I tried coax stubs several years ago. RG8 worked fine on the exciter.
When I fired up the amp there was a blinding flash at the base of the
tower. The RG8 had metamorphosed into charcoal. The coax was foam
dielectric; never thought of using teflon coax; that sounds like a great
idea, and it's proven.
At that time dollars were in short supply so I made a capacitor from window
glass and aluminum foil. I glued the foil onto the glass, leaving the last
couple of inches at the edge without foil. Used an air variable to get the
swr down, then measured it on a capacitance bridge, and then pulled foil
off the glass until the capacitance was the same as the air variable. I
don't know how many volts it would have withstood but I'm sure it's way up
there.
Another source of cheap caps is the vacuum padders which sometimes show up
at hamfests. That's what I've been using for the last ten years or so.
The fixed vacuum caps are much cheaper than the variables, and once the
gamma or omega is tuned you don't have to do it again unless there's a
major change in or on the tower. I'm using about 400 pf, consisting of
four padders in parallel. I think they were about $2 each in the flea
market at Dayton.
73
Larry, N9DX
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|