The first and foremost aspect to remember when it comes to choosing an antenna is that the quality, range, and capability of radio it's going to be serving will largely depend on the specs and details of the antenna. Even the most expensive radio will perform poorly with a second rate antenna, which while saving you money up front will result in frustrating low-quality performance later on.
Let's start with some basic quickies you should absolutely be looking for when shopping for an antenna.
The length of your antenna is of utmost importance when looking at getting the greatest range. The point actually is the height and upward reach of the antenna. So, in addition to getting a tall antenna, be sure to place it at the highest point possible on your boat. Case in point: an antenna at the top of a sailboat mast will broadcast farther than the same antenna mounted on a cabin side four feet above the water’s surface.
Sailboats. Sailboats will typically have a 3'-to-5' antenna mounted on the masthead. Some racing enthusiasts opt for an 8' antenna mounted on the stern. Either is acceptable, and the decision comes down to personal preference.
Powerboats. Power boats in the 16'-to-25' range should be using an 8' antenna. Larger vessels can afford larger antennas with more gain, but be sure to allow enough lay down room for clearing low bridges. In all cases, use a solid mounting arrangement to avoid damage to both, boat and antenna.
Something to keep in mind: the longer the wire run between the antenna and radio, the more signal loss you suffer. Make sure to choose the appropriate cable for the distance to minimize signal loss.
Gain is used to measure the transmitting distance of any antenna. A longer antenna mounted at equivalent height can have higher gain. The higher "gain" means a more focused area of transmission, or tighter beam, resulting in longer transmissions and less wasted power. We tell one antenna from another by its ERP (Effective Radiating Power), expressed in decibels (dB). Marine antennas most commonly rate as 3, 6 or 9dB, with the rule of thumb being that the higher the gain, the longer the range.
When choosing which gain you need, your vessel type should be taken into consideration, since the width of the transmission beam narrows as the gain increases. A 9dB antenna isn't going to justice to a 26-foot center console fishing boat that pitches, yaws and rolls, since the radio signal will fade as the antenna pirouettes in rough waters. Larger, more stable vessels can take advantage of 9dB or even higher gain antennas.
SWR (Standing Wave Ratio) rates exactly how much of the transmission signal will actually reach its destination. The term’s numeric representation is a ratio of numbers, and is indicative of how much of the signal actually gets radiated by the antenna. 1:1 would be perfect, while 3:1, 4:1 is telling you of high-reflected or wasted energy absorbed into the transmission media such as antenna mechanics, cable or connectors. "Wasted energy" are signals not reaching the final destination, resulting in poor transmission or reception of signals.
While you’re unlikely to find an antenna rated at 1:1, the industry tries hard to keep SWR under 1.5:1. Your goal as a buyer is to search for an antenna with as close to a 1:1 SWR rating as possible.
Another factor to look at is the shape of your transmission. With each different dB rating comes a different shape broadcast beam, some rounder and some more elongated. Since these beams can exit the antenna in many different directions, you should be looking for an antenna with a low angle of radiation. The lower the radiation angle, the closer to straight out to the horizon your signal will travel.
Power tends to get lost from the radio-signal along the way to the antenna. Use the largest 95% shielded coax that fits to keep the loss at an utmost minimum. PVC-coated (not plastic) cable RG-58 and RG-8X, frequently supplied with the antenna, are fine for cable runs up to 25'. For longer runs, use low-loss cables such as RG-8 or RG-213. (The RG-8U type is not recommended for marine applications because it is foam-filled and will collect moisture.)
Where NOT to Mount the Antenna
Antennas are good at what they're doing as long as you leave them and their signals alone. Fussy when it comes to their surroundings, here are some considerations of not-so-friendly neighbors who might interfere with the your antenna's performance.
Try to keep fluorescent lights away from your antenna. Route the antenna 3 feet away from plasma TV screens, other radio antenna cables, depth sounder transducer, engines, and other antennas.