"Tam/WB2TT" wrote in message
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"Buck" wrote in message
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An earlier discussion brought to mind the question of the 3/2 wl
antennas such as using a 40 meter dipole to operate on 15 meters. I
just modeled one using the following stats:...........................


Buck,
I ran EZNEC on your antenna, and the 1/4 wave section works like a charm.
However it does not do anything to bring down the frequency of the 3/2 mode.
You need to use another TV antenna trick. On a VHF antenna, elements that
are resonant on CH5 and CH6 do no good in the 3/2 mode because they are
resonant above CH13. What is done is this: you put a CAPACITOR in the
element, like a loading coil would be. This forces you to make the element
longer to work on the fundamental. The capacitor has less effect on the 3X
frequency; So, you can actually have the 3X resonance be less than 3 times
the 1X resonance.

Here is what I got:

Length +/- 34.5 feet
1/4 Wave section of 75 Ohm coax.
100 PF capacitor in each half of the dipole, about 5% in from each end.
SWR=1.5 at 7.0 MHz (50 Ohm)
SWR=1.1 at 21.2 MHz (50 Ohm)

I took a guess at the 100 PF number, and the capacitor placement; so, you
can optimize it. Note that the further you move the capacitors from the ends
of the antenna, the more effect they will have on the fundamental.

Tam/WB2TT