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Don Casey: VHF Basics

Boat Tech: Why you need a VHF

Foundation Findings: VHF Review - Jan. 1998

VHF Radios - updated September 2009
by Chuck Husick

When you are in trouble you need to attract someone's attention. In order to do this you need a properly installed marine VHF radio transceiver! The VHF radio should be the first piece of electronics you buy. Depending on your boat and where you use it your needs may be met with a hand-held VHF transceiver, but a fixed radio installation, plus a hand-held is a far better choice (regardless of how reliable radios are, two are far, far better than one when one MUST work).

A DSC radio must be "programmed" with the boat owner's MMSI- the Maritime Mobile Service Identity Number that can be obtained as a part of a radio station license or at no cost from BoatUS or others who have established the necessary protocol with the Coast Guard. Programming the MMSI into a new radio is not at all difficult, just follow the instructions to the letter.

Be sure to read and carefully follow the directions for entering your MMSI and be certain to verify that the number you have entered is complete and correct. The software in most radios will limit the number of times you can enter an MMSI and will prevent additional entries once the limit number of attempts has been reached. If this occurs contact the manufacturer to determine the method they use to unlock the radio to permit a re-entry of a corrected MMSI.

Although most VHF radios can be operated by an experienced mariner without reference to the owner's manual it WILL be necessary for even the most experienced master to read the book and PRACTICE with the DSC functions of the radio, especially the emergency, mayday, calling method. Although all DSC radios are equipped with a DISTRESS button or key (under a flip-up red cover) the steps needed to broadcast a distress call vary from one radio to another. Keep in mind that the life you save my be your own and everyone with you when an emergency arises.

Unless you buy a radio that has been on the shelf for a number of years, the radio will have some type of Digital Selective Calling (DSC) capability. DSC has been an important asset ever since its introduction and is now especially valuable since a DSC mayday call will instantly trigger a response from the Coast Guard's Rescue 21 system, now operational over a large part of our coastal boating waters.

A VHF radio will allow you to communicate with other radios within at least a 5 mile radius and frequently reach stations 25 or more miles distant. The VHF radio will put you in contact, either directly or through another station relaying your call, with people able to come to your assistance; the Coast Guard, Marine Police, commercial towing companies, marinas and other mariners. Marine VHF radios work as a party line, everyone who can be of assistance can listen in, offer suggestions and relay your requests onward to stations too far off for you to reach directly.

What should you look for when selecting a VHF radio from the numerous radios listed on this website? We recommend that you install a fixed mount Class "D" DSC radio, which will draw its operating power from the battery in the boat and will send and receive signals through an antenna mounted as high above the water as practical. Don't worry about the transmitter's power specification. All of these radios are rated to produce 25 watts of radio frequency energy when on the high power setting. By law, they must also provide a low power, 1 watt choice. Much of the time, the 1 watt setting will work just fine.

The specifications of greatest value in choosing a marine VHF set are those that describe the performance of the radio's receiver. Although many people think that the majority of the cost of the radio is in the transmitter, the receiver accounts for about 75% of the cost. The two main measures of receiver performance are sensitivity and selectivity. It is easy to build a very sensitive receiver. In fact, it is easy to build a too sensitive receiver. Virtually every radio on the market will provide more than sufficient sensitivity (typically 0.25ÁV). The most significant receiver performance measure is its ability to ensure you to hear only the signal you need to hear and to reject all of the interfering signals that crowd the airwaves. The greater the receiver's selectivity, the greater the cost. How much selectivity you need depends on where you usually use your boat. Three specifications define how well the receiver will do in ensuring that you hear what you need to hear; adjacent channel selectivity, intermodulation rejection ratio (IM) and spurious response rejection ratio. For a radio used in an unpopulated or radio interference free area ratings of at least 60db for each of these specifications will be satisfactory. If your boating takes place near a large city or in a busy port area you will want a receiver whose performance for these measures is at least 70 db. There may be a time when being able to clearly hear a weak signal may be vital to your safety of that of some other mariner. To ensure the best receiver performance buy a set whose specifications are 80 db for IM and 70 db for the other two measures.

Many of the other specifications are in the category of nice-to-have or bells-and-whistles. Most radios are capable of receiving every allowable NOAA weather channel. Radios will offer differing types of channel scanning, automatically listening to all or selected groups of channels and stopping the scan so you can hear what is being said on the first active channel found. Building the scanning capability into the radio is not at all expensive and in addition is usually of limited value. You will normally monitor channel 16, the calling and distress channel (or, in some designated areas, channel 9). Most of the time the last thing you want on your boat is a noisy radio blaring out the voices of others. The number of scanning tricks a radio can play is not a worthwhile measure of its worth.

As soon as you have your radio go to the BoatUS web site (BoatUS.com/mmsi) and log into the "Can You Hear Me?" tutorial. It will explain how Digital Selective Calling works and provide a wealth of information about installation and operation of the radio, both for routine and emergency calling. The BoatUS website will also make it easy for you to obtain the Maritime Mobile Service Identity number (MMSI) you will need in order to make your radio's digital selective calling functions work, at BoatUS.com/mmsi. Be sure to connect the radio to your GPS receiver, doing so will make the radio more valuable for all communications and might save your life in an emergency.





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