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Hi Mike et. al.
There are a couple of suggestions:
1. Extend the wire about 7 feet (from 128 to 135 feet) and put a shunt
capacitor across the feed point. Depending on ground losses, the
shunt capacitor might be anywhere from 800 to 1500 pfd. This will be
a 50 ohm match so you can run coax direct. (The extra wire is the
inductor of a L network)
2. Run about a 3/8 wave vertical, say 185 to 200 feet high. Tune out
the inductive reactance at the feed point with a series capacitor. A
200 or 250 pfd 10 KV vacuum would be close enough. Then you should
have a closer match to the open wire feed line, which should help the
tuner. Unless you are willing to give up the open wire feeder, this
is viable.
3. If you prefer a series capacitor, instead of a shunt capacitor,
there are a couple of methods:
A. Make the wire like solution two, and adjust the feed point
impedance for Z ant = 200 + Xj by adjusting the actual wire length.
Then you can use about a 200 or 250 vacuum variable to tune out the +
Xj, and a 4:1 UnUn to get to 50 ohm feed point.
B. Make the wire about 9/32 wave length (140-145 feet high) The
impedance, Z ant, should be about 50 + 100j. Then a series capacitor
of about 1000 pfd should give a 50 ohm match.
Good luck, George, K8GG
______________________________ Reply Separator _________________________________
Subject: TopBand: Vertical Matching
Author: "Mike Michon" <michon@eatel.net> at BC_INTERNET
To: <topband@contesting.com>
Date: 11/4/97 7:28 AM
Dear Antenna Experimenters,
For the 160 meter Contest in December we install a 1/4 wave balloon
vertical on a ridge in southern MS. We feed it with open wire line. A run
of about 700-800 feet. We have been using a Nye Viking tuner, but it isn't
real happy at times. Two years ago we had to add a small coil to tuner to
get it to work. Last year one of the fixed capacitor plate insulators
failed, and we repaired it with some sort of plastic we found. We have been
running 500 watts, but may be able to run 1500 watts this year and I don't
know if the tuner is going to take it.
So, is it possible to match this arrangement so we can eliminate the
tuner?? If so will we be able to have a broad enough match to accommodate
how spread out the band gets during the contest??
If we continue to use the tuner, can we do anything to make the match
closer to what the tuner can handle?? We usually have 40 60 radials of
various lengths for ground, and I think the soil in southern Mississippi
would be poor ground, although we are only about 15 miles from the
Mississippi river, but I am not real sure of ground conductivity.
Would a balun at the vertical do any good as far as matching. The feedline
is usually about 6 inches apart so I am guessing about 500-600 ohms for it,
but we can space it closer if necessary or beneficial.
Mike Michon
AB5XP
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