In article .com,
"jmorash" wrote:

Hi folks,

I've got some background in EE, but know very little about antennas
(though I have a copy of the ARRL Antenna Book I'm reading now), so
please bear with me if these are newbie questions.

I need to communicate with a device floating on the surface of the
ocean, at ranges of several miles or more, using a 900MHz radio link.
Vertically polarized seems to be the way to go, to get true
omnidirectional reception. My shore- or ship-side antenna can easily be
placed 20' or more above the surface of the water, and I can use an
off-the-shelf, moderate gain (5-6 dB) product with a nice fiberglass
radome, etc.

It's the remote side that's the problem. I need to fabricate my own
antenna, rather than buying one, for packaging and waterproofing
reasons. I have a 50 ohm coax transmission line coming out of the
electronics housing; right now I'm just modifying the end of that cable
into a "coaxial dipole" (design I found on the internet). This is a 1/2
wave section of core, with the corresponding 1/2 wave section of shield
folded back down over the feedline, to form a simple dipole.

The coaxial dipole works OK at short ranges (up to a mile or so), but
there must be a better way to do this. In fact, based on the stuff I'm
reading, it seems that a dipole is a "balanced" antenna, but a coax
feed is "unbalanced" ... would I be better off with a whip and small
metal ground plane? How would I match this type of antenna to 50 ohms?
What sort of instrument would I need in order to check the impedance?
Essentially I'm looking for something easy to build without much (if
any) tuning required, doesn't need to be high gain.

It will be tough to get the antenna more than a foot (maybe two) out of
the water, and the platform will be rolling and bobbing around a lot,
so I'm reluctant to use the seawater as a ground plane. Figure the
tuning would change too much. I also expect that in general, a low-gain
antenna on the remote side will be better - generous vertical
beamwidth.

thanks for any suggestions
--Jim Morash


You might want to look a a 1/2 Wave vertical base loaded antenna
similar to the 1/2 Wave used on sailboat mast tops. Morad makes
one for Vhf. Your real problem will be, when weather causes the
SeaState to be higher than the antenna above the wave troughs.
In this state you range is going to be considerably reduced whenever
the bouy is in a trough. You might just look at designing a
fiberglass antenna mast like 6 Ft long with a counterweight at the
bottom anchorpoint, the electronics package in the middle, and the
antenna at the top, which would give the bouy's antenna 3 Ft elevation
above the SeaState. In a past life, I designed some monitoring bouys
for NOAA, and this is how we solved the antenna problems. Still didn't
solve all the SeaState problems for Heavy Seas, but worked very well
for up to 4 Ft Seas. 900 Mhz isn't exactly the best choice for Maritime
bouys, for just these reasons. You ight also look at the transmission
data protocol, and see if it is compatable with lost packet recovery
techneques, as when the bouy is in moderate to heavy seas, you going
to lose about half the data packets due to SeaState Path losses, which
will seriuously cut into the data thruput your expecting.

Bruce in alaska
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