## Antennas for AIS Transmitters

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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### Antennas for AIS Transmitters

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.

DIFFERENT FREQUENCIES
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.

ANTENNA TYPES
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.

ANTENNA MATCH
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.

ANTENNA CONSIDERATION
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.

SUMMARY
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 an AIS transmitter and a VHF ship station transmitter albeit with some unavoidable conflicts if a wide-VSWR bandwidth antenna and a fast-acting automatic switching device are used.

jimh
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### Re: Antennas for AIS Transmitters

A further consideration on choice of an AIS antenna may be based on the capability of the AIS transmitter in use. Recently the AIS protocol has been expanded to allow transmission of two additional channels, designated as long-distance AIS and intended for reception by AIS receivers in orbit in satellites, thus satellite AIS transmissions. The new channels are located at VHF Marine Band Channel 75 (156.775-MHz) and Channel 76 (156.825-MHz), which were formerly GUARD BANDS for Channel 16 (156.800-MHz).

Typically only AIS CLASS-A devices or AIS CLASS-B SOTDMA devices will have the capability of transmitting on the two new channels in addition to the two original channels. If such devices are to be used, then the VSWR-bandwidth of the AIS antenna should be sufficiently broad to permit low VSWR at both 156-MHz and 162-MHz.

jimh
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### Re: Antennas for AIS Transmitters

For AIS-receive-only radios, use of an antenna optimized for ship station VHF voice transmission to receive AIS transmissions will not have any practical effect on the ability to receive ship AIS signals. I explain in more detail below.

The behavior of an antenna when transmitting or receiving is the same, so in that sense, if you use an antenna that is optimized for 156.8-MHz and want to receive at 162-MHz, the antenna behavior will not be exactly the same. There might be a loss of of signal of perhaps 1 dB at most.

When transmitting, the strength of the radiated signal is limited by the transmitter power.

But when receiving, the strength of the received signal can be amplified, and there is no limit to how much amplification can be used. (Well, there is a limit to the amount of amplification but it is related to the amount of noise generated by the receiver itself, but this is not a particularly import concern in this discussion.)

The typical VHF Marine Band radio receiver can amplify a signal from the antenna a factor of more than one-billion-times, 90 dB. Thus a very insignificant reduction in received signal at the antenna by a factor of 1-dB, will be extraordinarily unlikely to affect the strength of any received signals as this can easily be overcome with amplification in the receiver. This additional amplification can continue to grow with benefit to the receiver's sensitivity until the signal from the antenna falls below the level of the noise created by the first amplifier in receiver. Signals weaker than the noise created by the first amplifier will then be further masked in the noise output of that amplifier.

But in practical application on a boat where there is likely to be an engine operating with spark ignition and where there are likely to be other electronic devices such as a SONAR generating very strong wide-band pulses of energy, the level of radio-frequency noise generated on the boat itself by other devices will be the factor that will limit the ultimate sensitivity of any VHF receiver (voice or AIS) on that boat.

jimh
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### Re: Antennas for AIS Transmitters

A further consideration in selecting an antenna for use with an AIS transmitter is the introduction of two new frequencies for AIS transmission, the so-called AIS3 and AIS4 channels.

When AIS was initially introduced into the VHF Marine Band, AIS was allocated exclusive use of two channels (now referred to as AIS1 and AIS2) which are located at the extreme high frequency end of the band at 162-MHz. Several years ago at a global radio conference where such things as frequency allocations are discussed and worked out to the satisfaction of all nations, two new channels for AIS were allocated. The new AIS channels were allocated to the 156.8-MHz portion of the VHF Marine Band, about 5.2-MHz away from the original channels.

Not all AIS transmitters will be using the new AIS3 and AIS4. It it expected that all newer CLASS-A AIS transmitters will be able to use AIS3 and AIS4, and some newer models of CLASS-B SOTDMA transmitters will also be able to use AIS3 and AIS4. In order for those transmitters to work effectively, the antenna connected to them must be a true broadband antenna with a VSWR bandwidth sufficiently wide that at both 156.8 and 162-MHz operation the antenna will have a VSWR below 2:1 or lower. Correspondingly, an antenna that formerly might have been advertised as being "tuned for AIS" and is only able to have a low VSWR at 162-MHz will now NOT be compatible with these newer AIS transmitters that will be using AIS3 and AIS4.

A far as I know, at this time an AIS CLASS-B CSTDMA transmitter will not be using AIS3 and AIS4. They will continue to use only AIS1 and AIS2.

Regarding how an antenna becomes a broadband antenna, the usual approach is to increase the physical diameter of the radiating elements of the antenna. This is the simplest approach and is probably the one most used when trying to increase VSWR bandwidth. An alternative approach is to use a very well-designed antenna matching network that effective counteracts the antenna's inherent change in impedance with frequency so that a broader VSWR bandwidth is achieved. This approach is more difficult to design and construct, and is probably much less often employed.

Regarding the use of the AIS3 and AIS4 channels for AIS, they are intended to provide better results for long-range reception of AIS signals. Here the term long-range reception means mostly by satellite receivers in low-Earth orbits.

When AIS was first conceived as a maritime service, it was intended as a collision avoidance system. While AIS remains very useful for collision avoidance, it has also become a de facto method of tracking ship movement, and the people interested in tracking ship movement are sufficiently motivated that they have in essence caused a new industry to be created, the creation of an array of private satellites in low-Earth orbit designed to receive and relay AIS transmission on a global basis. Whether or not anyone involved in creating AIS initially could have foreseen this new industry is unknown to me, but I tend to think it was probably not considered.

The problem for AIS reception at long distances is that signals can be received from more than one self-organized network of ships, and thus there can be considerable overlap and interference. To mitigate this interference, the AIS protocol was amended to include a newer message format, MESSAGE 27. Message 27 was designed to reduce the probability of collisions with other AIS traffic when received at long distances. For more information see

https://www.navcen.uscg.gov/ais-class-a-reports#:~:text=Long%2Drange%20Automatic%20Identifcation%20System,vessels%20(typically%20by%20satellite).

For reference to the exact frequencies of AIS1, AIS2, AIS3, and AIS4, see my article

VHF Marine Band Channels--Simplified
https://continuouswave.com/forum/viewto ... =13&t=4897