I have installed the Raymarine AIS 250 with my RM C90W plotter.
Originally I also used the splitter function offered by the AIS 250:
- the masttop VHF antenna
cable into the AIS 250
- the antenna
out towards the marine
- even the FM radio antenna in connected to the dedicated OUT socket on the AIS 250
With this setup:
- the AIS reception is SUPER since it uses the 14m high masttop VHF antenna with low loss coaxial cable - receiving AIS data from very high distances > 30 NM
- no perceivable losses on reception signals on the marine
vHF over the splitter
- BUT interference
from the AIS onto marine VHF channels adjacent (e.g. european channel 27 which is a duplex channel often used by shore stations) to the 2 dedicated AIS frequencies (161.975 and 162.025 if I am not mistaken). This not only happens with the Icom
M80 marine VHF but also when using my Yaesu FT897D Ham transceiver on those frequencies.
Please check this if you use those adjacent channels often on reception!
I checked: US marine VHF channels 27,28 and 86 are close by AIS1 CH 161.975 MHz.
: I stopped using the AIS250 onboard antenna splitter and installed a dedicated AIS receiving antenna on the transom. Although far lower I still receive AIS signals from 20 NM away. Plenty. It also means I have a spare/emergency VHF antenna in case I lose the masttop antenna.
I did not check if installing choking toroids or moving the AIS250 further away (not practical in my case) helps.
MY advice: if adjacent channels are of no use for you, use the splitter and the masttop VHF antenna. If in doubt about insertion losses of the splitter: do a TX/RX test with a far away shore station but with a standard marine VHF you can only rely on the audio since it will have no S-meter. Inseertion losses on transmit will be next to nothing since you only pass a relay.
There has been a very popular thread on AIS spliiters and insertion losses on this forum.