I use THHN for antenna wire. For purposes of using NEC to design for
length, I need the diectric constant of the insulation used in THHN.
For #12, I've been using a thickness of 0.0445 inch and constant of
2.0. Is this a good first approximation?
Yes, I know I could hang the antenna, check the resonance, then lower
it and tweak the length. But this puppy has to be fished through some
trees, and it's a big deal to raise and lower it. So it's worth
getting as close as possible the first time.
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