Topband: HF-2V Coil Dimensions K7XQ Jeff

[Michael Sapp]

Hello Jeff & Group:  The HF-2V 160m coil kit is the TBR-160-S, an overview
is available at
www.bencher.com/pdfs/00237izv.pdf .  This is a parallel LC trap that  is
self-resonant at about
2.2 MHz. Butternet rates it for only 500 CW. I used that tid-bit of info a
few years ago  to home brew an LC trap
for my MFJ-1793 80/40/20 vertical, which is installed as a ground plane
antenna.
The first one I tried  was a 30 uh coil and 250pf ceramic transmitting cap
combination.
I had to tap down the top of the coil 3 or 4 turns to get a good match to
the antenna. I switched the 
LC trap with a vacuum relay.  This worked but once a year or so I was
replacing the tx cap from high current breakdown.
There was also a high current node at the tap point on the coil, which was
some old B&W stock with
12 gauge wire. The tap would "unsolder" itself at KW power levels, so
silver solder and pennys were
used for tap points.  Deciding this had to be improved upon last fall, I
made up a 22 uh coil from #6 AWG copper
on a telfon pipe form and used a Jennings UCSL-1000 vac variable. That
solved the QRO problems
and the vac variable lets me roam the band.  I also added a 1000 pf shunt
to ground on the input side
to get the SWR flat. I use a second relay to switch between the shunt caps
(160m)  and the shunt coil (80/40/20m).
The 2:1 SWR bandwidth is about 35KHz with the MFJ-1793 vertical. If you
were to parallel three or four
smaller value ceramic tx caps to get to the 250-500 pf range that may solve
the current issue without
the expense of a vacuum variable. I think the Nebraska surplus sales web
site has a listing of ceramic
tx caps with current ratings to help make some decisions on appropriate cap
selection.  Then go some
place that's more affordable to buy what you need.

                                        Mike WA3TTS