rgleason, I don't think there is a single
"preferred" way to mount the AIS antenna. Generally, higher is better, although with a stern-rail mount you will still probably get reasonable AIS reception
out to the horizon. This is adequate for collision-avoidance, although with the shorter range and weaker signal, it might take a while before the ship's name and callsign show up (these are only transmitted every six minutes).
With the spreader I do get better range, and I think my Class-B transponder is more likely to get picked up at a distance as well. There may be some interference
from the mast
, and I haven't done a careful 360-degree survey
. However, the shroud
, and even the mast, are pretty small in comparison to the wavelengths used for AIS. This means that a non-resonant small-diameter obstruction (such as the mast) will not have a big effect on the antenna pattern. It might slightly de-tune the antenna (but the SWR measurements were OK), and the pattern might be a bit distorted, but there should be no big dead zones.
I would not want to put the AIS and VHF
antennas right next to each other. They will affect each other's radiation patterns, tuning, and possibly overload the AIS and VHF
receivers. No doubt in many cases this arrangement will work well enough, but I prefer to keep these antennas apart. The upper spreader seemed to be a good compromise for me.
An above-the-radome location should be good too, as should be a stern-arch mount. Elevation is good, but even the stern-rail location is workable.