End-fed antenna for QRP portable ops

Some friends and I have been talking about dong a camping trip, and I decided this would be a good opportunity to try some portable ops with an end-fed half wave antenna. I’ve been doing a lot of reading on the subject, especially AA5TB’s excellent explanation of the theory behind a resonant EFHW antenna. I set out to build a coupler, something light and durable that I could throw in a bag and not worry about; ideally I wanted to tune 40 and 20 meters. The 30m band would be a bonus, but as I don’t yet have the hang of CW I wouldn’t get as much use out of that band.

I decided to cheat a little and build AA5TB’s “mini” 17-40m QRP coupler design. I recently picked up some polyvaricons from the De Anza College Electronics Flea Market, but I had no idea what the specs were; Googling for the markings turned up nothing useful. I decided to just build it and see what happened. As it turns out, I can tune 30-17, which isn’t bad for using unknown parts.

Math

28 turns on a T50-2 core gives 3.84uH of inductance, according to the calculator at toroids.info. At 14.250MHz (center of the voice band, my target), that’s 343.81Ω of reactance. To achieve resonance we need to match that with capacitive reactance; solving for C we get 32.5pF. At 18.139MHz, 437.65Ω; 20pF yields resonance. At 10.140MHz, 244.8Ω which yields a resonant capacitance of 64.1pF. I can just barely reach the top of the 17 meter band, so this tells me my polyvaricon doesn’t go much below that. I can go well below the 30m band but not quite get to 40 meters, so my guess is this is a 20-120pF unit. I’m not sure what range the built-in adjustment trimmer adds, but I couldn’t get enough range to tune both 40 and 17m.

Performance

To test performance, I picked up some nice wire from HRO, 26AWG black “polysheath”. I don’t know what the insulation is made of, but it’s strong, pretty flexible, and relatively smooth (but not Teflon slippery). I cut about 34′, soldered on a banana plug, and tossed it up in a tree, roughly in an inverted L formation. I dispensed with the counterpoise, hoping I’d get close enough to resonance that counterpoise currents wouldn’t be a big issue. Tuning was straightforward, once I remembered to switch the FT-817 to the rear antenna jack (oops).

Twiddling the varicap knob for maximum noise got me pretty close, and keying up at 2.5W for the final adjustment got me somewhere under 1.2:1 — my meter isn’t great at low power. I didn’t have time to try working any stations, as the ‘817 battery was nearly dead by the time I was done finding the “sweet spot”, but I heard a half a dozen states including Kansas and Washington. Next time I have a free hour, I’ll try some PSK and update this post with the results.

With the last bit of my battery I decided to try shortening the antenna and trying to tune 17m; I didn’t have as much luck with this band. I suspect this is because I did a bad job of estimating how much to shorten the antenna. I could easily cut another antenna for 17m; they pack down so small with the 26AWG wire, why not? I expect that with a resonant wire, finding a match on 17m would be trivial.

Photos

Conclusion

In all, I’m impressed with how easy it was both to build the coupler and to find a match. The end result is compact, flexible, and stealthy – the rope used to anchor the wire was far more visible than the wire itself! I could improve it by adding a ground lug, for attaching a clip-lead counterpoise. I’m a little disappointed by the fact that I can’t tune 40m; I may try adjusting the polyvaricon’s trimmer to give more margin in the 17m band, and then add a ceramic capacitor in parallel… without doing the math, I don’t know how successful this would be. Measuring the variable caps might make a good future post!