[AMRadio] How 3720 Killed my antenna

Jim Candela jcandela at prodigy.net
Thu Jan 25 16:03:39 EST 2007

A few weeks ago I decided to try 3720. This didn't work out as well as I hoped.

I use a full wave 80m loop that was adjusted to be resonant on 3880 at 80 ohms, resonant. I fed it with 450 ohm twin lead to a 1:1 current balun. From there I have RG-8X to the shack. The SWR was 1.5:1, so I put a tuner in line. Since that time the antenna fell down no less than 3 times, and each time it was a little shorter after the repair. I never went to check the resonant frequency again. Big Mistake! Unknown to me the resonant point had crept up to 4.1 Mhz, but the tuner kept the transmitter happy at 3.880 Mhz.

Prior to going to 3720 I put a new amplifier in line with a 8877 power triode in it. Anyway, my tune up was brief on 3720, swr would null with a bridge, but not at anything above 100 watts. Then it got worse. My 1:1 balun literally caught fire, and the antenna came down again where the wire had melted (16 awg) at what must have been a current maxima!! So much for a casual CQ on 3720! ;-)

I went back to my old trusted W6SAI Bill Orr 1:1 air core trifiliar balun, and added inductance in series with the feedline at the balun so I can pick 3880, or go down to 3720. Now the balun sees no more than 1.5:1 SWR at either frequency.

Is your Antenna 3720 or 3685 ready??


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