Hi Gents,
I'm building a clone of the famous BCS (Battle Creek special)
antenna. There are some design differences between my model
and the "standard" BCS.
Namely:
1. A "Hy-Gain style" trap for 40m. Yeah, not the most efficient, but
it was something I really wanted to try. So far no problem with
that....
works great.
2. The 80m trap, by design, resonants at ~3.3 MHz, about 5% below
the band instead of in the band.
3. I'm using 3, nearly equal lengths, of RG213/U for the capacitor
instead of 2 because I didn't want the coax ends sticking out
beyond the tubing end. My tubing section lengths are 3 feet
long instead of 6, made to fit in a golf club case.
The problem I'm having is the failure of the 80m coax trap capacitors.
The center insulation of the coax burns and shorts out by tracking/arcing
to the braid about 3/4" above the point where the braid separates from
the coax.
The braids connect to the lower end of the coil and the center conductors
continue on up to connect to the top end of the coil. There is no sign
of
tracking in or on the form itself. There was no damage to the center
coax
insulation before the failure. The connections are made inside the 2"
sked 80 PVC coil form. There was some condensate moisture inside the
coil
form. The coax is sealed where the braid breaks out with several coats
of brushed-on Plasti-Dip, curing between coats. The failure occured
above the Plasti-Dip coating.
Having seen this damage, I'm amazed that the "open" ends of the
coax capacitor didn't short out. The braid is skinned back about 3/8"
there
and the ends are dipped in the Plasti-Dip, covering the braid and
all.
The inductance of the trap coil is ~11.5 uHy and the Qu was measured
at ~255. The coax caps (total) measure ~220 pf. The trap started
to fail at only 800 watts while testing it on 160m (it was acting
as an inductance, not a trap, on 160). Obviously the RF
voltage across the trap (inductor) has exceded the voltage rating of
the coax! So I'm not so sure where I can go from here. Any
suggestions?
73,
Charlie, N0TT
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