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[TowerTalk] SWR shifts with power

Subject: [TowerTalk] SWR shifts with power
From: Bill Coleman <>
Date: Mon, 23 Jan 2006 21:05:41 -0500
List-post: <>
I'm having trouble with my shunt-fed 15m tower. I have an L matching  
network that is switched between 80 and 160m. A relay selects  
different taps on the inductor, and also adds capacitance for 160m.

If I use an MFJ 259 antenna analyzer, I can get a nice match on  
either band. And if I transmit with a few watts, I have no problem.  
Even the 20 watts that the KAT100 uses isn't a problem.

However, if I transmit about 70 watts or more, the SWR suddenly  
shifts to 3:1 or worse. It changes with each transmission.

I thought the caps might be arcing, so I replaced one of them with a  
wide-spaced variable, same problem.

Any suggestions?

Bill Coleman, AA4LR, PP-ASEL        Mail:
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901


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