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Owen Duffy · July 11th 07, 11:31 PM posted to rec.radio.amateur.antenna

....

I think you'll need to run a a simple calculation, based on the
frequencies you actually want to use.

What you'll want, is a wire whose length is not particularly close to
any multiple of 1/2 wavelength, on any frequency you want to use. A
wire which would match easily on 80 meters (e.g. 1/4 wavelength long)
would be a bad choice if you want to work on 40 meters as well, as
it'd be 1/2 wavelength long.

A simple program or spreadsheet ought to be able to do the necessary
calculations... try every wire length from 66 feet to 132 feet and see
if you can find a length which is a comfortable percentage away from
an even multiple of 1/2 wavelength on each frequency. Or,

You probably mean't any integral multiple of a half wave.

alternatively, iterate through each frequency, calculate the
1/2-wavelength multiples, and "blacklist" every possible length which
is too close to these multiples.

I have done just that, and searched for "sweet" wire lengths that aren't
within say 5% of band edge for all HF bands. It sounds like a solution to
the problem doesn't it. (5% implies that you have a pretty determinate
scenario, which is a big assumption. IIRC 10%+ will not give a practical
result on the higher bands.)

Problem is that it probably unecessarily constrains the solution.

The input impedance and the feed point voltage of an end fed wire at its
higher parallel resonances falls, so that whilst you might want to avoid
the first such resonance, the impedance (and feed point voltage) at the
third or higher resonance might well be low enough to not worry about it.

To demonstrate that life isn't simple, the antenna efficiency (Rr/Rtot at
the feedpoint) improves closer to those parallel resonances that everyone
wants to avoid.

Owen

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