This article will look at the requirements for an antenna for an AIS transmitter, and at the effects that occur if one antenna is to be used for both an AIS transmitter and for a VHF Marine Band ship station radio transmitter.
An AIS transmitter operates at 162-MHz, at the very high end of the VHF Marine Band. In contrast, a ship station VHF Marine Band transmitter operates at the very low end of the band, from 156 to 157.4-MHz.
Antennas sold as marine antennas for the VHF Marine Band are generally constructed to provide the best match to a 50-Ohm transmission line at the low end of the band, as that is where ship station transmitters operate. For the purpose of this discussion I will call such an antenna a "standard" VHF Marine band antenna.
The indicator for the degree of match between the antenna input impedance and the transmission line characteristic impedance is found by measuring the voltage standing wave ratio on the transmission line, called the VSWR for short, using a directional wattmeter or similar device (sometimes called a reflectometer). As the VSWR is a ratio, it is expressed numerically with values like 2:1, and often is just given as the first number in the ratio with the second number understood to be "1". The impedance can be assumed to be 50-Ohms, a standard impedance for both transmitter output, transmission line, and antenna input impedance.
A standard VHF Marine Band antenna is expected to produce a low VSWR in the low end of the band, and a typical value might be 1.2 (or 1.2:1). As a general rule, any VSWR below 2:1 is useful. As VSWR increases above 2:1, problems can occur.
EFFECT OF VSWR ON TRANSMITTER
A typical feature of modern transmitters is to have a sensing circuit at the antenna output jack on the transmitter that can sense the VSWR. If the VSWR is high, a modern transmitter will react by reducing its power output. It does this to protect the transmitter final amplifier circuit from harm. For example, if there were no antenna connected, a transmitter would be looking into an infinite impedance, and the VSWR would be infinite. This would result in no power being conducted away from the transmitter to the antenna, and that will cause the voltages in the output circuit to rise much higher than normal. In such a situation a modern transmitter will reduce its power output to (practically) zero, or to the lowest possible level needed to still sense the high VSWR. Because of a feature like this, a modern transmitter must be connected to an antenna that produces a low VSWR. And typically the threshold for reduction of power with excessively high VSWR will occur when the VSWR is greater than 2:1 or possibly at a higher threshold like 3:1.
If an AIS transmitter is connected to a standard VHF Marine Band antenna, the standard antenna will likely exhibit a VSWR of greater than 2:1, as the antenna was not designed to by used at 162-MHz. The result will be the AIS transmitter will sense this high VSWR and reduce its power output. Because of this typical behavior, an AIS transmitter needs to be connected to an antenna designed for the 162-MHz frequencies that AIS uses. Connecting an AIS transmitter to a standard VHF Marine Band antenna will result in reduced power output from the AIS or in some cases shut down of the AIS transmitter. The AIS transmitter will generally provide an alarm alert to the operator about this problem in order to let the operator know the AIS transmitter has stopped. This is reasonable because a ship with an AIS transmitter is anticipating that other ships will be able to receive those AIS transmission and use them for collision avoidance. Thus loss of an AIS transmitter represents a significant change in the ship's operation with regard to collision avoidance.
VHF Marine Band ship station radios generally also have similar protective circuits in their transmitter output, and will similarly reduce output power if connected to an antenna with a high VSWR, although the radio may not explicitly alert the operator of this problem.
USE OF SINGLE ANTENNA FOR BOTH AIS AND VHF SHIP STATION RADIO
At one time there was an expectation that any AIS transmitter would be connected to its own, dedicated, properly tuned antenna. However, as use of AIS transmitters increased on smaller boats, and because on smaller boats there is limited room for two antennas, the notion arose that perhaps both an VHF Marine Band ship radio and an AIS transmitter might be able to use just one antenna between them in a cooperative manner. There are two considerations in such a configuration: the antenna and its VSWR characteristics, and the device that will accomplish the antenna sharing.
If one antenna is to be used with both a VHF Marine band ship station radio and an AIS transmitter, the antenna must be a special model with broadband VSWR characteristics so that the VSWR for the antenna is below 2:1 at both ends of the band. Broadband VWSR antennas are often made with radiating elements of larger diameter than a standard antenna. The thickness of the radiating elements helps to create a broadband VSWR characteristic, and the antenna is constructed so the lowest VSWR occurs in the middle of the band (about 159-MHz).
ANTENNA SWITCHING DEVICE
For an AIS transmitter to use an antenna that will also be used by a VHF Marine band ship radio requires having a new device: an antenna switching device. These antenna switching devices are not manual devices, but instead are typically very fast-acting automatic devices that are able to sense which device, the radio or the AIS, is trying to transmit, and then switch the antenna exclusively to that device for the duration of the transmission.
Because both devices cannot transmit simultaneously, if one device begins to transmit before the other, that device captures the antenna. The second device must then be connected to a dummy load to absorb its transmitter output safely, until such time as the first device ends its transmission and the antenna becomes available for the second device to use. Or, the automatic device may include circuitry to give one transmitter priority in use of the antenna, and that transmitter will be able to interrupt the other transmitter's use of the antenna.
When neither device is transmitting, the automatic switch then connects both devices to the antenna simultaneously so they can receive simultaneously. Because connecting the antenna to two radios effectively reduces the signal level to each radio in half, some of these automatic antenna switching devices include an amplifier in this portion of their circuitry. The amplifier compensates for the signal loss in sharing the antenna with two simultaneously connected receivers.
In practice, the VHF Marine Band radio spends all most all of its time in receive, and only transmits sporadically. The AIS transmitter is more actively transmitting. Depending on the model and class, an AIS transmitter can be transmitting as often as every few seconds or as infrequently as every five or six minutes.
AIS transmission are very brief, lasting only about a tenth of a second. Such brief transmissions can be tolerated in the use of the VHF Marine band ship station radio for receiving, as they will just cause a very momentary loss of reception.
VHF Marine band ship station radio transmission tend to be much longer than AIS transmissions, often lasting ten seconds or longer. During these transmission the AIS receiver will not be connected to the antenna, and will not be able to receive any AIS transmissions from other vessels. AIS transmitters should never transmit blindly; they must listen before transmitting in order to determine when a time slot for their transmission will occur. Thus if the AIS is blocked from receiving by a long transmission from the ship radio transmitter, the AIS may not be able to determine when it is appropriate for it to transmit until reception is restored. The AIS will also miss transmissions from other vessels.
While some larger or fast-moving vessels make AIS transmission very frequently, as often as just a few seconds apart, those rapid transmission sequences do not contain all the information about the vessel. Vessels with AIS only send complete data sequences at much longer intervals. Because of this, an AIS receiver that is blocked from the antenna while the ship station transmitter is using the antenna could miss transmissions with data that will not be repeated again for several minutes.
VHF MARINE BAND RADIOS WITH AIS RECEIVERS
A very common situation now is for a VHF Marine Band ship radio to also incorporate an AIS receiver. The antenna used with such a ship radio does not need to take any particular concern for the AIS receiver. A standard VHF Marine Band antenna can be used.
The best antenna for an AIS transmitter-receiver is its own, dedicated, properly tuned antenna that it can use exclusively. However, on small boats that want to equip with an AIS transmitter and lack room for two antennas, a single antenna can be used in a cooperative manner with both and AIS transmitter and a VHF ship station transmitter with some unavoidable conflicts if a wide-VSWR bandwidth antenna and a fast-acting automatic switching device are used.
VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
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