Originally Posted by Emerald Sea
We have a Raymarine AIS650 fitted and Im getting the following error message "External AIS Unit Fault, Antenna VSWR Fault" with an alarm
going off every 10mins or so.....
What did I do before just this fault? I changed out the antenna, located on our mast
top as it blew off, and replaced it with a similar 6db one (open circuit). Also changed out the cable from RG58 to RG233...
If the antenna is located atop a mast, I assume the transmission
line length is rather long. If the previous transmission
line was RG-58/U cable, it was quite lossy. There was probably a loss of more than -3dB in the transmission line.
If you replaced the transmission line with RG-233/U able, you significantly decreased the loss by a factor of about ten, and transmission line loss is now probably less than -0.5dB.
This change in the loss in the transmission line between transmitter and antenna has caused the VSWR as seen at the transmitter to be closer to the actual VSWR at the antenna.
In case you are not familiar with radio
and transmission line theory, line loss can mask a high VSWR at the antenna. A simple way to consider this is as follows:
--assume an antenna has a VSWR 3.0 or 25-percent reflected power, that is, for every 100-Watt of incident power at the antenna there will b3 25 watts of reflected power.
--assume there is a -3dB transmission line loss
--assume the transmitter puts out 10-Watts (to keep the math simple)
--there is a -3dB loss in the transmission so only 5-Watts reaches the antenna;
--the antenna reflected power is 0.25 x 5 = 1.25-Watts back toward the transmitter; this is a VSWR of 3.0
--the reflected power is attentuated -3dB, so only 0.625-Watts reaches the transmitter
--the apparent VSWR at the transmitter is based on a reflected/forward power ratio of 0.625/10 = 0.0625, or a VSWR of 1.67
--the apparent VSWR is 1.67 but the real VSWR at the antenna is 3.0
In your case, the line loss of the RG-58 transmission line was probably more than -3dB, and this was masking the actual antenna VSWR. When you changed to a much lower loss transmission line, with a loss of only about one-tenth as much as before, the apparent VSWR at the transmitter increased significantly and now is more correlated to the actual antenna VSWR.
As a result, your AIS transmitter is now giving you an alarm that indicates the VSWR of the antenna is too high. This is probably a valid alarm because the VSWR of the actual antenna is probably too high, unless you happened to have purchased a special antenna that was specifically tuned for the AIS transmit frequencies.
Note that the AIS transmit frequencies are at the very high end of the VHF Marine
Band, about 162-MHz, while the usual vessel transmit frequencies are at the very low end of the VHF Marine
Band, about 156-MHz. Most vessel antennas are tuned for best VSWR at the 156-Mhz end of the band, not the 162-MHz end. This is why one sees specialty antennas sold for AIS transmitters.
This is just my armchair guess, but it is based on over 50-years of experience in radio communication, both as a radio amateur and in commercial
radio and broadcast radio.