VHF Marine Band Channels

by James W. Hebert

The rather confusing channel nomenclature and frequency allocation of channels in the VHF Marine Band are explained.

The Global VHF Marine Band

The VHF Marine Band is a globally allocated and coordinated radio service. Inasmuch as ships sail all around the world, it only makes sense that ship radios should be useful in all countries. The allocated band is organized into assigned channels, and marine radio dials almost always refer to the frequency in use by a channel designator instead of an actual frequency. Indeed, VHF Marine Band radios are almost universally tuned by jumping from one channel to the next, and seldom offer continuously variable tuning. The channel frequency assignments and the corresponding designators were developed according to a band plan, which has since been revised a few times from its initial state. I have not researched the precise history of the various plans, when they were proposed, when implemented, or when modified, but rather I am just attempting to show the underlying frequency allocations and methods which have been used to carve up the band into channels.

Original Band Plan

The VHF Marine Band is a global frequency allocation from 156.000-Mhz to about 162.0375-MHz. In the initial division of the band into channels, each channel was allowed a bandwidth of 0.050-MHz or 50-kHz. A channel is designated by its center frequency, and is considered to occupy 0.025-MHz or 25-kHz above and below its center frequency. The VHF Marine Band uses frequency modulation to convey voice or telephony. When the carrier wave is modulated, sideband signals are produced that deviate above and below the carrier frequency. To prevent signals from interfering with signals on adjacent channels, the spacing between channels is set to roughly twice the modulated signal width, or 50-kHz for a modulation bandwidth of 25-kHz of each carrier frequency. The wide spacing helps to insure that signals on adjacent channels do not interfere. There is also a band-edge guard channel of 25-kHz to prevent VHF Band Signals from interfering with adjacent services in other bands, as well as some in-band guard channels to protect certain important channels. With this in mind we can see the the logic of the initial assignment of channels in the VHF Marine Band, as shown in the table below.

So far the plan appears very logical. The designated "Channel-1" is at the low end of the band, and channel frequencies rise in step with the channel number. The plan appears to only use the lower portion of the allocated band. There is a good explanation for this. The band plan includes two types of channels: single frequency or simplex channels, where all stations listen and transmit on the same frequency, called simplex operation, and split-frequency channels, where ship stations transmit and receive on separate frequencies, and coast stations listen and transmit on the opposite pairs, often with simultaneous receive-transmit capability for the coast station, called semi-duplex operation. (If both stations on the split channel can transmit and receive simultaneously, then full duplex operation is possible.) The split between the transmit and receive pairs is set for 4.6-MHz separation, about a three-percent offset. The wider the offset, the easier for a station to obtain duplex operation, that is, to be able to transmit and not have its receiver desensitized by its own transmitter. The band plan calls for the ship stations to transmit on the lower frequency of the pair, and the coast stations to transmit on the upper frequency of the pair. Certain channels were designated for single-frequency operation and others for split-frequency operation. We now look again at the original channel assignments to see which ones are set up for split frequency. The second channel of the split frequency channel will be 4.6-MHz higher. This band plan is shown in the table below.

With the provision for split channels and 4.6-MHz offset between the receive and transmit pairs, the VHF Marine Band was more or less filled up with just 28 channels. Note that certain channels, notably 6 and 8-to-17 were reserved as single frequency channels, and the upper pair of the channel was not used in the band plan. Also note that there are no channels assigned between roughly 157.425-MHz to 160.625-MHz. This spectrum is likely in use for other services. In the USA, the range 157.450-MHz to 161.575-MHz is shared with several other mobile services. For more details about allocation, see the FCC's Table of Frequency Allocations.

Split-frequency Channels

Split-frequency channels permits the possibility for a coast station to be designed for simultaneous receive and transmit, or duplex operation. This permits the coast station to function better when relaying telephone calls. In the days before cellular telephone service, many of the VHF Marine Band channels were designated for operational use as Public Correspondence, and coast stations offered to relay telephone calls to ships. By using split channels, at least one side of the conversation--the shore side--could be provided with a duplex service. The caller from shore had to understand that the ship was operating simplex and the person on the ship could not hear them while the ship was transmitting.

The split-channels also offered some privacy of communication. If a ship were communicating to shore, say to its office or an agent, another ship listening to the channel with a VHF Marine Band radio would only be able to hear half of the conversation, that is, they would hear the coast station but not the other ship.

The band plan has ship stations transmitting over a narrower range of frequencies than coast stations. This is a good approach because the transmitter and antenna used on a ship will likely have a narrower operational bandwidth than the transmitter and antenna used at a coast station. (Antenna VSWR bandwidth is often a function of the diameter of the radiating elements. Ships tend to use antennas whose radiating elements are comparatively small diameter, while coast stations can use larger antennas.) The ship station only needs to be able to transmit from 156 to 157.4-MHz. A coast station needs to be able to transmit from 156 to 162-MHz, a much wider bandwidth.

Narrow Channels

Sometime after the original band plan was made, a change in channel bandwidth was made. The new plan provided for reducing the width of each channel, most likely in step with a change in the frequency modulation deviation to be used in the transmitters. Again, I am not researching the the exact history of these changes, just explaining the technical reasons behind them. Modern VHF Marine Band radios use frequency modulation that employs less deviation of the carrier than older radios. Modern VHF Marine Band radios transmit 16K0G3E modulation, or phase modulated telephony with a 16-kHz bandwidth. This easily fits into channel allocations that are 25-kHz wide. A set of new channels was overlaid on the first channels, using a 0.025-MHz or 25-kHz offset. The numbering plan appears to simply add the number 60 to the original channel number and apply the appropriate offset. The result is the plan shown below, with the original channels interleaved with the new channels.

The Missing Channels

The plans shown above gives us a channelization with channel numbers from 01 to 28 and 60 to 88. There are no channels (nor have there ever been any channels) numbered in the range of 29 to 59. Also, the VHF Marine Band has no channels in the frequency range of 160.950 to 161.500. These channels would have been the second frequency or coast station transmit channel if the associated lower frequency (Channel-8 through Channel-17) were used in a split-channel arrangement. Since they are designated for only single-frequency use, this has left the band 161.000 to 161.450 available for other services to use. (In North American it is often allocated for use by railways.)

Other channels were lost because they served as guard bands for more important channels. There are no Channel 75 or 76 for this reason; they were guard bands for Channel 16, which is the primary voice distress, safety, and calling channel. Omitting the adjacent channels provides greater protection against possible interference with the distress channel. At one time 75 and 76 may have been authorized for use but at only 1-Watt of transmitter power.

Three channels in the VHF Marine Band were recently reserved for specialized non-voice communication. The former Channel-70 (156.525-MHz) is now dedicated to digital selective calling (DSC) transmissions using data techniques. Former coast station transmit channels 87B (161.975-MHz) and 88B (162.025-MHz) are now reserved for automatic identification system (AIS) transmissions using data techniques. Their corresponding lower-frequency pairs, Channel-87A and Channel-88A are now single-frequency channels.

New Channel Numbering Plan

Recently there was a re-numbering of channels to eliminate the suffix A on former duplex channels that were and still are available as simplex channels. Several channels were eliminated, as well. Note this table is organized by channel number sequence, the earlier listings were organized by frequency. The table also adds the permitted uses; channels available to recreational boaters are noted in violet. Recreational boater can also use the Public Correspondence channels if they wish to place a telephone call to a landline via a commerical shore station that provides that service. (Today most boaters use cellular telephones instead.) VDSMS refers to a new service to use text messaging. A separate article discusses this mode.

Update November 2021: added new AIS3 and AIS4 channels.

Simplified Chart Recreational Channels

Below is a simplified table of VHF Marine Band channels available to recreational boats.

Even More Channels

The VHF Marine Band has been further channelized into even more closely spaced channels. Using a channel spacing of 0.0125 or 12.5-kHz, even more channels have been designated. The new channels are numbered by adding 200 to the existing channel number. For example, Channel 260 is created by adding 0.0125-MHz to Channel 60, producing a new channel at 156.0375-MHz.

An even further channelization has been proposed, where the VHF Marine Band would be allocated with 0.00625-MHz or 6.25-kHz spacing. The new channels are numbered by adding 100, 200, or 300 to the existing channels.

More details of the VHF Marine Band narrow band channel plans can be found at the U.S. Coast Guard NavCen website. I don't know what the status of these band plans might be at the moment or in the future.

National Band Plans

When the VHF Marine Band allocation was worked out, some of the lower marine channels were already in use in certain countries by other services. In the USA there were several VHF Marine Band channels which conflicted with existing users; those conflicting channels were then abandoned for marine use in the USA. This completely eliminated use of Channels 2, 3, 4, 60, 61, 62, 63, and 64, and caused geographic restrictions to be put in place for Channels 1, 5, 63, and 67.

The global plan for designation of single frequency and split frequency channels has also not been strictly followed in the USA or in Canada. Most notably, Channel 22 has been converted into a single frequency channel using the lower (or ship) frequency. In the USA this channel is designated as 22A. (The unused higher frequency coast station channel would be designated as 22B if used as a single frequency channel.) A number of other channels are similarly re-purposed for single-frequency use, i.e., 1A, 5A, 7A, 18A, 19A, 21A, 23A. 63A, 65A, 66A, 78A, 79A, 80A, 81A, 82A, and 83A.

As a result of the various national uses of the VHF Marine Band, modern radios are typically made with three distinct settings available. Each setting corresponds to the national use of the channels in a particular country. Most radios provide set-up for the USA plan, the Canadian plan, and an International plan. Each plan has a slightly different use of the available channels, with the differences generally in regard to use of certain channels for single-frequency operation as opposed to split-frequency operation.

Designated Operational Use

To this point we have only investigated the allocation of the VHF Marine Band into frequency channels or paired frequency channels. We have not investigated any restrictions on the use of a particular channel. Each regulating authority has imposed restrictions of the operational use of the available channels in the VHF Marine Band, and certain channels are designated for specific uses. For example, Channel 16 is designated as the primary voice distress, safety, and calling channel. A vessel equipped with a VHF Marine Band radio must maintain a radio watch on Channel 16 whenever the radio is not being used for other communications. This is a use restriction which is global in scope. Other use restrictions are part of national operational restrictions. For example, in the USA Channel-9 has been designated as a recreational boat calling channel. There are also geographic use restrictions. In the Great Lakes, for example, Channel 79 may be used for recreational ship-to-ship traffic, but elsewhere it is reserved for commercial use. Each regulating authority has produced its own operational use plan. Boaters in the USA can find details about the operational use of the VHF Marine Band from the United States Coast Guard NavCen website. Details about operational use in other countries are presented in a Wikipedia article.

AIS Antennas

The AIS channels are located at the extreme upper end of the Marine Band, at roughly 162-MHz. In contrast, a ship transmitter will generally only transmit in the range of 156 to 157.4. This is a rather large split, almost 6-MHz separation. Many ship antennas will not be sufficiently broad band to work well on 162-MHz if originally tuned for the 156-MHz range. The result has been a number of specialized antennas re-tuned for the 162-MHz range and marked as AIS antennas. The AIS antennas will provide a better match to a transmitter at 162-MHz than the typical narrow-band VHF Marine Band antenna.

Further Discussion

If this article has raised any questions or if you have a comment, feel free to post it to a discussion thread reserved for that purpose.

Weather Channels

The USA's National Oceanic and Atmosphereic Administration (NOAA) transmits weather information on seven channels near the VHF Marine Band, and these are usually available on most VHF Marine Band radios for reception. The channel numbers are not well correlated with frequency, as you can see by the listing below.

NOAA says the channel numbering is not significant and often varies with different receivers. In addition to NOAA, continuous marine weather information is also broadcast on two channels in the VHF Marine Band by the Canadian Coast Guard (CCG), using 21B (161.650-MHz) (often labeled WX8) and 83B (161.775-MHz) often labeled WX9. An addiitonal frequency, 163.275 MHz, often labeled WX10 is also used in some cases. It is often seen that VHF Marine Band radios provide all ten of these frequencies by calling them weather channels one to ten.

Copyright © 2008 by James W. Hebert. Unauthorized reproduction prohibited!

This is a verified HTML 4.01 document served to you from continuousWave


Author: James W. Hebert