[TowerTalk] antennas and towers near broadcast stations

[jimlux]

jim Jarvis wrote:
> Bottom post... see below:

> If we make a vertical resonant on our operating frequency, it is, de- 
> facto, non-resonant at the broadcast freq.
> i.e.  decoupled.    And we shouldn't have to worry.
> 

I think a dicier matter might be the ham with the tower supporting a 
Yagi that inadvertently winds up being resonant.  (hey, makes Rudy's 
recent questions a lot more relevant).

"
        (b) Construction near a directional AM station. Proponents of
the construction or significant modification of a tower which is
within the lesser of 10 wavelengths or 3 kilometers of the AM
station, and is taller than 36 electrical degrees at the AM
frequency,
"


36 degrees at 1500 kHz (lambda 200m) is only 20 meters (66 ft).. lots of 
towers in that height area, and a shorter tower with a yagi on top could 
easily have an electrical length of 20m. Or, a not sufficiently 
decoupled feedline could wind up being a problem.  I would think that 36 
degree electrical height (1/10th wavelength) would be pretty far from 
resonant, and a chore to measure.


I suppose the real issue is whether the ham's antenna does "bad things" 
to the DA pattern of the broadcast station, i.e. fill in a null.  I 
can't see a non-resonant (lambda/10) element 10 wavelengths away making 
a huge difference in main lobe levels, but I could easily see it causing 
a problem with a 20dB null, if such are ever required.