You asked for feedback on the homemade cap: Years ago (very late 50's) while in
college, I worked at a subscription sponsored FM station (KPFA) in Berkeley,
CA. Part of our engineering crew spent one summer in LAX building a 10KW tx on
Mt. Wilson for a new addition to the station's family (KPFK). The tx then was
semi-homebrew, and used four 4-1000's in parallel, at about 5.3KV and ~4.5 A
plate current, running class C.
The plate bypass cap was a real problem and we went through several. I finally
got a sheet of 1/4 inch teflon, about 2.5' square (just under the size of the
top of the pressurized PA enclosure, cemented a sheet of aluminum to it, and
cemented the teflon to the top of the enclosure.
1. First RF: Big electrical display ... stupid oversight ... aluminum
electrode was same size as teflon and the electricity managed to find it's way
around the edge.
2. Second try: Worked ok but corona around the sharp edges of the aluminum
plate and imperfections in the enclosure top. Rounded everything off and sanded
it down. (I was an elec engr student then, you'd think I might have know this
in advance!)
3. Third try: Worked like a charm (almost). PA settled down, neutralization
settled down, and no fireworks and blown caps.
4. Finally discovered that teflon was getting fairly warm (like discoloring),
so we ended up replacing it with a piece of window glass (maybe 3/32 or 1/8"
thick?). End of problems.
Can't see why it wouldn't work for a ham band antenna if you can get enough
capacity in a reasonably sized plate. Two pieces of glass with foil on the
outside and between them would double the capacity of course. In fact, I guess
one could sandwich several layers in so long as you can keep everything smooth
and separate the connections adequately.
73,
Fred, K6DGW
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