tiny toggle switches (Radio Shack catalog No. 275-645) to increase the total inductance.

If you use a similar coil switching scheme, wind the second coil with an inductance equal to that of the first, the third coil with an inductance equal to the sum of coils one and two, the fourth coil with an inductance equal to the sum of coils one, two, and three and so on. A smooth tuning range is obtained by throwing the appropriate combination of switches.

The inductance of powdered iron toroid inductors is estimated using the formula L=KN^2. L is inductance in nanohenries, N is the number of turns (how many times the wire passes through the center of the core) and K is a constant for a given type of toroid core. For a T68-2, K is 5.7. For a T50-2 K is 5 and for a T30-2 it is 4.3. For example, 14 turns on a T68-2 yields 5.7 multiplies by 14 squared, or 1117 nanohenries, or about 1.1 microhenries.

The tuner with expanded inductive range tuner works well with a G5RV and many other types antennas, including some random length wires (with an appropriate counterpoise). It tunes my coax fed 276 foot horizontal loop on 10 through 80 meters with ease. A resistive SWR bridge with LED indicator was built into this tuner, as well as virtually every other tiny tuner I've made.

To give additional versatility, I built an outboard 4:1 balun transformer for use with the tiny tuner. I use it with balanced feed lines, such as lightweight 300 ohm twin lead. The balun is wound on a FT50-43 ferrite toroid according to instructions found in the ARRL Antenna Book.

Skeptics among us will want to know how efficient these tiny tuners are. I confess that I am not certain. I'm sure they introduce some loss, but testing with simple equipment satisfied me that the loss introduced is not excessive.

These tiny tuners clearly met my goals for operation from the trail. They are smaller than any available commercial unit, they improve power transfer from a little transmitter to the feedline, they allow me to load antennas not otherwise useable, and they appear to improve receiver performance.

Tiny tuners have added confidence and convenience to my operation on the trail. They have been very satisfying homebrew projects.

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Dr. Bob Armstrong, N7XJ, a country doctor living in Manti, UT, is an expert outodoorsman, QRPer, avid builder and a contributing editor to The ARS Sojourner.

barmstrong@mail.manti.com