John:

1 to 7 MHz is almost 3 octaves, and no simple antenna is going to present a usable impedance match to the coax over that
range. Are you operating a spot frequency, a narrow band, or do you need the full range? If the latter, then you
better look at remote antenna tuners, although I do not know of any which go below 1.6 MHz. Because, no matter what
chokes, etc. you install, you will never decouple the outside of the coax from the antenna field enough to be able to
successfully run it in a tray with low level signals. Been there, tried to do that. One thing you will eventually
learn, forget the half-antenna (whip) and put up a dipole. Then, you have a fighting chance to keep the antenna
currents in the antenna where you want them, and not on the outside of the feedline, which thinks it is the other half
of the antenna when you employ a whip, even with radials. At 1 MHz, the radials need to be almost 300 feet long.
--
Crazy George
W5VPQ
Remove N O and S P A M imbedded in return address

"John - G0WPA" wrote in message ...
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.

Crazy George wrote:
John:

1 to 7 MHz is almost 3 octaves, and no simple antenna is going to
present a usable impedance match to the coax over that
range. Are you operating a spot frequency, a narrow band, or do you
need the full range? If the latter, then you
better look at remote antenna tuners, although I do not know of any
which go below 1.6 MHz. Because, no matter what
chokes, etc. you install, you will never decouple the outside of the
coax from the antenna field enough to be able to
successfully run it in a tray with low level signals. Been there,
tried to do that. One thing you will eventually
learn, forget the half-antenna (whip) and put up a dipole. Then, you
have a fighting chance to keep the antenna
currents in the antenna where you want them, and not on the outside of
the feedline, which thinks it is the other half
of the antenna when you employ a whip, even with radials. At 1 MHz,
the radials need to be almost 300 feet long.
--

All very good points. What it comes down to is that a whip always needs
some kind of ground/radials/counterpoise connection at its base. You
need something to feed the whip *against*.

Older marine transmitters is that they tended to take a good 'ground'
return for granted, so they only provided a single terminal for the
antenna wire. If you convert to a coax output, it won't make any
difference unless you have something to connect the shield of the coax
to, out at the base of the whip. That "something" has to collect all the
RF return current from the whip itself... and if it doesn't, all the
building wiring and the outside of the coax will collect those currents
instead.

If you think of your office building as a very small 'ship', but without
the benefit of all-metal construction or a decent salt-water ground,
this is not a promising situation. In today's electronic office
environment, I'm afraid you are likely to be fighting a perpetual losing
battle against EMC problems.

If this radio link matters, you'd do better to find another location for
the transmitter and antenna.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek