[Antennas] WTB: butterfly capacitor

2002年3月18日 03:45:53 EST

In a message dated 3/17/02 7:27:52 PM Pacific Standard Time, [email protected] 
writes:
<< I'm experimenting with a magnetic loop and have it working (sort of). I 
want
 to switch to a butterfly cap because of the lowered losses. I think I need
 something in the range of 5pF to 200pF. >>
I may be wrong - but don't you just want an old HV spaced, two gang
variable? Both stators are insulated from ground - you connect the loop
ends to the 2 stator contacts - the common rotors provide a variable
"third plate" between the two, and you have a balanced capacitor without
the loop having to use any moving contacts or wipers. The effective 
capacity is 1/2 the capacity of one section - so that is a problem, finding
a capacitor out there that is 2 gang and with enuff puffs. Actually, it's
tuff finding any, in kV type spacing, over about 200 pF, but you might.
This may limit your tuning range, or you could switch in a glass HV
vacuum cap, like about 100 pF or so, across the variable cap to
lower the bottom end tuning. If you want the loop to remain near
balanced, you have to use a coupling loop and not a high-pF series
cap. But you know that already.
Say, how big the loop, and what is it made out of, and how thick is the
wire or tubing? What holds it up?
I saved from the dumpster some ex-telecom wire, it's i think 8 coax
cables, maybe 10, in a common outer plastic carrier. I was thinking of 
wiring all braids on the coaxes together, at the ends, and using that
as a substitute for metal tubing in a magloop antenna. I still am looking
for the ideal capacitor! I'm not mechanical enuff, or have enuff tools, to
build from scratch with a vacuum variable.
Regards, Hue Miller