> I have read of passive antennas used as passive
> repeaters, so it does make sense that such a thing
> could occur ... but under what sort of circumstances?
I can see this on my little low band installation here. It's really
only a problem if you have resonant conductors of the same
polarization less than a wavelength from the antenna with the good
pattern.
The problem is that if you do what I've done, which is put up a 60
foot vertical and a flag RX antenna on a 40x90 lot, you necessarily
have your 160m/80m RX antenna with the good pattern a tiny fraction of
a wavelength from your resonant transmitting antenna; in my case my
flag is about 0.036wl from the transmitting antenna on 160m.
I did some tests on W1AW on 160.
Flag pointed at W1AW, TX antenna resonant or floating, W1AW steady S9
Flag pointed away from W1AW, TX antenna detuned (floated from ground),
W1AW audible but s0
Flag pointed away from W1AW, TX antenna resonant, W1AW s7!!
It bears mentioning that when the flag is pointed TOWARD the TX
antenna the problem is much worse. Even with the TX antenna in the
flag null, though, there's still some increase in band noise when the
TX vertical is resonant.
Fortunately, my band switch that connects the TX antenna to the
matching networks and the matching networks to the feedline is a wafer
switch with *continuous* rotation, so I can just go halfway between
switch positions to decouple the antenna from the radials when I'm
listening on the flag.
Anyway, the effect is huge here. Resonant conductors in the near
field of your directional antenna will change your pattern. That's
how you build a parasitic beam.
Dan
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