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Deciding on an antenna can quickly become a morass of technical jargon – overwhelming even the most knowledgeable boater. But considering that a marine radio is only as good as the antenna that supports it, it's important to understand what you are buying. Below, is a simple overview of the main points that boaters should consider when purchasing a marine antenna.
When it comes to choosing your antenna – mounting height is THE most important factor in getting the greatest signal range. Since VHF radios operate with a line of site principal – meaning the signal won't transmit through or around buildings or other natural obstructions – it's important to mount the antenna as high as possible on the boat. The higher you can put your antenna – the further your signal will carry.
As a general rule, sailboats have a 3' - 5' antenna mounted at the masthead. Even with their short length and low gain, the height advantage of the masthead mount gives these antennas great range.
Most powerboats from 16' to 25' in length use a “standard” 8' antenna, while bigger vessels have the option of larger antennas with more gain. Be sure to allow room to lay the antenna down, if necessary, for clearing low bridges or covered moorages. Your mounting arrangement should also be very sturdy to avoid damage to both boat and antenna when underway in rough seas.
After placing the antenna as high as practical, the next way to increase the range of your radio installation is to increase the gain of your antenna. Gain is measured in decibels (dB) and, generally speaking, the higher the gain, the greater the range. Marine antennas don't radiate energy equally in all directions – the signal “beam” is concentrated in the horizontal plane, parallel to the water. Gain can be thought of as a measure of the “focus” of this beam and its available energy. If the focus is narrow (high dB), like a lighthouse, the energy travels further. However, a narrowly focused signal beam may rise above or fall below the intended target in heavy seas causing signal loss or fading. Although lower gain antennas have a less focused signal and a shorter range, boats that roll or heel significantly often install them to ensure that even when heeled or rolling, some portion of the signal beam always points toward the target antenna.
9 db 3 db 6 db
Typical Radiation Patterns
(Radiating antenna is at center; horizontal axis of diagram is parallel to the surface of the water; boat is not heeled.)
Increased gain ratings almost always mean taller antennas. The structural integrity of the mounting installation and the antenna itself must be balanced against the quest for greater range. Sailboats and small lightweight powerboats, which roll excessively in heavy seas, normally do not use long antennas with gain ratings above 6dB. The more stable the platform of a vessel, the higher the gain that can effectively be used.
There are many styles of mounts available to suit your particular application and support needs. If you are using a two-piece antenna system greater than 10' in length, an upper support clamp is necessary. For antennas 14' to 18', the upper clamp should be 3' to 5' from the bottom. For antennas of 18' to 23', the upper clamp should be 4' to 8' from the bottom. Positioning the upper clamp too high or too low can significantly increase the potential for structural failure, so it's important to get this right.
The cable you choose can make or break your VHF system. There is nothing more frustrating than having a great radio and a great antenna, only to have terrible range because you choose the wrong type of cable for the length of your cable run. RG-58 cable is sufficient for cable runs up to 20'. For runs over 20', larger and better, low loss RG-8X, or RG-213 are preferable.
For optimum performance, the cable should be kept as short as possible and the connections should be high quality and as few in number as possible. It's imperative that if you have to make a new connection, you use high quality fittings and install them correctly. A poor connection can lower your transmitting power by a factor of 10 or more, so take the time to do this right.
While phased 1/2-wave collinear elements are the standard in high performance, high gain antennas, collinear phased 5/8-wave antennas, such as those made by Shakespeare, offer an increased level of performance by concentrating the radiated signal closer to the horizon, allowing for minimized fading and increased range, even during excessive vessel roll in turbulent seas. This is something you should look for when selecting an antenna.
So which antenna should you choose? Fisheries Supply features Shakespeare antennas as they represent the highest quality and most advanced antenna technology on the market. When looking at Shakespeare antennas, we recommend you consider a Good/Better/Best concept, and buy the best antenna your budget can afford.
We hope this has helped you choose the right antenna for your boat to make your VHF radio installation the best it can be. But if you still have questions or need more help, feel free to contact our product experts at (800) 426-6930.