Hi Dick:
I have had s shunt fed tower at five different QTH's since 1979. I have never
used anything other than a simple gamma match circuit utilizing one variable
capacitor. That variable capacitor has been in the range of 500 to 750 pf
depending upon the length of the tower and the conductors used for the gamma
wire.
At first my tower in MN was 60 feet high with a TH6 on top. The gamma wire was
three #14 wires twisted together and the wire was attached to an aluminum tube
at the very top of the tower. As the tower grew in height and the antennas got
larger, the "tap" was moved down to find the 50 ohm sweet spot. I eventually
replaced the wires with half inch CATV aluminum coax.
In my latest installation, the tower is 80 feet tall with a TH-11. I'm still
using CATV coax as the gamma arm and it is about at the 48 foot level. The
tuning capacitor was a 400 pf variable (about 2500 volt rating) paralled with
two, 200 pf mica capacitors. Several years ago, I replaced the air variable and
mica caps with a 750 pf vacuum capacitor.
I suspect that with the tower you decribed for 80M, the tap would be somewhere
down below the top of the aluminum tower section. Unfortunately, that means
climbing and experimentally locating the 50 ohm tap point - unless you use the
two capacitor Omega match approach. Still I believe that this is a less
complicated approach than going through the work of insulating the base. A
second benefit is that the tower is grounded for static and lightning
protection.
73 Dennis W0JX
Milan, OH
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