Topband: Detuning shunt fed tower and noise from a TV

[John Kaufmann]

Paul N1BUG wrote:

> I'm not sure what questions to ask at this point. Can anyone
> estimate how much capacitance I should be using in this detuning
> loop, or see some place where I have gone wrong?

Hi Paul,

I have successfully detuned 3 different towers using the same approach
(detuning loop with variable capacitor) as you.  In my situation the 160m
transmit antenna has some other nearby towers that I want to detune so that
they don't interact with the 160m system.  I excite the 160m transmit
antenna with RF (say 100W) and measure the RF current in the detuning loop
at the nearby tower.  You can do this by inserting an RF current meter in
series with the detuning capacitor or, probably better yet, using a clamp-on
RF current meter like the MFJ-854, which is what I use.  Tune the variable
capacitor for maximum (yes, maximum) current.

In my case the detuning loop spans about 20 feet of tower section at the
base.  I find that a 1000 pF variable capacitor is needed, and most of that
capacitance (about 800-900 pF) ends up being used.  The current maximum is
very, very sharp, so you must tune carefully or you may miss it.  The
currents can be quite high (several amps), depending on the amount of
transmit power being applied to the other antenna.

In your case if you can find some way to excite your tower with RF from an
external antenna system, you should be able to duplicate the results I get.

73, John W1FV