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Topband: shunt fed tower detuning

To: Topband <>
Subject: Topband: shunt fed tower detuning
From: Paul Kelley N1BUG <>
Date: Mon, 27 Aug 2007 07:47:03 -0400
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Once again summer is drawing to a close and I have not completed 
half what I set out to do. And once again I'm looking for a little 
nudge in the right direction to speed things up.

My new 160m transmit antenna had a rather severe impact on 
performance of my Beverages, which are in close proximity to it. The 
vertical is 98 feet of Rohn 25 with a short mast and 7 element 6 
meter beam sitting at 101 feet. My attempts to measure the resonant 
frequency of this structure were not successful. It is fed by a 
three wire gamma cage which extends to 60 feet. The tap to the tower 
is at 32 feet. I have two questions:

1) Any advice on how to go about detuning this? Perhaps someone has 
had experience with a similar structure? I had thought of trying to 
use the gamma itself to isolate the bottom 32 feet, by switching the 
appropriate capacitance from the bottom of the gamma section to the 
tower base. Even if that works it still leaves 69 feet of vertical 
structure with top loading. Should I be looking at isolating a 
section further up the tower instead? Any thoughts would be most 

2) Are there any issues I should be aware of in trying to switch the 
detuning network at QSK speeds with a control line of some length? 
The base of the tower is 250 feet from the shack.

Thanks & 73,
Paul N1BUG

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