RadioBanter - View Single Post

December 10th 04, 12:59 AM

On 09 Dec 2004 22:12:30 GMT, RM (John - G0WPA)
wrote:

Hi there,

I have a marine MF (1 to 7 Mhz) transmitter with only an antenna "screw
terminal" for a long wire connection on the back of the set, rather than an
SO239 or N type, and the set itself being grounded with copper sheet to a
decent earth. My problem is, I need to feed an MF whip on the roof, some 50
feet away, through the building, offices etc, and dont really want the EMC and
Health and Safety problems that would arise from 400W PEP happily radiating
indoors with the recommended wire lead-in. So I'll need to use a feeder. I have
decent low-loss coax (LMR type) but how do I couple it to the set? ..and to the
whip (just a long-ish wire in fiberglass) at the other end. Im thinking an
unbalanced to unbalanced transformer at both ends of the coax should do it,
with both the tower and the set in the workshop grounded but the coax outer
isolated. I have toroids that will do it, but how should I wind them? Im
thinking MF ununs could be made by winding 10 or 12 turns of coax on suitable
toroids. Is this reasonable or am I way off.

ALL suggestions, even those calling me a muppet , will be very gratefully
received.

Thanks, John G0WPA.

Hi John,

If you are adventurous enough to open her up, replace the "screw
terminal" with an honest SO239 (or "N" or "BNC" or "RCA") bulkhead
connector. This will require you to probably open up the hole to
match connector dimensions and perhaps drill the bulkhead mating screw
holes. None of this is terribly hard.

Or simply get a SO239 (etc.) bulkhead connector and solder two wires
to it: one into the metal through one of the mounting holes, the other
to the conventional center conductor solder cup. Connect those two
wires to the set: the center conductor wire to the existing "screw
terminal" and the mounting hole wire to the chassis (add a sheet metal
self tapping screw nearby the "screw terminal", and make sure the
screw does not puncture anything on the other side of the chassis
face). Insulate exposed wiring with tape. Leads should be kept
short, less than 4 inches or so. Make sure these connections are
hardy, one lead open can take you right back to RF in the Office.

As to your other question, add a 1:1 Current BalUn (or UnUn if you
prefer) at the antenna end of your cable. Make sure it is adequate
for your operational frequencies.

73's
Richard Clark, KB7QHC