W8JI noted:
> (In many circumstances antennas interact
> noticeably with even a wavelength of separation, so you can imagine
> the coupling at .1wl or less spacing.)
For those of you who wonder how much separation is enough to avoid
interaction and distortion of the radiation pattern, I offer this story from
my earlier career in broadcast consulting:
After running a "proof of performance" series of field measurements on
a supposedly non-directional AM radio station, the results showed a
3 dB null (~70% field intensity) in one of the eight directions that were
measured. No obvious cause was found, and after ruling out all local
effects (power poles, guy wire re-radiation, etc.), we finally took the
field meter to a cable TV receiving tower that was about 1-1/4 miles
away on the right bearing. This tower was in fact the culprit, very "hot"
with re-radiated RF, despite being NINE WAVELENGTHS from the
transmitting antenna.
While this is the most dramatic case I experienced, others have been
reported that are equally unexpected. Tom is right in pointing out that we
rarely know exactly what our antenna systems are doing, since hams
don't normally run a series of field measurements to verify performance!
73, Gary
K9AY
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