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The Ins & Outs of Marine VHF

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(Editor's note: PMM took an in-depth look at marine VHF in the October '05 issue with an article that focused on radio waves, decibels, and the various features and functions of VHF radios. As technology in the marine industry continues to change and improve, we will keep you up to date in all areas of marine electronics.)

Twenty to thirty years ago, if you were cruising on a pleasure boat, odds were you would not find a VHF radio aboard. If you did have a radio on the boat, it was probably large and expensive and operated on only a few channels. Handhelds were relatively unheard of, except on commercial craft.

Today, things are different. Radios have been updated with all the bells and whistles, including access to all the marine channels, as well as loudhailers, foghorns, and remote speakers/microphones. The latest models are not much larger than a paperback novel, and they're amazingly inexpensive, considering their sophistication. Almost every vessel, large or small, has one or more VHF radios installed. These improvements, coupled with ease of use and the waiving of VHF licensing requirements for recreational boats in U.S. waters, may contribute to people's tendency to think of these radios as little more than an extension of a household telephone. In reality, marine VHF radios offer much more, and there is considerably more to know about their operation than meets the eye.


First, let's take a look at the characteristics of the radio frequency spectrum, or the band of frequencies, used by any VHF (very high frequency) radio. Technically speaking, the VHF radio spectrum is defined as all frequencies between 30 and 300MHz, but marine VHF only makes use of frequencies between 156 and 174MHz. Unlike radio signals in the lower HF band, VHF radio transmissions are not reflected back to Earth by the ionosphere. A VHF signal must travel in a straight line between points, a characteristic that often is referred to as "line of sight." Although this term is not strictly accurate in all cases, it does help to explain how communications take place on VHF frequencies.

Variations in frequency and signal strength can create exceptions or variations to the line-of-sight rule. For example, a commercial FM broadcast station, which transmits at the lower end of the VHF band, can send a radio signal through the wall of a building by using a powerful transmitter. Normally, a VHF radio signal will be blocked by an obstruction such as a building or high terrain, but a surprising amount of bending can take place over or around objects in the signal path. Transmitter power, which is limited to 25 watts on marine VHF frequencies, also can limit range, but usually the limiting factor is the curvature of the Earth. A radio signal traveling in a straight line is unable to follow the curving surface of the Earth and cannot be received at sites where the receiving antenna is below the horizon when viewed from the transmitting antenna.

If the Earth's curvature is the limiting factor, then the theoretical range (in miles) for a VHF transmission can be determined by multiplying the square root of the antenna height (in feet) by 1.42. Obviously, the higher the antenna, the better the range, but the heights of the transmitting and receiving antennas determine the actual communication range. This can be found by calculating the line-of-sight range for each antenna and then adding the figures. Antenna heights vary with each installation, and other factors, including transmission power levels and antenna gain, also must be taken into consideration. So it's difficult, if not impossible, to predict what range can be expected with a typical marine VHF installation. As a rule of thumb, you can anticipate a range of about 5 nautical miles with both antennas near sea level and a maximum range of 60 nautical miles or so with both antennas mounted well above sea level. Most of us will be somewhere in between, and 10/15 nautical miles isn't an unreasonable expectation for small-boat installations.

Two unusual atmospheric conditions can extend VHF range to hundreds of miles. The more common of the two is tropospheric ducting, a phenomenon that causes VHF signals to follow the curve of the Earth instead of traveling in a straight line and being lost in space. Ducting takes place when a large temperature inversion forms in the Earth's atmosphere (the temperature of the atmosphere rises, rather than falls, as elevation increases). The warm air layer aloft creates a "duct" between the warm layer and the Earth's surface that has unusual refractive properties and, in effect, bends the radio waves, back to Earth, greatly increasing communication range. Tropospheric ducting is not uncommon. Temperature inversions form in many atmospheric conditions, including when an approaching cold front lifts the warmer air in front of it, when a cool sea breeze exists along the coast, and when large, stable high-pressure systems dominate a region in the summertime.

The second phenomenon is far less common but may be widespread when it occurs. It's called "sporadic E," which refers to the E layer of the ionosphere. When a large sunspot eruption occurs, the Earth's atmosphere is showered with a high concentration of charged particles. This can cause the formation of a dense area of ions that reflects VHF radio signals back to Earth rather than allowing them to continue in a straight line and leave the atmosphere.


Marine VHF radios use frequency modulation (FM) as a method of adding information to the basic signal or carrier wave transmitted by the radio. Unlike AM, in which the amplitude or strength of the carrier is modified to impose speech and other information on the signal, FM alters the frequency of the carrier to accomplish this. Frequency modulation adds some very desirable enhancements to radio communications, but not without a price. Almost all electrical and atmospheric noise is amplitude modulated. Because FM receivers are immune to AM noise, marine VHF users enjoy virtually interference-free communication. Another plus is frequency modulation's ability to reproduce audio with greater fidelity than AM or SSB, making FM radio transmissions more intelligible.

On the other side of the coin, a phenomenon known as "capture effect," which is associated with FM communication, means that only one signal can be heard at a time. If two radios are transmitting at the same time on the same frequency, only the stronger of the two will be heard. The weaker signal will be lost, and the listener will never know it was there.

While not strictly a function of frequency modulation, VHF FM receivers are sensitive to both cosmic noise and noise generated internally by the receiver itself. The result is continual and somewhat annoying background white noise. To make this less troublesome, radios employ a manually controlled squelch circuit to suppress background noise. When a reasonably strong signal from another radio is present, background noise won't be heard. If no signal is being received at the moment, the squelch circuit functions to mask the background noise. The squelch control should be set at the point where the white noise is just eliminated, but this may result in annoying bursts from weaker signals. These can be eliminated with a higher squelch setting, but at the risk of missing weaker calls.


Duplex communication is easy to understand. We experience it every day on the telephone, and it's characterized by the ability of both parties to hear and be heard simultaneously. Simplex communication, on the other hand, has two somewhat contradictory definitions. The first describes simplex as a one-way street where communication can take place only in one direction. A radio receiver listening to a broadcast station is a good example of this.

The second definition refers to communication that takes place only in one direction at a time but can occur in either direction. If you are talking, you won't be able to hear any other station until your transmission has ended. Likewise, the station you are communicating with won't be able to hear you during its transmission. If communication is taking place on a single frequency, other listeners on the frequency will be able to hear both sides of your conversation. This method of communicating one way at a time is also referred to as "half duplex." To differentiate between half duplex and duplex, the term "full duplex" may be used when referring to full-time two-way communication.


There are three primary sets of marine channels in use in the world: a U.S. group, a Canadian group, and an international group. Some channels are shared by all three groups; some are shared by two of the groups; others are utilized by only one of the groups. Modern marine VHF radios allow the user to select between each group by name. However, the radio operator may not be fully aware of the nuances and complexities of the channel grouping system, and this can lead to some interesting communications mix-ups around the world.

In each group, some channels are duplex and some are simplex. The duplex/simplex designation may be different for the same channel in a different group. When a duplex frequency is selected on a radio designed for onboard marine applications, it transmits on one frequency but listens on an entirely different frequency. This is imperceptible to the operator, but you may notice that duplex channels are used only to communicate with shore-based marine stations. In order for this to work, the shore-based station that you are calling uses the same two frequencies but in the opposite sequence. In other words, the shore-based marine station receives on the ship's transmitting frequency and transmits on the ship's listening frequency.

This is duplex operation, marine radio style. You cannot transmit and receive at the same time, so, technically, it's really half duplex, and this produces some interesting effects. You won't be able to hear other vessels calling the shore station, and if you are monitoring the frequency, you will quickly notice that you only hear half of the conversation. It's also impossible to communicate with another vessel on a duplex frequency, because that vessel will be listening on a different frequency, not the one on which you are transmitting.

To further confuse the issue, in some regions the decision was made to use only one of the two frequencies assigned to a marine duplex channel. Referred to as "simplex operation on a duplex frequency," this means that on some duplex channels, marine radios both transmit and receive on just one of the two frequencies reserved for the channel. The other frequency remains unused. This might be easier to understand if everyone adopted this idea, but only Canada and the United States have implemented it in their respective channel groupings, and not necessarily on the same channels.

As a user, about the only thing you may notice that's different is the addition of the letter "A" after some channels when either the U.S. or Canadian channel group has been selected. This indicates that simplex, single-frequency communication is being conducted on one of the two frequencies available for use on this duplex channel. It also means that the frequency being used is the one originally designated as the ship's transmitting frequency when it was used as a duplex channel. You may also see a "B" written after some channel numbers, which indicates that the frequency being used for both transmitting and receiving is the one originally used for receiving on the vessel's radio when the channel was used in the duplex mode.

Channels in the international group are either simplex or duplex and do not carry a letter suffix. The use of only one frequency on a duplex channel makes these channels incompatible with the original version used internationally. If your radio is set to the U.S. channel grouping and transmitting on a channel with the letter "A" after its number, you won't get an answer because you're listening on the wrong frequency. For example, marine VHF Channel 63 is not used in Canada, but it is used in the duplex mode for international port operations, ship movements, and marine operator calls. In the U.S. channel group, it becomes Channel 63A, where the shipboard duplex transmission frequency is used in the single-frequency simplex mode for port operations and Vessel Traffic Service. Selecting the wrong channel group will prevent communications entirely on Channel 63; consequently, it's illegal to use channels with an "A" suffix except in areas approved for their use.

An examination of the table of VHF marine frequencies (which can be viewed at under "Web Extras") reveals several pitfalls for the those who fail to select the correct group of frequencies for the geographical area in which they are cruising. It's important to realize that while recommended usage for international VHF channels is shown in this table, many regions and countries have their own scheme for channel assignments, and some even make use of channels not listed here. In the U.K., there are three or four marine channels that won't be found on a radio sold in the United States, even if it's set to the international channel group. While Channel 16, the international distress, safety, and calling channel, is one of the few recognized worldwide, it's not authorized for hailing marinas in Canada. In other countries, you may even find that marine channels are used by local fire departments, law enforcement agencies, and occasionally restaurants (for reading their daily list of specials). To avoid embarrassment, gaffes, and possibly legal penalties, it pays to be familiar with local channel assignments in areas where you're cruising.


In the United States, the price of a radio is all that's required to operate a marine VHF in local waters. In many countries, specific training must be completed successfully before an applicant can obtain a marine radio license. Internationally, boaters who meet these training requirements, as well as government agencies, are well versed in proper operating techniques and tend to frown on those who take liberties with radio procedures. Good operating techniques are simple and are developed from experience. Misuse of the VHF marine radio is a growing problem on the water today.

Simply stated, a marine radio is not a telephone. Regulations require radio conversations to be for "operational" purposes, but there is a tendency for boaters to take a very liberal view of this term. Forwarding weather information, arranging a mutual location to meet, and inquiring about local conditions may well fall within the definition, but sharing recipes, negotiating the menu with a dinner guest, and chatting idly about sporting and social events go beyond even the loosest interpretation.

While this regulation is widely ignored, there are valid reasons for the use of proper radio procedure on marine radio bands. The first and foremost reason is always safety. Consider that a vessel transmitting with a nominal range of 20 miles blankets 1,256 square miles with its signal. During the time the transmission takes place, no other vessel in this area, including the vessel that is calling, can hear anything else on the frequency. Using low power helps, but you'd be surprised how far a signal travels even then.

Radio procedures were developed with two major goals in mind: brevity and clarity. Being brief means being well acquainted with standard phraseology that is designed to convey your meaning simply and succinctly, so everyone will have better access to radio communications. It's really not snobbish to use correct terminology. These phrases were developed to avoid misunderstandings when communicating, and if you ever get into trouble on the water, you'll fully appreciate the importance of having your message properly interpreted. A single word that conveys your meaning will be more readily understood than a longer, more ambiguous phrase. Take a look at some of the terms that are designed to convey a clear-cut meaning in a single word. (A VHF radio glossary can be found at under "Web Extras.")

"Roger." This term simply means that you understand a message. It doesn't imply that you agree with it or will comply, just that you comprehend. This is an important concept when instructions are sent by radio.

"Wilco." Short for "will comply," the use of this word indicates that you have both understood and will comply with the instructions received. It's incorrect to combine the two terms, despite what Hollywood says: it's never "Roger, wilco." Use one or the other.

"Over." Because radio communication is so often a one-way conversation, terminology was developed to indicate when you are through speaking and ready for a reply. It saves a lot of interruptions. When you are ready to listen, just say, "Over." "Did you copy?" or "Come on back to me" and similar phrases just take up more airtime. Remember: brevity counts.

"Out." This is another phrase that was designed to help streamline the flow of radio communications. Use this term when you end communication with a specific station. It's contradictory to say, "Over and out." Use one or the other.

"Affirmative." This is how you say "yes" in such a way that there can be no doubt about your meaning. It's amazing how important this can be. Casual phrases like "you betcha" and "alrighty" are not what the listener is expecting and are easily misunderstood.

"Negative." The opposite of affirmative and by far the best way to simply say "no." "Negatory," "no way, man," and similar phrases are unacceptable because they're too easily misinterpreted.

"Niner." It has been repeatedly demonstrated that the number nine is very easily misunderstood when spoken over the radio, so the word "niner" was developed for use in its place. If you don't want search and rescue looking for you several miles from your actual position, say "niner" instead of "nine" when reporting your position.


The Coast Guard is well on its way to implementing a massive upgrade of its coastal communications and distress system. Among the improvements will be automatic identification and location of automated emergency calls coming from boats equipped with DSC (digital selective calling) radios. But, even when the new system is implemented, Channel 16 will remain the mainstay of emergency communication by VHF radio. Cell phones will never be a satisfactory replacement, if for no other reason than the fact that a phone doesn't broadcast your message. With a phone, you can communicate with only one party; if you're cut off, there is no provision for someone else to pick up the call.

There are two very important things to remember regarding the use of a shared distress frequency. First, everyone on the water with a marine VHF radio should be monitoring Channel 16 continuously. We are our own safety net on the water. VHF range is short, and one day you could be the vital link in an emergency situation when you relay a distress call that no one else heard. Second, it is vital that Channel 16 be kept open and available to any vessel in distress. While Channel 16 is designated as a hailing frequency, boaters should arrange to monitor another working frequency when communicating with friends. Most shore-based stations, including marinas and the USCG, publish other frequencies that can be used to make routine contact. Idle chatter, fishing reports, and radio checks have no place on Channel 16, and you almost certainly will be admonished by the Coast Guard if you tie up communications on this important frequency.

If you do use Channel 16, limit the preliminary call to 30 seconds, and wait two minutes before repeating your call. If you don't get a reply after two attempts, make the call again later. Don't monopolize the frequency by continuing to call endlessly. Be brief. Extra phrases like "Do you read me?" or "Are you out there?" serve no purpose. If you do get a reply, shift immediately to a mutually agreed upon working channel.

In day-to-day communications, remember that it's a party line. Everyone within range will hear your conversation. Be concise, keep it simple, and leave a little time for someone else. Avoid CB lingo. Things like "Breaker, breaker. Hey, you got your ears on there, Billy?" and "10-4, good buddy, come on back" will go right over the heads of vessel operators from other countries, and there are more foreign boaters cruising U.S. waters than many realize. If nothing else, this sort of language is completely contrary to the goals of clarity and brevity. As the operator of a marine radio, you are subject to Federal Communications Commission rules and regulations.

Know your channels, and verify that you have the right channel group selected for your region. In U.S. waters, the following list applies:

Channel 16 is the distress channel. If you are in trouble on the water, use this channel to call for help. The Coast Guard continuously monitors Channel 16 and has no plans to discontinue monitoring it in the future. For nonemergency calls, you must change to an agreed upon working channel once contact is made.

Channel 9 is also a calling/hailing channel. It's not a distress channel. It is a good choice when establishing a call-up frequency with another vessel. Once you've established contact, switch to a working channel.

Channel 13 is used by bridge tenders and lockmasters and is monitored by all vessels longer than 65.6 feet (20 meters) in U.S. waters. This channel also is used for bridge-to-bridge communications.

Channel 22A is a working channel and is the right choice for routine contacts with the Coast Guard.

Channels 68, 69, 71, 72, and 78A are noncommercial working channels for ship-to-ship and ship-to-shore communications. They can be used by recreational boaters for routine boat-to-boat communications.

Channels 79A and 80A are noncommercial working channels but only for the Great Lakes, where they can be used by recreational boaters.

Channel 70 was a working channel but has been reassigned and is now reserved for DSC. It may be possible to select Channel 70 on some radios, but it's illegal to use it for any type of voice communications.


A strict protocol for communicating in an emergency has been established to provide a method for clearly and concisely conveying vital information to rescue services. To help define the type of emergency, three levels of emergency calls have been designated. In all cases, the first step is to select Channel 16, where you can broadcast your distress message to all vessels and stations monitoring the frequency.

For the direst emergencies, when your vessel and crew are in danger, the proper distress call is "Mayday." Clearly say the word three times.

For urgent situations, the phrase is "Pan-pan" (pronounced "pon-pon"). Again, repeat the phrase, stating it three times. Use this expression to indicate a serious but not life-threatening situation in which assistance is required. Examples include a serious injury or illness, or a mechanical failure that could lead to a more urgent situation in the near future.

To warn other vessels of conditions that may adversely affect them, say the word "Securité" (pronounced "say-cure-it-tay") three times. This is often used to alert others to navigational hazards, severe weather, or vessels that may pose a hazard to others.

After you have stated the appropriate emergency term three times, say, "This is" and your vessel's name three times. Then state your call sign (if one has been assigned) one time.

Repeat "Mayday" and name of your vessel once.

Give the position of your vessel. The importance of accurately stating your position cannot be overemphasized. Use whatever means is available to you; if you have a GPS or loran, read the latitude and longitude coordinates directly from the screen, if possible. If you don't use any form of navigation receiver, learn to keep track of your approximate position at all times and be aware that you may not be the one who has to make the distress call. Write your location down for other crew members to use.

Vague statements such as "We're off the beach" only serve to delay efforts to assist you, and in dire situations you cannot afford to waste valuable time.

State the nature of your distress. For example, "boat on fire," "sinking," "adrift," etc. Don't elaborate beyond the essentials at this time, as still more information needs to be communicated.

State the type of assistance you require. Medical assistance, towing, evacuation, and so forth.

State the number of people on board and any injuries among the crew.

Provide any additional information that's available. A description of your boat, including its size, color, type, and manufacturer, can be very useful to those searching for you. It may be the only way the Coast Guard has of locating you.

Say, "Over."

If possible, stand by on Channel 16 after the distress message has been acknowledged. You may need to provide more information, and it may be necessary to transmit at intervals to allow rescuers to home in on your radio signal. As boaters, we all share the airwaves. It's important to know the rules and etiquette when operating a marine VHF radio so that we can cruise safely and communicate effectively on the water.

In an upcoming article, we'll delve into the major restructuring of the Coast Guard's coastal facilities. The Rescue 21 initiative, which aims to take the "search" out of "search and rescue," is another step toward safer boating.