[TowerTalk] tower resonances

[D.W. Fearn]

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