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Re: [TowerTalk] Tower resonance measurements

To: "joe johnson" <joe.k3rr@gmail.com>
Subject: Re: [TowerTalk] Tower resonance measurements
From: "Rudy Severns" <n6lf@arrl.net>
Date: Wed, 25 Feb 2009 09:48:37 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Hi Joe,

To measure tower resonances, all I'm doing is measuring the series impedance 
of the sensing loop while it is attached to the tower.

Without the tower the loop is just an inductor up to the point where the 
loop length approaches 1/4-wave resonance where it starts acting as a two 
wire shorted transmission line.  The coupling to the tower is very loose so 
when you go past a tower resonance all you see is a small bump in the 
impedance.  In fact the deviation in |Z| (magnitude) is almost too small to 
see, that's why I use the phase of Z.  the deviation in phase is much easier 
to see but it is still quite small.  That's why I use a VNA for the 
measurements.

There may be other instruments which will work but I haven't had time to 
think about them.  An AIM4170 impedance meter is good enough for the job, 
but it's $400+.

This is why I've been very careful to tell everyone:  my technique works but 
is not, repeat not, a general answer to the problem.  It's requires 
knowledge and instrumentation that the average ham may not have.  I'm hoping 
that I or someone else will figure out a better and more general way to do 
this.  Actually I'm hard at work on that, lots of ideas but no 
breakthroughs.  For now the present less the optimum solution is it.

GL and 73, Rudy N6LF 

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