Topband: Antenna matching question
billw at waveform.net
Tue Mar 18 16:28:43 EDT 2014
The inductors are homebrew, but beefy. Basically I use 1/8" thick G10 phenolic with holes drilled in a cross vise (to maintain spacing), 8 gauge wire for the windings, and three aluminum brackets shaped like this \___/ arranged like a triangle on each end to hold the G10 in place. It ends up making a very sturdy assembly with an almost entirely air core. I'm not worried about heating and there isn't much conductive material in the area of the coil. The mounting plate for the matching network is glastic ('micarta'), and the entire thing is in a large PVC electrical enclosure.
You mention keeping the highest impedance part "clean" electrically, which would make me think putting the small coil nearest the anteanna would be best. Whichever part of the inductor is shunted out is essentially just adding stray capacitance, that keeps the shorted turns part of the inductors towards the feedline. Since most or all of the large inductor will be shorted out when on 40M, it will act essentially as extra capacitance in the matching network on that band.
The capacitors and vacuum relays are Russian surplus with 15kV ratings. I expect around 7kv or so at the base of the antenna while transmitting.
I should probably post a pic of one of the inductors sometime. It's a simple construction method that is very robust and easy to do at home even if you don't have machine shop facilities handy.
> It really depends on the parts.
> We always want the highest impedance part of the system to have the least
> unnecessary stuff hanging from it, and to have the highest voltage rating
> for contacts and arc paths.
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