In most VHF Marine Band ship station radiotelephone transmitter installations there is not much concern about the antenna VSWR because the band of frequencies that will be used for transmitting is relatively narrow (156.1 to 157.8-MHz) and the typical VHF Marine Band antenna is tuned precisely for that range; as a result the VSWR will always be quite low (or good).
On the other hand, with an AIS transmitter, the frequency of transmission will be at the very high end of the VHF Marine Band (using AIS1 at 161.975 and AIS2 at 162.025MHz-MHz), and in the case of some newer Class-B AIS transceivers using the self-organized time-domain multiple access (SOTDMA) protocol, the newly allocated AIS3 (156.775-MHz) and AIS4 (156.825-MHz) frequencies at the other end of the band. In either instance, the VSWR bandwidth of the antenna to be used with the AIS transceiver becomes an important consideration. The reason this becomes important is because many AIS transmitters will not transmit of they are seeing an antenna load with a VSWR greater than about 2.0:1. With a VHF Marine band radiotelephone, the
With an AIS transceiver, the device is measuring the VSWR at the point where the antenna transmission line is connected to the transceiver. Because of this, the measured VSWR will ALWAYS be slighter better--that is the VSWR will be a lower numerical ratio--than the actual VSWR at the antenna due to the effect of loss in the transmission line.
Measurement of the VSWR at the transceiver is the simplest way to measure the VSWR, and that VSWR is what the actual transmitter will be seeing.
The reason the VSWR will be higher at the antenna is the effect of the line loss is to attenuate the amount of forward power that arrives at the antenna, and also to attenuate the amount of reflected power that returns to be measured at the directional wattmeter.
A common practice is to accept some transmission line loss in exchange for the convenience of avoiding having to move the transceiver to the antenna location and directly connect the transceiver to the antenna. The loss in the transmission should be kept to a minimum as much as possible, but a loss of 1-dB is probably still considerable tolerable. Many VHF Marine Band antennas will be connected to their transceivers with a 15-foot length of RG-58C/U cable, which at 150-MHz will be inserting a loss of about 1-dB.
The effect of a 1-dB loss in transmission line on the VSWR that will be measured at the transmitter end will be demonstrated in the example below
EFFECT ON VSWR OF TRANSMISSION LINE LOSS
--Assume the actual VSWR at the antenna is 2:1
--Assume the transmitter produces 20-Watts of power into the transmission line. This is the forward power for the VSWR measurement. Then:
--Because of a 1-dB loss only 15.9-Watts reaches the antenna.
--The power reflected at the antenna will then be 1.8-Watts due to the antenna having an actual 2:1 VSWR.
--This reflected power will also be attenuated by 1-dB on the return path through the transmission line.
--The reflected power at the transmitter will then be 1.43-Watts.
--A directional wattmeter measuring 20-Watts forward and 1.43-Watts reflected will indicate a VSWR of 1.73:1
So when someone notes that their AIS antenna seems to have a better VSWR bandwidth than specified, the improvement is probably due to measuring the antenna VSWR with some transmission line loss inserted into the test set-up.
If the transmission line loss increases, even greater masking of the VSWR at the antenna occurs.
For example, assume the transmission line has 3-dB of loss, and there is nothing connected at the far end.
If 100-Watts of power is input into the line, only 50-Watts reaches the end.
With nothing connected, all power is reflected back, all 50-Watts.
But the reflected lower is also attenuated by 3-dB, so only 25-Watts returns to be measured.
The directional wattmeter indications are 100-Watts forward and 25-Watts reflected, for a measured VSWR of 3:1.
In this extreme example we see that an INFINITE VSWR is converted to a 3:1 VSWR due to measuring with a transmission line loss of 3-dB.
This relationship leads to the conventional wisdom that if a VHF antenna at the end of a long transmission line has a VSWR approaching 3:1 there is probably no antenna connected to the transmission line.