I suggest you look closely at em-trak. em-trak is the house brand of SRT Marine
Systems. SRT is more or less the gold standard in AIS devices. They design the actual silocon chips used in AIS, and they are the original equipment
manufacturer (OEM) for many products sold under private label brands.
The em-trak Class-B model B100 used to retail for about $550, but the price
has recently been reduced. You can find this Class-B AIS tranceiver for under $350. It may be
headed for end-of-life and replacement with a newer model, but it represents a best-buy at this moment. See:
[No affiliation with this vendor]
There is a more recently introduced model, the B300, which sells for about $480.
[No affiliation with the vendor]
At one time I was trying to keep abreast of Class-B transponders, and I wrote and updated an article on them. The article is a bit out of date now, but it may be useful for you. See
AIS Class B Transceiver Round Up
continuousWave: Whaler: Reference: AIS Class-B Transceiver Round Up
If you do get a Class-B transceiver (or transponder), I suggest using a dedicated antenna for it, rather than using an automatic antenna switching device to share an antenna. The AIS frequencies are at the very high end of the VHF Marine Band at 162-MHz. Most ship station antennas are tuned for the very low end of the band at 156-MHz, where all ship transmitters operate, and their VSWR bandwidth is usually not very broad.
AIS transmitters often will object to operating into a transmission
line with a VSWR above about 2:1, and may not work
very well trying to feed an antenna tuned for the wrong end of the band.
Also, the transmission
interval of Class-B is rather infrequent, and it would be a shame to miss an opportunity to transmit when scheduled due to the shared antenna being in use by the other transmitter. The whole point of AIS is to transmit your position, and impeding that by sharing an antenna seems to be at odds with the point of having an AIS transmitter. A Class-B transmitter is just operating at a few Watts of power. You ought to provide the transmitter with a low-loss, low-VSWR transmission line to a well-tuned antenna.
The better antenna sharing automatic switching devices cost more than a second antenna will cost. They also add complexity to the system. Adding a second antenna for AIS will add redundancy to the system; you could is the AIS antenna for the voice transmitter in an emergency
For those reasons, I suggest having a dedicated AIS antenna that is tuned specifically for 162-MHz.
If you must use a shared antenna, it would be best to replace the current
antenna with a new antenna that is designed to have a wide VSWR bandwidth and is useful on both voice and AIS. Those antennas usually are made with larger diameter radiating elements to produce a broader VSWR bandwidth. Of course, they cost more than single-purpose antennas. Again, the dedicated AIS antenna seems like a good choice.