On 2019-03-23 1:57 PM, Herbert Schoenbohm wrote:
My tower is a Rohn 45 90 footer and there must be some way of
detuning it via the skirt adjustment means to make it non-resonant at
1825 kHz This is similar to electrical towers being detuned near a
direction AM station so their pattern is FCC compliant.
Herb,
Calculate the impedance of your skirt and tower as a coaxial line
with outer diameter = diameter of the skirt and inner diameter = the
effective diameter of the tower. Once you know the Z of the trans-
mission line, calculate the inductive impedance of that length of
shorted transmission line. When you have the XL, calculate the
capacitor necessary across he open end of that line to create a
parallel resonant network. You have detuned (or anti-tuned) the
tower (the shirt/tower/cap becomes a parallel tuned circuit between
the feedline and any part of the tower (or antennas) above the
connection of the top of the skirt.
You can, of course, use a variable cap and connect an antenna analyzer
across the cap and adjust the cap for maximum impedance/zero reactance
if you want to be "right on the nose" (that's what the broadcast guys
would do - using a bridge an tuning for minimum on frequency RF).
73,
... Joe, W4TV
On 2019-03-23 1:57 PM, Herbert Schoenbohm wrote:
My tower is a Rohn 45 90 footer and there must be some way of detuning it
via the skirt adjustment means to make it non-resonant at 1825 kHz This is
similar to electrical towers being detuned near a direction AM station so
their pattern is FCC compliant. In some cases, the power companies used
some drop wires to get the electrical pole to resonate outside the AM
frequencies range. But we are talking about a pattern shift here and not
reradiated noise. There must be a way other than moving the RX antennas
250 feet away from the tower.
Herb, KV4FZ.'
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