I do not have any experience with detuning amateur antennas, but I
have done it for several AM broadcast stations. The way we determined
if the tower was re-radiating (and affecting the pattern of the AM
station) was to use a Potomac field intensity meter (FIM), which is
the standard measurement device for field strength measurements. It
is simply a calibrated receiver with a shielded loop antenna. After
calibrating the FIM at a point near the suspected tower, the meter
was moved slowly towards the tower as the field strength was
measured. If the tower was re-radiating, the FIM reading would go up.
I would think you could get a useful sense of which tower was causing
the problem using a portable receiver with a receiving loop. The
exact field strength would not be important, just the change as you
neared the suspect tower. Sometimes the loop would have to be
practically in contact with the tower to see the effect.
If you know the chief engineer (or contract engineer) at a local AM
station, you might persuade him to loan the meter or help you make
the measurements. The older Potomac meters covered 160M but not 80M.
The newer ones
http://www.pi-usa.com/fim2241/fim2241g.htm will cover 80M.
In one case I recall, the offending tower was over a mile away, but
almost perfectly resonant at the station's frequency.
Doug K3KW
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