How to Choose a VHF Antenna and a Sonar Transducer
At first blush, one story combining discussions of VHF antennas and sonar transducers might seem like a stretch. But these marine electronics accessories share more in common than you might initially think. Both are used to transmit and receive. VHF antennas do that with radio waves over the air, while transducers use sound waves through the water.
Just as importantly, both antennas and transducers prove indispensable to the proper operation of their connected equipment. Neither a fixed-mount VHF radio nor a sonar unit can perform its functions without a matching antenna or transducer. You can also apply the adage that these electronics work only as well as the antennas or transducers connected to them. That is, both accessories can greatly influence the performance of and your satisfaction with the radio or sonar itself.
What’s more, VHF antennas and transducers are often sold separately from the actual electronics units, leaving the boater to research and decide which antenna or transducer to buy.
Don’t spare the dollars when it comes to a VHF antenna; buy the best-quality model you can afford. Manufacturers such as AirWave, Digital, Glomex and Shakespeare offer antenna models at a variety of price points to suit different budgets.
The benefits of high-end models include more-durable components, an improved signal-to-strength ratio for more-efficient broadcasting, and thicker fiberglass outer sheaths for longer service life. Within each price range, various types of VHF antennas can accommodate different boat styles and communication needs.
Go the Distance
A major measure of performance for antennas is distance—how far you can reach and communicate with another VHF. Since the Federal Communications Commission limits fixed-mount radios to 25 watts, the antenna serves as a pivotal factor in determining distance. The taller the antenna, the greater the communication range. That’s because VHF waves travel via line of sight. A taller antenna can see farther over the horizon.
Also consider the gain. Measured in decibels (dB), antenna gain represents the VHF transmission pattern, and it too can affect range. Antennas vary from 3 dB for a 4-foot model to 9 dB for a 12-to-23-foot antenna. The higher the dB, the greater the range, but with a caveat: Higher gains broadcast at a narrower pattern than lower gains. So as a boat rolls and pitches, as smaller boats are more prone to doing in rough seas, the higher gains intermittently angle downward into the water or upward into the sky rather than toward the horizon. This reduces range.
As a consequence, small-craft owners should opt for medium gain. An 8-foot, 6 dB antenna represents the most popular compromise on 20-to-35-foot boats. That antenna offers less range than a 9 dB version, but it’s also less susceptible to the effects of pitch and roll.
Can I DIY?
Optimal performance for VHF antennas hinges on proper installation. As discussed earlier, height can increase the range of VHF communication. That’s why boaters often mount antennas on T-tops or towers.
Because antennas extend the vessel’s height—and affect its bridge clearance—boaters secure them to ratcheting bases. Loosening the ratchet handle easily swings the antenna downward to clear obstructions on the water or on the road while trailering the boat to and from a launch ramp.
Recently, a new motorized VHF antenna mount hit the market. Taco Marine’s electric mount raises and lowers an antenna at the push of a button, eliminating the need to climb on the hardtop to manually perform this task.
Boaters should also consider quality when buying a sonar transducer. Marine electronics brands such as Furuno, Garmin, Humminbird, Lowrance, Raymarine and Simrad offer transducers at a wide range of prices; higher-priced models deliver greater size and power.
For example, when shopping for a through-hull chirp medium-frequency transducer for a Garmin fish finder, you can opt for the Airmar B150M (sold through Garmin for $449.99) with 300 watts of power. Or you can choose the Airmar B175M (sold by Garmin for $1,199.99) with 1,000 watts of power and larger elements. While the price for the latter is more than double that of the former, the greater investment in power buys you greater satisfaction when it comes to finding bottom structures and fish.
Depth or Distance
Sonar specifications usually report functionality in terms of depth, but in reality, these sonars measure distance—the distance from the transducer to the bottom, to a fish or to another object. Unlike VHFs, sonars can vary in power output; the greater the power, the greater the distance (depth) it can read.
Most recreational-boat sonar units max out at 1,000 watts. To utilize that power, the unit must match with a transducer delivering the same power rating. Otherwise, the sonar scales back power to match the transducer.
The frequency of the transducer also helps determine how far down (or out, in the case of scanning sonar) it can collect data. The lower the frequency, such as 50 kHz, the farther or deeper a sonar can read. But a lower frequency delivers less display resolution. Higher-frequency signals, such as 200 kHz, can’t reach as far but offer more detail and resolution.
Finding the proper installation location proves even more critical for transducers than antennas. Transducers need to send and receive signals that are free of interference caused by the aeration of a boat hull at speed. Air of any kind under the transducer face blocks signals. Hull type, propulsion and transducer-mounting type should all be considered when choosing the best location.
For example, through-hull and in-hull transducers on non-stepped outboard and sterndrive boats usually work best when mounted as close to the centerline and as far aft as feasible. That positions the transducer face as deep in the water as the hull allows. In addition, installers should avoid positioning a transducer directly abaft hull strakes, through-hull fittings and other objects that can generate aerated water.
Stepped hulls present a quandary, since steps are designed to generate a thin layer of aeration under the running surface from amidships to the transom while underway. On these vessels, install through-hull and in-hull transducers just forward of the steps.
On an inboard boat, situate a through-hull or in-hull transducer forward of the running gear. Propellers, rudders, prop shafts and struts create aeration under the aft portion of the hull.
If you want help ensuring you buy and properly install the right transducer or antenna, look for qualified technicians through local electronics and marine dealers.
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