Antenna Gain and VHF Transmission Range

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Recreational marine VHF antennas are usually broken down into three categories: 3- and 4-foot sailboat antennas (3dB gain), 8-foot powerboat antennas (6 dB gain) and 16-plus-foot, long-stick antennas (9+ dB gain) that are popular on larger, long-range craft. Antenna gain is a ratio related to an antennas effective radiated power (ERP) instead of a fixed quantitative value.

An antennas decibel rating can be thought of as the design of its transmit and receive footprint, or its radiation pattern. A 3dB gain antenna has a nice circular transmit and receive pattern, whereas a 6dB gain antenna operates in more of an oval vertical plane.

For every 3dB increase in an antennas gain rating, the effected radiated power of the antenna doubles. Thus, a 6dB gain, 8-foot VHF antenna has double the ERP of a 3dB gain sailboat antenna, and the elliptical shape of the signal radiation pattern of a 9dB gain long stick antenna has three times the ERP of the 3-foot sail antenna.

A 9dB gain antenna will communicate farther than a 3dB or 6dB gain antenna mounted at the same height abovedeck, on a calm day. But with the increase in antenna gain, attributed to the antennas elongated radiated beam pattern, comes a tradeoff of signal fading when in a rolling sea. Because the beam pattern of a 9dB gain antenna is elongated, when a vessel rolls, the service area of the 9dB gain antenna dips toward the waterline and effectively puts VHF communications out of range until the antenna is once again level with the horizon.

Communication in the 150Mhz marine VHF band is strictly line of sight. And although it is important to install an antenna that has as much gain as your particular vessel can physically and practically support, antenna height remains the single most important factor in effective transmission of VHF signals. A 3dB gain antenna on top of a sailboat mast will easily outperform a 6dB gain antenna on a center console powerboat.

To determine communication range expressed in miles, you take the square root of the height of the VHF antenna above the waterline and multiply it by a factor of (1.42). Do this same calculation for the shore station or other boat that you are trying to talk to and add the two numbers together to arrive at the maximum distance that the two radios can communicate.

Mounting an antenna higher on a boat will usually require a longer coaxial cable run, and a longer coaxial cable means increased signal loss. In typical installations (RG-58 coax), adding elevation will outweigh these signal losses. Ideally, however, you should select coaxial cable that keeps signal loss to less than 2.2 dB.

Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida.

2 COMMENTS

  1. Doesn’t the db gain of an antenna have more to do with transmitted and received signal strength rather than envelope patterns? A 3db gain doubling the sensitivity. A 6db gain doubling again and a 9db doubling again. Each increase in gain provides a stronger signal to work with. I do not believe the increased gain has much to do with the radiated direction of received and transmitted signals.

  2. As with everything else in life it’s a compromise – an aerial with gain in some directions will get that gain by being less sensitive in others, hence talking about the shapes where the aerial is more sensitive.

    A 3dB gain antenna will be twice as sensitive to signals (coming from the right direction) to a theoretical aerial that has the same sensitivity to signals coming from any direction. It’s like cupping your hands over your ears to hear a distant sound – if you cup you ears towards the sound your hearing is more sensitive in that direction than it would have been without cupping your hands, and less sensitive in other directions.

    In the same way an aerial with gain squeezes more transmit power in some directions that others. Imagine using a rolled up sheet of paper as a megaphone – your voice will be louder in front of the megaphone than it would have been without it, and and quieter behind, as the sound energy has been squeezed into a narrower beam.

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