Topband: Antenna interaction

[K9AY]

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