On Fri, 28 May 2004 18:58:25 -0400, "Jack Painter" wrote:


good stuff by Meindert snipped

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


Jack, I too wonder about the matching of (short) backstay HF antennas.
The thing that occurs to me is that trying to match the ATU to the antenna
isn't really the goal. The ATU *IS* the matching network. By feeding the
backstay with a coax, the excess capacitance (due to the coax) is just
another reactance the ATU must try to "tune out". Using coax is equivalent
to conncting shunt capacitors from there to ground.
My opinion is that the lead, whatever it is, between the ATU and the
*real* antenna, becomes part of the antenna. To me it makes sense
to use something like GTO-15 between the ATU and backstay.
We also must remember that matching the ATU to the backstay is only
part of the job. The ATU must present a proper impedance to
the transceiver. If the antenna is a horrible match, and the ATU
runs out of "range", then the impedance presented to the transceiver must
suffer also.
Be nice to put a network analyzer on a backstay and see what it
really looks like, eh? Be an opportunity to experiment with
different grounding schemes also. I'm convinced that salt water
is the best possible ground....coupling/connecting to it is the challange.
My 2-bits worth...
Norm B