Don't listen to Guy. I too have my AIS antenna mounted on the spreader and it works fine.
It's the Shakespeare 5215AIS which is a bit under a metre tall. I made a bracket from stainless plate and had my rigger rivet that to the spreader half way between mast and spreader tip. The bracket positions the antenna almost level with the front of the mast and the forward diamond protects it from the genoa
I know you didn't ask about splitters or separate antennas, but no doubt you will get more suggestions to go the splitter route
. I put a lot of thought into whether it was better to have a splitter or a separate antenna. As with everything on a boat, there are compromises. The separate antenna worked out a little dearer after buying
cable and the time for the rigger to fit it, but the difference was not significant. So for me it was a close decision, but came down to usage in practice.
The splitter argument
With a splitter, you would be using the best located antenna (masthead for me), so more range. But long range in AIS is not important for me so I discounted this benefit. As it turns out, I have tried swapping antennas and the difference in reception
is marginal. Most splitters reduce the signal a little so it would be worthwhile to find one that minimises this (e.g. Vesper).
What I saw as a downside for the splitter solution was the fact that AIS communication is disconnected while you are on the radio. So if you happen to be on the radio when your AIS transmits, the signal will not go out, and you will also not receive incoming signals. You will not know this has happened.
There could be rare occasions when the AIS is effectively dumb and blind. Maybe you are on the VHF trying to raise the very vessel you want to see or be seen by on AIS.
The separate antenna argument
In this case you have redundcancy and abiliity to easily swap cables
. The downside is you will not know if the mast shadow is interfering with transmission
or reception. I haven't seen any evidence of this happening but it must affect the signal if the antenna is hidden by the mast. There is a small angle abeam where signal could be affected. As you get closer, with stronger signal strength this effect should reduce. Mounting on the starbord spreader could be a slight advantage.
So each solution has benefits and disadvantages.
Back to your question ..... yes a spreader mounted AIS antenna works fine.