Hi Jim

Along with comments from Dave I thought it might be worth mentioning a
few more points.

Think of 900MHz as line of sight and any obstruction is an issue to plan
for. It is worth keeping the antenna gain low because of wave slap
action tilting the structure and radiation pattern. Also consider
mounting it as high as you can off the water surface, say a few metres.
Hopefully this will allow for the device rolling into a wave trough
whilst still giving you reasonable range. The best analogy here is to
replace the antenna with a lamp and se how far you can see it from.

If you end up needing high antenna gain consider gimballing the antenna
to keep it oriented properly. I'd stick to providing the gain at the
ship/shore end though.

If multipath reflections and cancellations are a problem (and honestly I
dont think they are in a water/wave environment - more an issue between
trees and buildings) then consider a horizontal antenna like a halo,
bent dipole or "3 leaf clover". The plus with a horizontal is that the
antenna aperture is wide rather than high so phase cancellations are
much less of a problem.

You should also consider that what ever data you are sending might be
lost periodically so whatever it is make sure there is a forward error
correction or retry mechanism where needed.

Cheers Bob VK2YQA

jmorash wrote:


thanks for any suggestions
--Jim Morash