Thread: AIS Questions
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Old 04-14-2021, 04:23 AM
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Originally Posted by jhebert View Post

If you have two VHF Marine Band DSC radios, you need two antennas.

If you have an AIS transmitter-receiver, you need a third antenna or you need a very fast-acting automatic antenna switch that will switch one antenna between the AIS transmitter and one of the DSC radios.

Although these fast-acting automatic antenna switches are often sold as "splitters", during transmission of one transmitter connected to the antenna "splitter" the other transmitter will not be connect to the antenna at all.

In the case of transmitting on the VHF voice radio, that means the AIS receiver and transmitter will not be connected, and when the AIS is transmitting, the voice receiver and transmitter won't be connected. The antenna is "shared" only when neither the AIS transmitter or the voice transmitter is transmitting.

The AIS transmitter operates at a different part of the band than the ship voice radio transmitters, so the antenna for the AIS transmitter must be tuned differently than the antennas for the ship voice transmitters.

If a shared antenna is used, that antenna must be a special wide-band antenna that can work on both voice and AIS frequencies, and not an antenna that is narrow-band and tuned for just voice frequencies or just AIS frequencies.

Regarding antennas for VHF Marine Band--either voice or AIS--the most important element for the antenna is the height of the mounting base. On small boats, increasing the antenna height is more important than the length of the antenna. To increase the boat's range of radio communication, mount the antenna base as high as possible.

With regard to using an fast-acting automatic antenna switch to let a radio and an AIS use one antenna, consider the following:

--the cost of the automatic switch may be greater than the cost of a separate antenna;

--the cost of a special wide-band antenna will be greater than the cost of a standard antenna that is designed for just voice VHF or just AIS;

The essential advantage of using the fast-acting automatic antenna switch is that it eliminates a second antenna; this can be advantageous on small boats where room for antennas at high mounting height can be very limited.
I am sure you are correct in a narrow technical sense and I respect the radio expertise you show in your posts. However, I have never been able to discern any impact whatsoever on my VHF performance from the active antenna switch built in to the AIT5000. Since I have two radios with identical antennas mounted at the same height, it is easy for me to compare. One radio is sharing an antenna through the AIT5000 and the other is connected directly to its antenna. When the Sea Tow radio check station was still operating in my area, I ran a number of side-by-side tests of the two radios. They both could be heard and receive the response at virtually identical range. I have never had an issue with anyone with whom I communicated via VHF on the water with one radio doing better than the other. So what ever difference may exist in theory, on the water with properly installed cables and connectors and a quality antenna, it seems to make no difference.

As for the need for an AIS specific tuned antenna, that is also something that in real life adds limited value. My AIS transponder has no trouble at all receiving transmissions from large ships 30-40 miles away using the conventional VHF antenna. My signal shows up on the Marine Traffic ground station that is 30 miles away and friends have seen me on their AIS receivers over 10 miles away. An interesting question is whether the benefit of using a 17' VHF antenna mounted on a base 12 feet above the water outweighs the benefit of a shorter 4 or 8' AIS tuned antenna. On my boat, the space in the tower legs for cable is at a premium and my two big VHF antennas use leave no good place for a dedicated AIS antenna with the recommended separation so the active switch was a good solution that as I noted, appears to have had no negative impact on performance of either the VHF or the AIS transponder.