I have been using a shunt fed tower on 160 since 1979. My first was a 60 foot
Universal with a TH6 on top. The shunt wire was three #14 wires twisted
together and tied to a three foot aluminum tube very near the top of the tower.
It was fed through a simple, single variable capacitor with a voltage rating of
about 3000 volts.
The second version was a 70 foot Universal with the TH6 and later a stack of
three beams. With a taller tower the shunt tap came down to about 56 feet using
the twisted wires.
The third version is the current 80 foot Universal. The shunt feed was changed
to 1/2 aluminum CATV hardline. This year I changed the bread slicer to a vacuum
variable (approx 600 pf) and use an air variable as an Omega capacitor. My tap
is now at the 48 foot level. My tower appears to have a natural resonance
around 1765 khz.
After years of turning on the rig and send 20 watts out to the gamma, and then
running outside to tune for minimum SWR, I had a brain spasm this year and took
the MFJ SWR analyzer out to the capacitor box and hooked it directly to the
coax connector. Then I tuned the vacuum variable and omega cap for minimum
reactance and SWR, 1 to 1. Works like a champ!
73, Dennis W0JX/8
Milan, OH
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