Thread: AIS Questions
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Old 04-13-2021, 05:23 AM
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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.

Last edited by jhebert; 04-13-2021 at 09:43 AM.