It's obvious that splitting the antenna output two ways would reduce the strength by 3 dB. As you point out, S/N is just as important, and is why I have my home TV antenna amplifier right at the antenna (so I'm amplifying the purest signal, and not amplifying the noise that my wires pick up).
Exactly. That's the same reason satellite dishes put LNAs at the feed, that GPS "antennas" are really the antenna and receiver integrated together, and even that depth sounders (acoustic vice electromagnetic) now have demodulation built into the sounder. Noise and attenuation are less critical at lower frequencies like VHF and HF, but are none the less still important. At VHF we worry more about impedance (my earlier note on connectors) and attenuation (why RG-213, RG-8X, or LMR-400 are better choices for VHF than RG-58). At HF we worry about inadvertent shielding, multipath, and interference so tuner location and grounding become the first order factors.
As a technical matter, the GX2150 does have a splitter somewhere in there, since there are at least 3 radios - the standard VHF radio, and the two separate receivers dedicated to AIS channels 87 and 88.
I don't have a circuit diagram at hand, but the GX-2150 is a Class D VHF radio so there should be a receiver for voice VHF and a separate receiver for DSC on VHF channel 70; I don't know how they implement AIS reception - it could be a separate, third radio or switch the voice receiver. If Standard Horizon included a circuit diagram in your manual you can fax it to me or scan and e-mail it so I can tell you.
But unlike when you mix and match two devices from two different manufacturers, the GX2150 is designed, tested, warranteed, and reviewed as a single unit, so the complications that you describe are addressed by the manufacturer.
Of course. I haven't done the comparison, but we can look at a comparison of sensitivity and selectivity AND WHERE IT IS MEASURED between single purpose VHF radios and VHF-AIS combinations. It would be interesting. What is clear is that no manufacturer can reinvent the laws of physics. Somewhere in my pile of business cards are some contacts inside Standard Horizon. When I get a chance I'll send them a note and ask for their technical position.
... as a practical matter I've never heard any complaints about the GX2150 having "all the same deficits." In fact, the reviews that I have seen have been overwhelmingly positive. Can you point me to some test results that show that the GX2150 has difficulties hailing dockmasters at marinas?
I can't point to such results because there are few if any side-by-side evaluations in real time and reviews are mostly qualitative anyway. Again, the physics are clear. If you have splitter in your system, internal or external, and have a high-noise or low-signal path then your system will not perform as well as a VHF without a splitter. If you get past the sales people at any radio manufacturer they will tell you the same thing.
As for your suggestion about installing an AIS antenna on the spreader of a small boat, since small boats have short spreaders, the antenna would have to be pretty close to the mast. Wouldn't the presence of the mast so close to the antenna create a blind spot due to shadowing of the signal?
No. Masts and rigging on a single boat have limited impact on field of view. Edge diffraction puts significant energy into the element. Multipath is always a factor, but it's a factor anyway even for masthead antennas. That's different from the effect that we get from hills, buildings, and huge numbers of masts and rigging in a marina.
Nothing is perfect. As many have noted there are compromises in everything. What is very real is that the compromises of a splitter, internal or external, are more significant than most sailors realize.
The place for an AIS receiver to be integrated is in a chartplotter. Many units, especially smaller ones, have already integrated GPS receivers. An AIS receiver would be an easy addition and avoid all the problems of integration with a fixed VHF. If you find one on the market next year remember you heard it here first.