Effect of Antenna Length on Range of AIS

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
Jefecinco
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Effect of Antenna Length on Range of AIS

Postby Jefecinco » Sun Oct 15, 2017 10:28 am

Jim--recreational AIS antennas sold at West Marine range from 36 to 96-inches long. Is AIS transmitting range dependent on antenna length?

If so, is long range transmission important to recreational boaters?

I've assumed distant reception range is essentially unimportant to recreational boaters so a 36-inch antenna is sufficient for safety. Do you concur?
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jimh
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Re: Effect of Antenna Length on Range of AIS

Postby jimh » Sun Oct 15, 2017 10:45 am

Radio range is NEVER a function of antenna length. Radio range is a combination of the two stations, their transmitter power, the path loss, their radio horizons, the propagation conditions, the antenna gains, and their receiver sensitivities.

For small recreational boats that can alter course quickly, AIS receive-only for collision avoidance should be useful with short range reception. I think notice of an approaching ship that is a mile or two away would be sufficient to allow a small boat to get out its way.

But most AIS receivers are integrated with the communication radio on small boats, so the communication radio antenna will be used. A separate and dedicated AIS receiver antenna is probably not going to be needed, unless you plan to install a CLASS-B AIS Transponder on your boat.

If you are asking about a separate antenna for use with an AIS transmitter (often called a transponder), the most import aspect of the AIS antenna for transmitting is the tuning of the antenna for the AIS frequencies. The two AIS frequencies are at the very high end of the VHF Marine Band, about 6-MHz higher than the usual tuning for a ship station antenna, which favors the lowest end of the band.

CLASS-B AIS is typically only 2-Watts, so the range of a ship statioin with CLASS-B AIS is already, by design of the overall system, intended to be less than CLASS-A AIS ships, which use higher power, 12.5-Watts.

As in any VHF Marine Band radio range estimate, the height of the antenna above the sea is more perhaps the most important factor in determining communication range. Higher is always better, and particularly when starting from a low height, as would be seen on a small boat.