"Jack Painter" wrote in message
news:luPtc.221$Y21.126@lakeread02...
Thanks very much, that was a lightbulb going off (duh) that the backstay
on
less than a 70' yacht is going to have a seriously short antenna WRT
wavelength!

Precisely! :-)

My wires and dipole are of course half wave devices and at
desired frequencies do not even require a tuner at all.

Aha, there's the catch..

And yes I do use a 1:1 Balun (isolation only on the tunes dipole, 4:1 on
random wires). And
just because the specs of my Sunair Coupler _could_ deal with any wire 30'
or longer, that would be a frivolous effort to try to tune, say 2182khz on
so short a wire with 50ohm coax. It does work mediocre on an 80' wire but
I
am still somewhat surprised that any sailing vessel could get much
performance (if any do) on MF from a (relatively short) backstay antenna.

It all depends on proper tuning. Your 4:1 balun might not be enough on short
wires. Try a 9:1 balun. But anyway, it is always betten not to use a low
impedant cable like coax between a tuner and an antenna.
This might explain your wonder about the ability of vessels to get good
performance. At a vessel, the cable run from tuner to antenna is mostly (I
hope :-) ) short and made of GTO15 or similar stuff. If you, on the other
hand, use coax to the antenna, even with a 4:1 balun which is not
transforming high enough, you have a problem tuning it properly.

If you have the chance, try a balun (1:1) directly after the tuner, better
yet, use a symmetrical tuner, and feed a dipole with open line (two wires,
spaced 3" apart). This will give you:
A) better tuneability and B) better surpression of man-made noise. To
prevent the feeders from radiating, you could add a common-mode choke in the
feeder (sometimes called a current-balun).

Closer to the 1/2 wavelength, I would think that coax would be more
appropriate to the ATU-to-Antenna match than this GTO-15. Correct?

Only if you feed the 1/2 in the low impedance point, which is halfway in the
middel. Since this is impractical on a boat, feed it at the endpoint. But at
the endpoint of a 1/2 wave, the impedance is very high so you need the least
capacitance you can get at the antenna feedpoint. So GT15 is ok, but keep it
1" away from any grounded or other conductive area (like strapping the GTO15
to the uninsulated lower part of the backstay with tie-raps... BAD
PRACTICE).

Meindert
PE1GRV