VHF Marine Band Antennas

[jimh]

For a ship station in the VHF Marine Band in the USA, the radio transmitting frequencies range from 156.025-MHz (Channel 60) to 157.425-MHz (Channel 88). This is a bandwidth of 1.4-MHz at a nominal center frequency of 156.8-MHz, or a bandwidth of less than one-percent (0.89-percent). This represents a very narrow bandwidth for an antenna to cover. In actual practice, the transmitting frequency range is even smaller, mostly around 156.8-MHz and a few adjacent channels. On this basis there is really no justification to purchase specialized (and extra-expensive) wide-band antennas to cover this spectrum for the typical VHF Marine Band ship station transmitter. Most commercially made VHF Marine Band antennas for the recreational boat market are nominally tuned to have lowest VSWR at 156.8-MHz, which is also Channel 16, the emergency voice monitoring and alerting channel.

The receiving frequencies used in VHF Marine Band ship station radios vary over a much wider range. The frequencies to be received include AIS channels around 162.0-MHz and NOAA Weather Radio Broadcast transmission at 162.550-MHz. This is a much wider bandwidth than the transmitter needs, but there is very little concern about a decrease in performance from the antenna having been tuned for best VSWR at 156.8-MHz. While a small addition signal loss can occur if the transmission line VSWR is higher, for all practical purposes there is really no effect on receiver performance if the receiving antenna has a VWSR higher than 2:1, as long as the VSWR is not extremely bad, such a 10:1.

On this basis, there is no point in buying a special wide-band antenna just because you plan to listen to AIS. People have been listening to NOAA Weather Radio Broadcast transmissions at 162.550-MHz for decades with their standard VHF Marine Band antenna tuned for 156.8-MHz and never gave a moment's thought about it. Now that people may have an AIS receiver that will listen at 161.975-MHz and 162.025-MHz for signals, there is not a sudden need to get a different antenna.

On the other hand, if one plans to transmit with an AIS transponder, there is a need for a specialized antenna. If an AIS transponder is used with a standard-tuned antenna, it is almost certain that the VSWR of that standard antenna on the AIS channels will be greater than 2:1. Most AIS transponders have an output circuit that measures the VSWR and will complain about a VSWR greater than 2:1 by setting off an alarm condition or, in some instances, will stop transmitting entirely. In this instance, use of an antenna that is specifically tuned for AIS transmit channels is much preferred.

Having a wide-band antenna that can have a low VSWR on both the normal ship transmit channels and on the AIS transponder channels is normally not necessary. Usually an AIS transponder will have its own dedicated antenna. If, for some reason, only one antenna must be used for both an AIS transponder and the ship station radio—a practice that is probably not common or desirable—then perhaps the use of a wide-band antenna could be justified. I don't see that as a good method for these reasons. First, the cost of a splitter device than can handle two transmitters feeding one antenna is quite high, much more than the cost of a second antenna. And the cost of a special wide-band antenna is also higher, perhaps as much as having two antennas of narrow-band but specific tuning.

For example, a splitter device to permit an AIS transponder to use the same antennas a the ship VHF radio can cost $250. See

http://www.vespermarine.com/antenna-splitter-sp160.html

A specialized wide-band antenna capable of having low VSWR for both normal ship radio and for AIS transmission can cost $250, too. See

http://shakespeare-ce.com/marine/product/6396-ais-ais-antenna/

Whether it makes sense to spend $500 to avoid spending perhaps $125 for a dedicated antenna is left to the individual to rationalize.

[jimh]

The VHF Marine Band is represented graphically below, with annotations of where the various ship station transmitter and receiver frequencies are located:

VHF Marine Band Spectrum

vhfMarineBandSpectrum.png (14.9 KiB) Viewed 5091 times

As you can see, the portion of the VHF Marine Band in which a ship station will transmit is very narrow, confined to the lower portion of the band. The only exception to this occurs with an automatic identification system (AIS) transponder, which will transmit at the very high end of the marine band. This graphic representation should help readers understand the need for a specialized antenna for AIS transponder transmitters.

Note also that the NOAA Weather Radio band extends farther than the AIS frequencies, and for decades no one ever suggested that a wide-band antenna was necessary to properly receive weather radio broadcasts. Also note that shore stations in the public service using duplex transmit-receive are being received by the ship at the very high end of the band, too. Again, no one ever suggested that in order to communicate with a duplex shore station you should have a wide-band antenna to improve reception.

[Don McIntyre - MI]

Regarding the upper portion of the band, used by the marine [shore] services: I wonder if anyone with one of the better scanners has monitored that band segment, and could determine the frequency of use of that band? Since the introduction of cellular telephone, I think the usage of that service has dwindled significantly.

Regards - Don

[jimh]

I also suspect that VHF Marine Band ship-to-shore traffic with public correspondence stations is almost non-existent today in North America. The last time I made a telephone call using a public correspondence shore station was in 1988, as best I can recall. But there are some private shore stations still working their commercial fleets with those channels. They like the semi-duplex operation, and it also gives them a bit of privacy as other ships cannot hear their ship's side of the conversation.

The VHF Marine Band allocation is on a global basis. Here in the USA we have already split up some segments and re-allocated them to other services. The railroads use some of the frequencies in the middle of the VHF Marine Band. And some other services are also allocated some of those vacant frequencies in the middle of the band. Spectrum allocation is always a fight, and once a particular service has some spectrum, they will fight and resist giving it up. So even though those shore station transmit frequencies are not in heavy use, I don't think they're going away soon.

[richv]

For a ship station in the VHF Marine Band in the USA, the radio transmitting frequencies range from 156.025-MHz (Channel 60) to 157.425-MHz (Channel 88)...

Your analysis of the effects of VSWR on receiving, while technically incorrect is, practically, pretty accurate. A VSWR of 2:1 is going to result in, perhaps, a .5db reduction in received signal strength, but with modern receivers, that's not going to make the difference in whether or not you receive an AIS signal. Right now, the ISS is running an experiment involving the reception of AIS signals. The ISS orbits at 249 miles, but it has no trouble picking up stations near its far horizon, which is several thousand miles away. Surface-to-surface, you're not likely to see more than 20 miles, unless you have a very tall mast. On the other hand, most modern transmitters are easily able to handle up to 3:1 VSWR safely, and if you use good coaxial cable, a VSWR of 3:1 still doesn't cost you that much extra power. With a VSWR of 3:1, 100' of RG-8 adds only .886 db of loss, or an additional -8.5%.

The main reason for using a splitter rather than two antennae is simple; if the two antennae are closer than one wavelength (λ = 6.3 feet or 2 m), transmissions from one may desensitize the receiver of the other. On a practical basis, it's unlikely that FM marine voice transmissions will occur often enough on a small boat to cause a practical problem. I'd advise keeping the two antennae at least λ/2 (3.15' or 1 m) apart to prevent the transmitter from overloading the receiver of the other unit. Overloading the receiver can cause it to become non-linear and re-transmit some of the incoming RF energy as spurious signals on many harmonics. Thus, you want your antennae to be separated as much as possible.

[jimh]

Your analysis of the effects of VSWR on receiving, while technically incorrect...

I did not offer a technical explanation of the effect of VSWR on reception, only a practical observation, so I am somewhat confused how something I did not describe could be incorrect.

How much additional loss occurs in a transmission line due to effects of VSWR is dependent on how much initial loss would occur with a VSWR of 1.0, so one cannot say that a VSWR of 3-to-1 would always cause a certain amount of added loss.

In an ideal situation, one would avoid installing two antennas that were both resonant on the same frequency in any sort of proximity. The effect a second antenna has on the first is a function of their mutual impedance, which varies with spacing and how closely they resonate. Generally greater spacing reduces the effect. Antennas that are larger in their capture area need more spacing to avoid being affected.

Unless the laws of Physics have changed, the effects of VSWR are the same for transmit as receive.

In the case of radio aerials, the plural of antenna is antennas. Insects have antennae.

Whether a nearby receiver will be overloaded by a transmitter depends on many factors. One cannot say that a certain spacing is sufficient to avoid overload. The design of the receiver is most important, but so is the gain of each antenna, the amount of frequency separation, and the transmitter power. For example, a RADAR might transmit with a 4-kW peak pulse and have no effect on a GNSS receiver close-by. A 1-Watt transmitter transmitting ony 25-kHz away from a receiver whose antenna was 1-meter away might completely desensitize the receiver.