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[TowerTalk] Sudden shifts in shunt matching network?

To: towertalk@contesting.com
Subject: [TowerTalk] Sudden shifts in shunt matching network?
From: Bill Coleman <aa4lr@arrl.net>
Date: Sun, 4 Dec 2005 11:07:54 -0500
List-post: <mailto:towertalk@contesting.com>
I use a shunt-fed 15m high tower on 80m and 160m. The matching  
network for 160m consists of an L-network. The series inductor is  
tapped about 45 turns on two stacked T200-2 cores. Probably about 70  
uH max. Shunt C is on the rig side, consists of a 560 pf 1kV silver  
mica and an 150 pf air variable. This network allowed me to tune for  
a 1:1 match at 1825 kHz. I use an automatic antenna tuner in the  
shack to bend the match to other frequencies in the band. I only run  
100 watts

During the ARRL 160m the first night, I was calling CQ for about 5  
minutes around 1820 kHz. Suddenly, the tuner showed a mis-match.  
Hitting the tune button couldn't restore the match.

Whipping out the antenna analyzer, I found that the match frequency  
had jumped up to 1875 kHz. I eventually moved up into that part of  
the band, and had a lot of fun for about 3 hours calling CQ around  
1870 kHz.

Second night, I found a pretty good match at around 1825 kHz. Called  
CQ for about an hour when the SWR jumped out again. This time, the  
antenna analyzer showed a resonant frequency of 1980 kHz. I went  
outside in the cold and attempted to adjust the matching network. I  
had to add inductance, but I ran out of capacitance. Couldn't move  
the match further down than about 1935 kHz. I had to QRT early that  
night.

I'm trying to figure out what might have caused these sudden shifts  
in the match. When I touched the components of the network the second  
night, none of them were hot, so I don't think there were excessive  
losses involved.

I'm wondering if either component might have been affected by the  
constant transmitting of calling CQ. Could the circulating currents  
be too high for the Silver Mica and caused it to lose some of its value?

Does anyone have any suggestions for solving this mystery?  
Suggestions for beefing up the matching network? What kind of fixed  
capacitors should I be using in this circuit?

I have really been impressed with the capabilities of the shunt-fed  
tower. I was able to call CQ and run about 70-100/hr during the  
contest. I worked 44 states, 5 provinces, and a couple of DX  
stations, including the Czech Republic, in just a few hours of  
operating.

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

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