Tuesday, December 8, 2009

Progress in 160m

This 50' aluminum vertical with auto-tuner at its base has served me reasonably well in QRP contesting, even on bands like 15m where its pattern should be poor DX. However, while it tunes nicely from 10-80m, 160m has eluded me. Two years ago, I rigged a wire up to the top of it and make a sort of parallel inverted L, but the Winter winds soon make short shrift of this. The snow and ice loads down the wire, and that, being horizontal, pulls down the entire vertical.

This time around I settled for base loading. Not having an antenna analyzer, I used the demo mode of EZNec to guess the capacitive impedance of the vertical. Finding an unused water bottle to use as a former, I wound about 20 turns of about #14 enamel wire over its 3.5" diameter.

I didn't bother with a transformer tap, hoping that the SMC autotuner would deal with the rest of the matching. While initially I did get a good match, I was disappointed to see that this did not prevail across the band. The tuning was very touchy, indeed.

However, when I got transmitting during the contest, the coil had obviously made a difference. Running QRP, I could work anyone with a S9 or greater signal, and even snagged my first DX on this band, a QSO with Turks and Caicos.

Obviously, what's happened is that I've improved the SWR on the feedline (RG-8) from an absolutely rediculous value to something terrible, but not so terrible that the RF is lost in the feedline. Since feedline losses are comparatively low on 160m, I still get some RF into the antenna.

I have three improvements in mind. First, for contesting, it would be great to be able to switch this coil in and out of the circuit remotely. A micro-controller with RF link or XBee would do just fine. Secondly, a more perfect match would improve things more. But I think realistically I'd need an antenna analyzer to do this well. Perhaps the greatest improvement in performance might be gained by replacing the feedline with LMR-400. This would better my signal on all bands, after all.

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