I often talk with people who are using small "rubber duck" style antennas and getting poor reception or transmit range.
These types of antennas are designed to be as tiny as possible (often only 200mm or so) and are sometimes used as emergency or dinghy antennas. They usually don't have much bandwidth (remember AIS is at the extreme upper end of the VHF marine band) and some are rated at -2dB gain. That means that they will lose nearly 1/2 the signal strength. And finally, some only work when they are mounted on a large metal ground plane.
As a result, we strongly urge our customers to consider an antenna that is better suited for use with AIS transponders. Things to look for are 1/2 wave, +3dB gain (more is not necessarily better), omni-directional and these will tend to be at least 1m (3') long. It doesn't necessarily need to be AIS-tuned, but that will result in the best performance. But if it isn't AIS-tuned, check the manufacturer's VSWR curve or bandwidth figures. You want it to be less than 2:1 at 162 MHz.
Even if you don't have a "rubber duck" but your antenna is old or of unknown origin, then consider replacing it. They deteriorate in the sun and weather and some are prone to getting water into them.
The performance of your AIS system is only as good as the weakest link. If you own a Vesper Marine AIS you already have the most sensitive and highest quality AIS there is. But it can't overcome a low performing antenna system. Our recommendation is to always use the best quality cables, connectors and antennas you can. And usually they don't cost much more either.