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
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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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