You may be asking for help with a solution that isn't necessary, and
also may not be sufficient to accomplish what you want. If the
transmitter is capable of matching a variety of loads, it may be
sufficient to simply run the coax from the transmitter to the roof,
connecting at the transmitter end to the screw terminal (to the center
coax conductor) and the transmitter's ground/chassis (to the outer coax
conductor). At the roof end, connect the center conductor to the whip,
and the outer to a ground plane...at least some radial wires, if there
is no large metallic plate to connect to. But how will you know if the
transmitter has properly matched to the load presented by the
coax+antenna? It's usual to have some sort of tuner (perhaps already
in the transmitter?), and perhaps an antenna at the other end of the
coax that has a reasonable feedpoint impedance. It could result in a
fairly high SWR on the line and still be reasonable, so long as the
transmitter can match it. But I don't see that an "un-un" would do you
much good in this case, unless you need galvanic isolation between the
transmitter and the antenna. -- It also may be convenient to mount a
different antenna connector to the transmitter, either in the existing
sheet metal or on a small bracket attached near the existing screw
terminal.

Cheers,
Tom

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.