Jon, AA1K asked what effect a 120' support tower would have on the
"squashed" delta loop I posted earlier. Adding the tower had such an
interesting effect that I thought I'd share this with everyone.
As you recall, if supported by a non-metallic mast, the "squashed" delta
loop had a gain of 1.9 dBi @ 22.8 degrees and a feedpoint resistance of
106 ohms.
Replacing the 120' non-metallic mast with a metal tower spaced 6" from
the delta loop caused the delta loop resonance to shift downward in
frequency. It was brought back into resonance at 1830kHz by shortening
the bottom wire from the original 230 feet to 226.5 feet. Doing so also
shortened the sides to 159.25 feet each for a total wire length of 545
feet (the "circumference" with the non-metallic support was 551 feet).
With this configuration, the feedpoint resistance was 49 ohms, a very
close match for direct feed with 50-ohm coax. EZNEC showed the gain to
be 3.28 dBi @ 22.7 degrees and showed a significant amount of current
flowing in the 120' grounded tower.
Someone with knowledge of EZNEC might be of help with a paradox in
modeling this antenna. EZNEC requires that grounded "wires" be modeled
using the "Mini-nec" ground method. For best accuracy when modeling
wires close to the ground, EZNEC requires using the "real ground,
high-accuracy" method. In this model, there is the close-to-the-ground
loop, and the grounded tower. Both types of ground cannot be modeled
simultaneously.
I used the "real, high accuracy" EZNEC ground for the models which
yielded the above gain values. Using the "mini-nec" ground yielded gain
figures which were unrealistic, i.e., gain with non-metallic support =
2.85 dBi, with tower support = 7.65 dBi.
So it's an appealing-looking topband antenna, but with debatable modeling
results.
73, de Earl, K6SE
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