Finally someone mentinoed the curvature of the sea...

Answers:

The 1/4 wave ground plane with a flat plate or 3-5 radials [drooping or
not]; the coaxial "sleeve" dipole and the "J" are all pretty much the same
thing / pattern-wise. The "J" and sleeve/coaxial dipole may be easier to
get up higher, however.

The sleeve dipole is a 1/2 wave TOTAL, just like a regular dipole and coax
feed is standard. The "j" is nothing more than an end fed 1/2 wave (where
the 1/2 wave dipole is a center fed half-wave).
These are all fairly simple to make and the "J" shouldn't be a big problem,
even with it's much debated so-called feed-line radiation 'problem'. They
work just fine.
I think the main issues a
1- As Richard correctly points out, sea / earth curvature means there is a
minimum altitude required for a given receiver distance. With this case of
the low "bouy", it will be the larger of determining factors and the other
end will require greater altitude to make up for small changes at the
"bouy".

2- Sea action means an omni gain antenna is contraindicated (not a good
idea) because there will be larger variations in signal strength as it bobs
around. This is because this "gain" is obtained by compressing the
radiation pattern more toward the horizontal and as the "bouy" rocks, you
more quickly get to the points where the signal drops off - above and below
the horizontal where the signal peaks. Trying to get a "stable" platform
with non active means (perhaps a weighted pendulub under the antenna on a
gimble) won't work well due to the accelelrations involved - it may rock
more than the "bouy". I would first try a stabilizing "keel" if this is an
option, to provide a little stability to the "bouy". Depending on its
construction, it may rock moerthan the surface, but be stabilized by being
"anchored" to the water below (which win't be moving as much as teh "bouy"
can.

Just like in real estate, the three most inportant things in antennas are
altitude, altitude altitude...

Hope this helps & good luck
73, Steve, K,9.D;C'I









"jmorash" wrote in message
ups.com...
Richard (& Bob),

I didn't think of simply calculating the distance to the horizon...
oops.

The problem is that this is a mobile device that will spend most of its
time underwater. It will not be particularly stable at the free
surface, hence it will be tough to support a tall antenna mast without
it waving around wildly. The taller it is, the more it's going to move
around.