Topband: BCS Clone 80m trap problem

[Ulrich Weiss]

hello Charlie,

following Tom's advice about the inefficiency of coax-traps I built a
conventional 80m trap using a Russian 50 pF capacitor (door knob type) and
resonated it with an air-wound coil below 3.5 Mhz (just as you) for my
version of the BCS... the trap is housed in a plastic box at the top of a
17m aluminium mast of 70 mm OD with insulated guying...
80m resonance can be varied between 3.5 and 3.8 Mhz by changing the angle
between a short L-wire (about 2m attached just below the trap) and the
mast... this can be done by moving the anchor point of the guy-wire to the
L-wire  towards or away from the mast...
160m resonance is achieved by 2 sloping T-wires of about 15m attached above
the trap...
it was actually an experimental set-up... but it worked so well that I
haven't yet bothered to build the final version (with a 40m trap)...
meanwhile it has survived three WWDX contest seasons in the HP category...
fed by a homebrew 2:1 unun against a ground system of about 20 (more to
come) buried  radials it shows a good match (< 2) in a BW of about 40 khz on
160m and about 100 khz on 80...
I'd just replace your coax-trap by a conventional one with adequate
components in a weather-proof housing...

good luck

Uli, DJ2YA

----- Original Message ----- 
From: "Missouri Guy, N0TT" <n0tt1 at juno.com>
To: <Topband at contesting.com>
Sent: Friday, November 12, 2004 3:58 AM
Subject: Topband: BCS Clone 80m trap problem


> 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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