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[TowerTalk] tower resonances

To: "towertalk" <towertalk@contesting.com>
Subject: [TowerTalk] tower resonances
From: "Rudy Severns" <n6lf@arrl.net>
Date: Tue, 10 Feb 2009 14:17:57 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
I must apologize to the reflector for asking my original question in such a 
broad way when my interest is actually much narrower.

Obviously there is a vast literature on tower detuning, especially relating 
to AM broadcast.  My question relates to what seems to be not so common: the 
details on how you determine you have a problem in the first place.  Do you 
even need to detune anything?

A first step would be to determine the tower resonances, there will be 
several, and if they are anywhere near the desired operating frequencies. 
The standard advice is to install a short arm at the top of the tower and 
drop down a wire to the base.  Supposedly if you install a small loop at the 
bottom you can use a dipper to find the resonances.  I tried this under very 
controlled conditions where I knew the resonances accurately ahead of time. 
This process didn't seem to work well at all and of course has the problem 
you need to climb the tower first.

The usual case is that the tower is firmly grounded with lots of antennas 
and cables on it.  You can't just pop an insulator in the base and measure 
the resonances.  What I want is a technique where I can walk up to the tower 
and determine the resonances without having to climb, at least more that a 
few steps.  I have a way to accomplish this and have been gleefully 
measuring all the local towers I can get to.  But my approach requires a 
field portable network analyzer.  This is nice but totally impractical for 
most amateurs.  My approach, while it works great, has no broad application.

My question really should have been: are there other ways to determine tower 
resonances that do not require elegant instrumentation but actually work?

Thanks and 73, Rudy N6LF 

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