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Electronics

Choose the Right VHF Antenna

Loud and Clear

Get the right antenna for the job and mount it properly, and your voice will be heard.

Boaters need to take many things into consideration, but the type of boat and style of cruising they’re doing are critical factors when choosing a VHF antenna. Here are some suggestions.

Antenna gain  Sportfisherman ➤
  Offshore Cruiser ➤
  Center Console ➤
  Small Boats ➤

There’s an old radio operator’s saying that “every dollar on the antenna is worth a hundred on the radio.” While the currencies and ratios vary, the point is always the same: A little money spent on a good antenna will do more than a lot spent on a better radio.

But surely an antenna is just a bit of wire? Up to a point, that’s true. But designing a good antenna is a mixture of art and science—some math, some intuition, and some trial and error.

To see why, we should start at the back of the radio, where the antenna socket puts out AC electricity. It’s like your domestic power supply, but at lower voltage and much higher frequency—about 32 volts, and around 156,000,000 cycles per second (or 156 MHz). 

At the antenna, that alternating current pushes electrons up towards the tip, then pulls them back down. If the frequency of the push-pull process matches the speed at which the electrons can flow up and down the antenna, it will be efficient—like gently pushing a child on a swing. But if the antenna is too long or too short, it will be as counterproductive as trying to make the child swing faster by pushing forward before the swing has finished swinging back.

The optimum length of a VHF antenna is generally about 3 feet, but an antenna engineer can vary that by adding other electronic components such as capacitors and chokes, by bending or coiling it, or by stacking several short antennas in line with each other in a “collinear array.”

But why would they bother? The radiation pattern from a 3- to 4-foot antenna is roughly doughnut-shaped. There’s very little radiation from the ends of the antenna (creating the hole in the doughnut) but the doughnut itself is quite thick—meaning that if the antenna is vertical, a lot of energy is wasted by being transmitted away from the horizontal plane, up into the sky, or down toward the surface.

Longer antennas have more “gain.” There is the same amount of energy in the radiation doughnut, but the pattern is flattened to focus the energy towards the horizon. So while a 3-foot antenna might be described as 3dBi gain, with most of its energy transmitted within about 40 degrees of horizontal, an 8-foot version  typically has 6dBi gain, and directs most of its energy within about 20 degrees of horizontal, and a 16-foot model has 9dBi gain, focusing its energy within about ten degrees of horizontal. Fitting a 9dBi antenna instead of a 3dBi antenna could—entirely legally—quadruple the effective output of your radio and potentially double its range, so long as it’s within 10 degrees of truly vertical. But lean that antenna back to match the angle of your tower’s leg and you could be talking to the sky instead of your buddy at water level.

Boaters need to take many things into consideration, but the type of boat and style of cruising they’re doing are critical factors when choosing a VHF antenna. Here are some suggestions.  

This article originally appeared in the September 2012 issue of Power & Motoryacht magazine.

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