Bill Capps and Tom, W8JI wrote:
Bill:
"i was reading your post to topband tonite. Believe it or not, i built up
a inverted "U" today. I had been using a inverted "L".but i wanted to try
lengthening the antenna, thus i created a inverted "U". Mine is up 55'
over maybe 40' and down 50'."
Tom:
"Grounding the far end only works when the antenna is 1/2 wl from
end-to-end, unless you "tune it" at the feedpoint or the far end, or
both."
==========
Actually, if you make the total length of the inverted "U" 1/2-wave, say,
65 feet up, 130 feet across, then 65 feet down, and fed at one end at
ground level, you'll have what amounts to a pair of 65' verticals fed in
phase, with maximum radiation perpendicular to the direction of the
horizontal portion. Only the low angle radiation from the vertical
portions would contribute to the signal because any horizontal high angle
radiation from the horizontal part of the antenna would be at a minimum
due to cancellation.
This can be visualized as half of a quad element (with ground being the
other half). Feedpoint resistance with good ground sytems should be
close to 50 ohms.
Because of the close spacing (1/4-wave) of the "verticals" in the above
example, gain in the broadside direction would be only about 1 dB above a
single vertical, and it would be down only about 2 dB off of the ends,
i.e., an oval, egg-shaped pattern. More directivity could be achieved
(more gain broadside) if the horizontal spacing of the "verticals" was
made greater with an inverted "U", say 45 feet up, 170 feet across, then
45 feet down (still a total of 1/2 wavelenth of wire).
73, de Earl, K6SE
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