Sonar
In 1957, Carl Lowrance began marketing his original Red Box and Green Box. Using a technology developed for the military, sonar has since taught generations of anglers how to locate fish and how to interpret bottom features represented by various models.

Does sonar technology help anglers locate more fish? That depends on how well it’s understood and used. Without factoring in all the other elements covered in this book, a sonar display won’t get you very far—you’ll be too busy checking out unproductive parts of the lake and then puzzling over display results, to do much productive fishing. But used as an adjunct to what your eyes tell you about lake or river structure, weather, water temperature, and where fish are in relation to the Calendar Period, sonar units can fine-tune the task of locating largemouths.

Original sonars used a lighted wheel, called a flasher. Paper graphs soon followed, drawing intricate pictures of the bottom and fish. Many anglers found them difficult to operate and maintain, however. Yet avid structure fishermen recognized them as far more accurate than other methods.

Today’s sonar units have now come a long way, with greatly improved picture definition, thanks to new screen technologies. New electronic circuitry has improved the speed of readings, approaching “real time.” The latest generation includes new colored displays featuring TFT technologies (thin film transistors) that enhance screen clarity in bright sunlight. This technology is the same as in your flat-screen TV.

Transducers
For most anglers, transducers are those things mounted on the back of boats or placed somewhere in their hulls. They get blamed for poor images, poor high-speed performance, and no picture. But if you know how these small but important objects work and how to make them work better, they will become the objects of interest they deserve to be.

All transducers have one thing in common: They contain a crystal that vibrates in response to electrical current. The crystal converts that current to sound energy, which is emitted at a particular frequency and direction. In the case of fishing sonar, the operating frequency from 50 kHz to 400 kHz is aimed at the lake bottom. When the sound energy bounces back—whether from fish, or from bottom and other structure—it’s reconverted (transduced) to electrical energy. This travels to the locator head, where it’s displayed as an image. Screen images are displayed as cross-sections of depth, and objects that have impeded the sonar unit’s sound waves are displayed at their respective depths.

Discussions of transducers usually center on their frequency and cone angle (beam width). Most freshwater sonar units operate in the 50- to 200-kHz range. Every frequency within this spectrum offers advantages and disadvantages. Manufacturers choose frequencies to optimize particular performances and functions. It’s important to know what these are before choosing a unit.

Cone angle or beam width refers to the diameter of the three-dimensional cone of water covered by the unit’s sonar at a particular depth, which is usually referred to as the “half-power point,” or –3 dB. Think of the cone angle as an inverted sugar cone, with the point as the transducer. A narrow cone angle looks like a narrow ice cream cone, while a wide cone angle looks like a broad one. Depending on frequency, cone angles typically range from about 8 to 50 degrees. Usually the cone angle is narrower at high operating frequency and broader at low frequency. These are physical limits that design can’t overcome.

A Narrow Cone Angle

A cone angle of say, one of fewer than 20 degrees provides more accurate bottom detail with less coverage, while a wide cone angle displays a larger area with perhaps more targets. But those targets are spread out over a larger area, and it’s unclear exactly how close they are to your boat. When you’ve located a fish with a narrow-beam transducer, you know that it’s near or under your boat. Measured at –3 dB, an 8-degree transducer covers an area whose diameter is about 1/6 of water depth (scans a 3-foot circle in 18 feet of water); a 20-degree transducer covers an area whose diameter is about 1/3 of water depth (6-foot circle); a 38-degree transducer, one whose diameter is about 2/3 of water depth (12-foot circle).

The disadvantages of low-frequency systems are that they usually don’t work well in water less than 10 to 15 feet, and that they penetrate deeper water more weakly than high-frequency systems. But a narrow-beam transducer (say, 8 degrees) can concentrate sound energy and reach deep water. In several hundred feet of water, even narrow beams penetrate to significant depths. For example, deep-water anglers on the Great Lakes often prefer low-frequency units. High-frequency systems, however, usually offer better target separation.

Because there are advantages to both designs, a few manufacturers produce sonar units able to operate at dual frequencies. A common dual frequency transducer operates at 50/200 kHz. A second design offers a dual-beam transducer whose frequency stays the same but whose cone width can be set for either 9 or 18 degrees. Yet another design is built around transducers containing multiple crystals of the same frequency; each scans in a different direction—right, left, or center—thus creating a wider beam of coverage.

Transducer Power
Power determines how deeply sonar penetrates; a more powerful unit can send its signal deeper into water. For deep fishing, a 3,000-watt unit performs better than a 200-watt one. The latest units boast up to 8,000 watts peak-to-peak. But how much power do you need? Bottom hardness, fresh or salt water, plankton concentration, interference, and receiver sensitivity are factors affecting the depth to which a unit can penetrate.

Two aspects of sonar power should be factored into your decisions about how much sonar to buy: edge detection and target separation.

Edge detection—The strongest signal is along the axis of the transducer. On the edges of that sound cone, energy decreases. Powerful locators— sonar units— help detect targets at the edges of the cone better than less powerful ones.

Target separation—Another aspect of sonar power is a unit’s ability to detect and display objects like fish, rocks, and weeds. Greater power can drive sound waves to lake bottom in deeper water, making possible the separation of objects that are close together. Most units with midrange frequency can separate targets that are about 3 inches apart in shallow water. That is, if a fish is 4 inches above bottom, a midrange unit can show the fish and bottom as distinct objects. But if a fish lies 2 inches off bottom, fish and bottom are likely to blend into a single image. Similarly, if your boat is floating over three fish all the same distance from the transducer, even though they’re at different depths only one fish will be displayed. Target separation widens in deeper water, so the more powerful a unit is, the greater likelihood that it can bounce off bottom and separate objects in close proximity to each other.

How much edge and target separation to buy depends on where you do most of your fishing. If you rarely fish deeper than 30 feet and bottom there is relatively firm, 300 watts may be adequate for you. If you fish soft-bottomed lakes or deeper water, you’ll get more satisfaction from a 2,000- to 3,000-watt unit.