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Can you hear me now? Selecting the correct marine radio antenna

Can you hear me now? Selecting the correct marine radio antenna Can you hear me now? Selecting the correct marine radio antenna
By Dave Adams

Most mariners envision the VHF (very high frequency) marine radio as an invisible super-signal sender. It must be able to tell our fishing buddy, who is 20 feet away, that we caught more fish than they did. And in times of peril, it must communicate with a distant United States Coast Guard station. It must, also, - although only during certain atmospheric conditions - leap around the earth's curvature and tell someone that the 49ers are better than the Chargers, but not as good as the Raiders.

The broadcasting and receiving aspects of a VHF marine radio are amazing. The normal maximum range is about 20 miles. But signals might sometimes be heard at distances of 200 miles or more under "freak" conditions; this usually occurs during temperature inversions when a "duct" is formed between the earth and air layers at several thousand feet that are warmer, rather than colder, than conditions at the surface.

But the best radio (transmitter) is only as good as the antenna (receiver). Also, the transmitter-receiver combination can be severely handicapped, or even worthless, if it isn't connected to an efficient feedline (coaxial cable).

What type of antenna is best? Antenna construction usually consists of three main styles of electrical elements: those using cut lengths of coax cable, those using a simple brass radiator, and those using a more complicated copper and brass radiator. While all provide acceptable performance, the best antennas use brass or copper inside the fiberglass for maximum strength and durability.

If any part of the antenna's construction fails, however, efficiency is lost. For example, a cracked outer casing could expose the ground system, which along with the antenna length gives you the correct gain.

The gain of an antenna is its ability to increase effective radiated power; it is measured in decibels (dB).

The gain of an antenna translates to greater distance for transmission and reception. A gain of 6dB usually results in a performance increase of four times the radio's power output and eight times with a 9dB gain.

Higher gain antennas achieve greater range by "squeezing down" the radiation pattern; increasing signals at lower, desirable angles and decreasing the power that is wasted in upward, unproductive angles. But, as the boat rolls and pitches, an antenna with too much gain (too flat a radiation pattern) can cause the signal to go above the receiving antenna. The light beam from a flashlight is a good example; just under the flashlight it`s dark, but a few feet away, the light illuminates the object as it receives the beam.

What dB antenna is best? A good general rule for antenna selection is to use a 3 dB antenna for sailboats and either a 3 or 6 dB antenna for power boats. 9 dB antennas should be reserved for use on land.

Other factors can be equally important when purchasing an antenna; the tuned frequency of the antenna and its SWR (standing wave ratio). Ask if the antenna is tuned in accordance with the median frequency of usage, (i.e., VHF will be tuned for 156.8 MHz [megahertz]). The SWR ratio should approach 1:1.

The feedline (coaxial cable) is specially designed to transfer your radio signal to your antenna with as little power loss as possible. There is a loss of power that is proportional to length; coaxial cables are rated as so many "dB per foot". For shorts runs, up to 20 feet, RG-58 coaxial cable is satisfactory. For longer runs, RG-8 coaxial cable will conserve an appreciable amount of power for radiation by the antenna. Also, when installing the cable, avoid sharp bends, kinks, or strains.

The connection between your radio and antenna is critical to your VHF system's operation. After running the cable, the coaxial connector must be correctly installed because any interruption in continuity will cause power to be reflected back to the transmitter and lost. Marine-grade crimp type connectors, with the proper crimping tools, can be used. But soldering is best; use 60/40 rosin core solder.

While cell phones have become increasingly popular for onboard communications, a VHF radio is still the primary means for boaters to reach each other. And when your radio fails, or operates poorly, it can get very lonesome out there in a hurry.

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Shagster | Posted: March 11, 2003

Good basic intro into antennas for a first-time buyer.