"Meindert Sprang" wrote in message
...
"Jack Painter" wrote in message
newsxstc.52$9h.43@lakeread02...

Hi Meindert, I don't understand your reasoning there, sorry. And Doug
too,
who referenced a steel ship, which is my reference as well. I have seen
hardline (still 50ohm coax) in shipboard installations using the same
Sunair
ATU that I use, connected to the wire HF antennas.

A backstay antenna is relatively short compared to the wavelength. It
therefore has a high impedance. To match it to the 50 ohm of the
transceiver, the impedance has to be transformed by an L-circuit with the
capacitance at the low impedant side to ground and the inductance from the
low impedance "hot" side to the antenna. If you would use coax at the high
impedant antenna side, you get a terrible mismatch. The capacitance of
this
pice of coax adds to the L circuit at the wrong side, effectively giving
you
a PI circuit which is unable to match the high impedant backstay to the 50
ohms of the transceiver.

It appears (to me) no
different that the ungrounded dipole that I feed with coax from my land
station tuners.

Theoretically no. But your land dipole is probably much longer than a
backstay and therefore has a lower impedance. By the way, does your coax
connect directly to the dipole or do you have a balun (with a possible
impedance transformation wich makes the coax have less influence)?

Meindert,

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! My wires and dipole are of course half wave devices and at
desired frequencies do not even require a tuner at all. 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.
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? And a 4:1
balun would in other cases make the match even more feasable, as well as the
desirable electrical isolation from noise that a Balun can provide.

73

Jack Painter
Virginia Beach, Va