Larry wrote:
wrote in news:1180601311.461858.45960
@r19g2000prf.googlegroups.com:

Does this sound like a good idea?



Well, not really.

I did it with shrouds for a while and it did work, somewhat. I could
tune it but radiation proved less than the insulated backstay when my
captain switched boats. Here's why....

If we feed the backstay (or shrouds) from the insulated-by-fiberglass end
on the stern, the instantaneous antenna current goes, for this instant,
up the backstay and we'll call this radiated wave because of this
current, "in phase". At the top of the backstay, connected to the mast,
the instantaneous current now flows DOWN the mast, OUT OF PHASE with the
backstay current, creating a wavefront that is "out of phase" with the
wavefront created by the backstay.....cancelling lots of it because there
is a significant difference in distance and angle with the backstay.
This creates a radiation pattern that has many small lobes and many deep
nulls, all rotating around as the boat swings, which is not good. The
current will also flow down all the shrouds and forestay, making the
radiation pattern even more complex.

It'll radiate, but not as good as an insulated backstay, which is really
cheap to accomplish.

The foregoing notwithstanding, it is
important to maintain focus on what is
meant by "good" when speaking of radiation.

All of the metal rigging on a sailboat
is in EVERY antenna's near-field.
Currents will be induced in ALL of it
whether grounded or not. The resulting
radiation patterns in the horizontal and
vertical planes will be modified by all
of this rigging.

Only a perfectly vertical radiator on a
boat with no metallic rigging will have
an omnidirectional pattern in the
horizontal plane. Backstays, whips,
etc., will all have non-uniform
radiation to some degree in the
horizontal plane. For antennas with
elements that are neither perpendicular
to the sea nor parallel to it (like a
backstay), there will be a mix of
horizontally and vertically polarized
radiation with generally different
horizontal and vertical patterns that
vary with frequency.

When feeding the mast at its base (or
via one of the shrouds), with the
forestay and backstay ungrounded, the
stays act as a "capacitative hat" and
make the mast appear longer electrically
than it is physically. Good for the
lower frequencies.

If the stays are grounded at stem and
stern, in the above example, two
vertical "loops" are formed with the
mast common to both. Toss in the shrouds
and modeling is required to gain
insights into the resulting radiation
pattern.




I'd rather be feeding the mast from the bilge with insulators on all
shrouds and stays, myself, using the iron or lead keel as a significant
ground plane, but that's not very practical as it would be hard to
accomplish and even harder to keep from corroding into oblivion. All
boat antennas are a great compromise. It's amazing they radiate on HF at
all...(c;

Larry

Almost anything can be made to radiate
efficiently. A great deal of effort and
sometimes costly hardware is required to
achieve that for some radiators.

Sal****er sure helps low-angle radiation
though.

Chuck



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