VHF ANTENNAS FOR SMALL CRAFT
By Capt Ed Wiser for Coastal Angler, May 2013
The legend goes that “God made some men tall and some men small, but Colonel Colt made them all equal.” Alas, there is no Colonel Colt for VHF antennas. They come in various lengths, materials, broadcast arcs, and mounting types. There are good reasons for the diversity.
Antenna selection, location, and installation are critical factors for your safety and to get the most out of your VHF radio and must be carefully considered.
VHF radio signal is “line-of-sight”, meaning the signal travels in a direct line from the top of one antenna to the top of the other station’s antenna. Other factors being equal, the taller the antenna, the farther the signal can reach over the horizon. If a 3-foot antenna and an 8-foot antenna are mounted at the same height on a center console the taller antenna will give greater range.
Ironically, a 3-foot antenna mounted on the bridge of a 60-foot Hatteras may not have the range of the lower 8-foot antenna on the center console fishing boat. That is because there is more to consider than just length.
Antennas are rated by gain, measured in decibels (dB), 3, 6, 9 being the most common values. VHFs are limited to 25 watts broadcast power so the amount of power available from each radio is the same. How that power is projected is a matter of decibel rating. The lower the decibel rating the broader the arc of broadcast and the less focused the beam. Consider a beam of light from your searchlight. A 3-decibel beam is projected over a very large angle, is not focused, and hence much of its power gets shot into the stratosphere or the water. It is like using the “area” setting on your light. This limits range considerably. So why use it? 3-decibel antennas are the familiar stainless steel whips 3-feet long. They are easy to mount on a center console and still be under the bimini. They are inexpensive. They last for ages. They don’t snap when you trailer your boat under a tree. They are very suitable for sailboats because ragbaggers often cruise along at a list. They need a broad signal arc because a highly focused beam might shoot above or below the intended recipient. The reduction in range is partially mitigated by placing the antenna at the masthead.
Also important to sailboaters is the increased cable run from the VHF radio through the cabin, and up the mast. This very long run mandates a far heavier cable than found on most powerboats. Using powerboat coax cable can significantly reduce the signal strength before it even reaches the antenna.
The 8-foot long, 6-decibel antenna is the most common and for good reason. It focuses the broadcast beam much like the Fresnel lens in a lighthouse focuses the beam of light. The results are similar to the lighthouse as well. The broadcast is intensified but in a very narrow, flat arc. This gives greater intensity and range. Problem is it requires a relatively stable platform that can keep the antenna perpendicular to the surface of the water and is thus less suitable for sailboats. These antennas can be had for $60 and up depending upon quality of internal element and exterior finish. They are made of fiberglass or similar materials and are a good compromise between the short range stainless whip and the 9-dB type described below.
The 9-decibel units are well suited for sportfishermen, motor yachts, and commercial craft like tow boats. These are much longer than the 6-dB, generally 18-25 feet and come in two sections. Because of the narrowly focused beam the range and strength of signal is superior to the lower rated types. It does require a more stable platform to be of real value, however. A 9-dB antenna on a 20-foot boat is generally a waste of money, even if you can find a way to mount it. Same thing applies for a 3-foot whip on a flybridge yacht
Internal element and exterior finish are important considerations. Elements come in three types: simple coax cable, brass rod, or combination brass and copper. The copper-brass type is the best. Coax cable in a saltwater environment tends to absorb salt and corrode and signal strength will deteriorate with age. Finish goes far to slowing or eliminating internal corrosion and exterior “bloom.” Bloom is the decomposition of fiberglass fibers due to UV exposure and appears as a fuzzy, splinter-like coating on the antenna. When you see bloom it is time to replace the antenna immediately.
Range on a good installation depends on antenna height above the water, gain, and sea state. Rough seas may cause highly focused beams to give intermittent reception at extreme range. Generally, a 25-foot center console with a 6-dB fiberglass antenna communicating with a similar vessel can expect a range of about 10 miles for two-way conversation. Range will increase as antenna height increases on one or both stations. Weather broadcasts on those same boats may be received over a distance of 50 miles depending upon weather, quality of components and installation, and sea state.
Land interference also impacts range. I recently conducted an experiment aboard a 58′ Hatteras in Bimini. We had an older Standard Horizon VHF/DSC radio and an old 19-foot 9-dB antenna that was due for replacement. At the dock inside Bimini harbor we could not receive Miami weather because the central ridge of the island blocked the flat line-of-sight signal. When we moved into the open ocean the NOAA weather broadcast came in well though the station was over 50 nautical miles away.