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ContinuousWave: Small Boat Electrical
VHF Marine Band Antenna Designated for AIS Use
|Author||Topic: VHF Marine Band Antenna Designated for AIS Use|
posted 03-22-2010 08:44 AM ET (US)
My 2005 Montauk 170 has a Shakespeare 4' Galaxy 5400-XT VHF antenna that was put on the boat when new, mounted to the center console. The antenna needs to be replaced it has a crack where the fiberglass is attached to the ferrule.
I just installed the new Standard Horizon GX2100 Matrix AIS VHF. I noticed that Shakespeare has a 4' Galaxy 5396-AIS that they market for improved performance for AIS. Any thoughts about the differences between the two antennas?
I was also considering the GAM SS-2. Will I have any performance differences with the Galaxy 4' 3db antenna vs the 3' 6db GAM SS-2?
posted 03-22-2010 12:00 PM ET (US)
You might find this recent thread, as well the thread referenced, useful.
posted 03-22-2010 09:55 PM ET (US)
VHF Marine Band antennas that are designated as AIS antennas are most likely slightly different from standard VHF Marine Band in the tuning of the antenna for best VSWR match versus frequency.
The VHF Marine band frequencies range from 156 to 162.05-MHz. A standard antenna is probably tuned for best VSWR around 156.8-MHz. An AIS antenna is probably tuned for best VSWR around 162.0-MHz. Or, the AIS antenna may be designed to have a broader VSWR bandwidth, so that it can be used with transmitters on both extremes of the VHF Marine Band.
Since you mention use of the antenna in conjunction with an AIS-receive only radio, the antenna VSWR on the AIS frequencies will not be of much concern. I will explain:
The VSWR on a transmission line affects the loss of the transmission line. On a transmission line with a high standing wave ratio, the loss due to line attenuation will increase. Thus, to obtain the lowest loss in the transmission line, it is desirable to keep the VSWR as low as possible. Generally a VSWR of 2:1 is considered acceptable.
On transmit, line loss is very important because any power lost in the line is prevented from reaching the antenna. The effective radiated power is reduced. In Marine Band Radio service the transmitter power is limited to 25-watts output at the transmitter. Any power lost in the transmission line is therefore lost power.
On receive, line loss is not as important. In a receiver there is plenty of surplus gain. If there is some extra line loss it can be compensated by extra gain in the receiver. However, this is not a limitless situation. Line loss adds directly to the receiver noise factor--a figure of merit for the minimum signal that can be detected--so too much line loss will limit receiver sensitivity.
posted 03-22-2010 10:01 PM ET (US)
Regarding claims of manufacturers for gain from their antennas, I have little faith in them. Instead I rely on two factors:
--physical length of the radiating element
In regard to the GAM SS-2 antenna, the length of the radiating element is about 36-inches, or about a half-wavelength at the VHF Marine Band frequency. Therefore I would give it a rating of unity gain with respect to a half-wavelength dipole. I would also say a "four-foot" fiberglass tube enclosed antenna was likely to have a radiating element of about the same length, and it probably has the same unity gain, assuming it is also well constructed and properly tuned.
posted 03-22-2010 10:02 PM ET (US)
I have written more about antenna gain in another article:
You should read that article to gain a better understanding of antenna gain.
posted 03-23-2010 03:05 PM ET (US)
Since your unit is only an AIS receiver, you don't need a "special" AIS antenna.
Any good marine VHF antenna is perfectly capable of receiving AIS transmissions.
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