Joe Subich, W4TV wrote:
>> Do the "other elements" actually absorb some of the power
>> creating an "IR???" loss of the source power?
>
> The current in the trapped, parasitic element creates an
> I^2R loss in the traps of that element. The decrease in
> current available for re-radiation represents an effective
> I^2R loss to the driven element because there is less total
> power available for radiation.
Same is true for the multi monoband on a beam untrapped design. Currents
in the other elements will have loss. I think, overall, that loss
source is probably a wash either way.
I would guess that the directivity being lower on a trapped antenna is
because the elements are physically shorter for the lower frequencies.
There's also the effects of having to pick a single spacing for all 3
bands (although, the SteppIR does this, and still gives better
directivity, at the expense of smaller instantaneous bandwidth).
On a trapped antenna, one might choose to make the spacing optimized for
20 (where the elements are short) and accept the non-optimimum spacing
for 10 (where the elements are the right length) so you get more uniform
performance.
OTOH, maybe it's not worth it to worry about 10m gain, and concentrate
on 10m bandwidth. It's seemed to me that propagation on 10 is like a
switch. Either the band is open and there's propagation, and you can
work the world on a watt, or there isn't, and a few dB of gain isn't
going to make any difference, so you might as well design for operating
convenience (10% bandwidth)
>
> ANY loss in the antenna system represents an overall decrease
> in efficiency. The effective gain of an antenna is equal to
> the directivity times the antenna efficiency.
>
> 73,
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