At 12:34 PM 5/16/2005, AA6DX - Mark wrote:
>OK .. that's interesting .. If you would Rick, please `splain more on the
>separate amps causing probs w/mutual coupling ... I have ran a contest
>station w/separate amps to antennas, and... well, it worked... maybe I was
>missing something, though, for I never analyzed it as to the coupling.
I assume you're talking about multiple antennas/amps on the same
band/frequency?
As long as the phase relationships among the antennas remain fixed, the
mutual impedances would remain fixed, and you'd probably just suck the
variation up into when you tuned the output of the amps. If the antennas
were directional, and pointed different directions (or stacked, etc.) so
that the coupling was low, you might never notice it.
If you had FWD and REV wattmeters in all the lines, you might also notice
that the reflected power on antenna 1 would change when the forward power
on antenna 2 changed.
It's the general case where you want to adjust the phase (and current) of
each antenna independently where it gets trickier.
Just for an example, I modeled a pair of 20m 3-element Yagi's separated by
10m vertically (both yagis pointed the same direction, and perfectly parallel)
The coupling from one antenna to the other is about -18 dB. (that is, if I
send a kilowatt to antenna 1, and hook a load up to the other one, I'll see
about 16 W in that load.
If I were pushing a kilowatt into both, and they were otherwise perfectly
matched, that 16 W coming back would make me think I had a mismatch of
about 1.3:1... In reality, too, you'd have some phase shift between them,
and you might actually wind up with an apparently better match (if the
power coupled from the other antenna happens to be out of phase with the
power reflected by a mismatched antenna, the total reflected power would be
less).
In my example model, the current induced by the "other" antenna is 183
degrees away from the current from the feed line (because they're roughly a
half wavelength apart).
In any event, getting the most (however you want to define "most") out of
an array driven with multiple amplifiers is a somewhat tricky
proposition. If nothing else, having separate amplifiers that you want to
run at full tilt leads to some optimizations for patterns that run counter
to traditional approaches which tend to try to maximize overall RF
efficiency (that is, power radiated in the desired direction over RF power
put into the system).
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