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post #25 of Old 10-24-2013
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Re: VHF AIS radios

Originally Posted by Classic30 View Post
Originally Posted by SVAuspicious View Post
Splitters are fundamentally bad. The best thing you can do is have independent VHF antennas for VHF and AIS.
Yep, understand that. It just not practical on small(er) boats..
I recommend a masthead antenna for your VHF and an AIS/VHF antenna on your pushpit. As an alternative, especially if you haul for the winter with mast down, put the AIS antenna on a spreader. How is that not practical on a small boat?

Originally Posted by TakeFive View Post
Not sure if you noticed that the built-in splitter is circuit-protected, which I presume to mean that it blocks the VHF radio's 25W transmissions so it won't fry the electronics. I know this does not address all the issues with splitters, but it would seem to be a significant improvement vs. someone who unknowingly puts a passive splitter in place without realizing the damage that it can do when he presses the button on his mic.
Any splitter that will be at all functional if either leg includes a transmitter must have some active component.

Splitters function by dividing the energy gathered by the antenna between two downleads to the receivers, in this case the fixed VHF and an AIS. The only difference in the GX-2150 and similar is that all the connections are internal to the radio. You still take a 3 dB attenuation of the signal, which means half the power absent the splitter. Remember, decibels (dB) measure the ratio of power - db = 10 log (P1/P2) so small numbers make a big difference. PL-259 and SO-239 connectors are not so great either so an external splitter will be even worse with additional attenuation of 0.5 - 1.0 dB (depending on the skill of the connector installer) from the extra two connectors. The impedance bump (you can't count on a continuous 50 ohm characteristic impedance through the connectors and splitter) results in increased SWR which on most radios will roll back power output on transmit.

Now on the subject of transmit, most splitters I am familiar with have a 1.5 - 1.8 dB attenuation on transmit.

In commercial (like cable TV) and government (like SIGINT) applications where splitters are used to share very expensive, high performance antennas there is a low noise amplifier (LNA) ahead of the "splitter" and a carefully implemented, constant impedance divider network, often followed by another LNA. Noise levels are critical - you can't just amplify the signal, you have to amplify the signal without increasing the noise floor. You do not want to think about what those cost. *grin*

Transmitters get dedicated antennas. In applications that spend serious money on signal distribution, don't you think that if there was a fundamentally good way to use a splitter with a transmitter they would?

Regardless the physics and the engineering are clear. Splitters, internal or external, at the price point available to the recreational market introduce significant signal loss AND increase the noise floor.

Now for the "compromises are necessary" contingent the question becomes whether it is significant. How big a difference does it make?

Fixed radio to fixed radio you probably won't be able to tell most of the time. Talking to the USCG over Rescue 21? Probably not discernible. However, calling a marina when the dockmaster has a handheld and is out walking the docks? It will make a difference that may mean the difference between contact and not. Entering an inlet where your local knowledge is over a hill and the other side of a forest of masts? Could be a problem. Trying to maintain contact with the Towboat/US tow boat on it's way to haul you off a soft grounding? There may be delays as you relay through the tow base.

I've been working with radio systems for a long time both professionally and as an avocation. Splitters are not a good compromise. I have yet to see a boat that can't manage a proper installation of separate antennas for fixed VHF and AIS.

Originally Posted by TakeFive View Post
One of the reasons I decided to go with the GX2150 is because it did not require a splitter and all the potential problems that go along with it.
Don't kid yourself. The GX-2150 does have a splitter. It's built in to the radio. All the same deficits apply.

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Last edited by SVAuspicious; 10-24-2013 at 07:45 AM.
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