Just to throw this into the mix... here's the solution I used:
Kenwood TM-D710 ->
A/B Switch -> 1/2 wave antenna at mast
Class B -------->
MT-AIO APRS ---> 1/4 wave (19 inch) antenna on the bow pulpit (for a ground plane)
Standard Horizon Marine
Band HT (with DSC) ---> 1/2 wave antenna on the stern toerail
Both 1/2 wave antennas require no ground plane and are Shakespeare "squatty body" antennas.
The advantages were:
1) I seldom use the Kenwood on 2 meters while underway. If I ever need 50 watts ERP and 40 miles line-of-sight while underway, it's probably an emergency
and I can switch on the Kenwood and use it on the marine
band. Otherwise, the AIS
gets the range and the preferred antenna. There's no problem with the AIS transponder seeing infinite VSWR (the AIS specification says it must tolerate an antenna dead short/open). It just keeps trying until I throw the A/B switch back to it. I find the squatty body antenna works great on both 2m and 70 cm when I need it.
2) I hear way too much
on the Marine HT here in the San Francisco
Bay area using the masthead antenna. I really don't need to receive other vessel's hails to a portmaster 40 miles away. Channel 16 is too chatty and a real distraction with that much receive range. But when I'm offshore
- I really DO want to know about a container ship that's closing on me at 30 knots that's 40 miles away.
3) If I ever need to, I can hit the Big Red Button (DSC Distress) on the HT, and still talk on channel 16 with the Kenwood while the HT repeats the DSC distress
call every 4 minutes (until acknowledged). The Coast Guard antennas are on towers with good line of sight to my antenna 5 feet above the water
, and any vessels that can't hear the DSC distress
call are probably to far away to provide any assistance.
4) The APRS, AIS, and HT antennas have the maximum physical separation.
All three antennas have lightning
suppressors (Alpha Delta) and are bonded to a ground strap that runs the length of the bilge
and up to the mast base (where the masthead antenna coax terminates at a lightning suppressor). The ground strap is then terminated at a 1 square foot, 1/8 inch think zinc plate that I toss in the water
at the slip/anchor -- and that I also toss in the water when underway when lightning is anywhere nearby.
The solution works for me. YAMMV.
BTW, I tried an antenna splitter, and I was very disappointed with its performance. You can't get somethin' for nothin'.