Originally Posted by seegem
At the moment, this is how the antennas are configured:
Mast antenna: VHF and AIS, using the Raymarine automatic splitter
FM separate whip installed on a rail
SSB: HF backstay, no antenna on the ICOM M802 DSC.
What I want to do is:
1. Improve my FM radio reception
2. Connect my ICOM M802 SSB DSC receiver antenna (for redundancy and and also for proper DSC function via the M802)
Thus, I am considering two options:
Two mast antennas: Ant #1 for VHF and AIS. ANt #2 for FM receive and ICOM M802 DSC receive). This option requires me to install a second mast antenna.
In antenna #2 you are mixing completely different bands (HF for the 802 DSC Rx and VHF for broadcast FM) that require completely different antennas. If you want to move broadcast FM Rx to the masthead then you can use Vesperīs AIS / VHF / FM antenna splitter together with an antenna tuned somewhere between VHF ad AIS bands, which will be good enough for broadcast FM Rx.
It is also a bad idea to have VHF/AIS Tx antenna close to a separate FM Rx antenna because it is possible to push lots of Tx power into the FM receiver. It is much better to either have FM in a separate antenna that is a few wavelengths away, or have it in the same antenna but protected by a splitter that will cut off the FM reception when you are transmitting AIS or VHF.
Originally Posted by seegem
1 mast antenna: VHF Tx/Rx, AIS Tx, FM Rx, ICOM M802 DSC Rx. This would require a 4 way automatic splitter--do these even exist? Is option 2 even feasible?
I do not think it is possible to find or even make such a 4-way splitter that will work well across HF and VHF.
It would be better to keep the 802 DSC Rx antenna separate and lower (say mount it in the pushpit). By the way, there is no benefit from height for the 802s HF reception.
FM will typically work fine off a masthead antena that is tuned to VHF/AIS and an adequate splitter. If you do not need the height (eg in the Caribbean
where the FM Tx antennas are high enough to give you line of sight for dozens of miles) you can just make a simple dipole using duplex wire that is sliced alongside for 1/4 wavelength and stretched apart; all this hidden behind the panel (as in many Beneteaus).
IMHO the two points directly above are slam dunks and the only real question is whether you want to use the same antenna (and coax) for VHF and AIS, which are both critical systems. I am not too fussed about the small difference in frequency (156.8 for Ch 16 and about 162 for AIS) because most antennas have enough bandwidth to cover that range. Another story is that a passive splitter (ie without an amplifier inside and a 12V input to feed it) will hurt your Rx ranges, but the Vesper active splitter will not have that problem.
All that said, you could install VHF or AIS in a separate antenna in a mast spreader (as in some Antares
44i boats) and then have two antennas that are high, separate and identical and could give you redundancy if a coax fails when you cannot replace it.
My base setup would be one masthead antenna for VHF, FM and AIS with a 3-way Vesper splitter and a separate HF DSC Rx antenna mounted in the pushpit or stern arch, etc. A possible tweak would be to save the splitter (cost and power consumption), use a cheap
wire dipole for VHF and have AIS and VHF in separate antennas , one in a spreader and the other one in the masthead.