May 13, 2020
By Dr. Jason Halfen
A fishfinder is by far the most important difference-maker you have on your boat. No other piece of equipment has the singular ability to turn a bad day on the water into a good one, or to convert a fun trip into one of epic proportions.
Contemporary fishfinders all trace their roots to 1959 and the classic “green box”—the Lowrance Fish LO-K-TOR, which was the first commercial sonar unit to be marketed to sport anglers. In the modern age, marine electronics possess a fantastic, ever-expanding collection of features, helping anglers to find fish quickly, navigate to their destination safely, and enjoy their on-the-water experience more completely than ever before. Fishfinders have come a long way.
In my experience, however, the vast majority of anglers know little more than how to turn their fishfinder on and off, thus failing to leverage its most powerful features. Your fishfinder is an incredibly powerful tool capable of teaching you a dazzling array of secrets about your favorite lake, river or reservoir—if you’re willing to learn a few key facts about its operation.
SONAR by the Numbers
Fishfinders locate underwater objects by using SONAR. An electromechanical manifestation of the intricate biological echolocation system used by whales and dolphins, SONAR is an acronym that stands for SOund NAvigation and Ranging.
Using ceramic materials in its transducer, your fishfinder transmits sound energy into the water, and then listens for echoes of that transmitted sound. Each echo arises when the transmitted sound waves strike a subsurface object—like the bottom, structure or a fish—which reflects the sound through the water column and back toward the transducer. By measuring the time that elapses between the initial sound transmission and reception of the echo, and considering the speed of sound through water, your fishfinder can accurately determine each underwater object’s depth.
The frequency of the transmitted sound energy has important implications for your fishfinder’s performance. The benchmark SONAR frequency, dating all the way back to the green box, is 200 kHz. Depending on the specific capabilities of your fishfinder and its connected transducer, it may be possible to transmit other SONAR beams as well, including those with lower (50 or 83 kHz) or higher (455, 800 or 1200 kHz) frequencies.
Each transmitted SONAR frequency has its own associated depth penetration and resolution. In general terms, as the frequency of the sonar beam increases, resolution—also described as target separation—increases, while depth penetration decreases. This means low frequency SONAR, especially 50 and 83 kHz, is best for investigating very deep water, like you might find in the Great Lakes or the bluewater of the Gulf of Mexico; higher frequency SONAR, such as 800 and 1200 kHz, is more appropriate in the shallower environments of your favorite lake, river or reservoir. At the same time, super-high frequency SONAR beams, especially 1200 kHz (or 1.2 megahertz, MHz) provide the sharpest, highest-resolution, easiest-to-interpret images possible, while the lower frequency SONAR techniques provide “blob-o-grams” that are subject to guesswork.
For simplicity, I divide SONAR frequencies into two categories. Low-frequency beams—50, 83 and 200 kHz—have general utility in all of the typical depth ranges and are especially useful in very deep water. These frequencies provide traditional, low-resolution SONAR images that often require an experienced eye to interpret reliably. Higher-frequency SONAR beams—455, 800 and 1200 kHz—are associated with high-resolution imaging techniques, like Side Imaging and Down Imaging, and provide picture-like images that are generally much easier to interpret without guesswork. Keep in mind, however, these high-frequency techniques have a more limited useful range than the low-frequency SONAR beams, with their range becoming shorter as their frequency increases.
Coverage and Cone Angles
By understanding the strengths and weaknesses of the common SONAR frequencies, we can consider the coverage provided by each. Recognize that SONAR can be transmitted in different directions, interrogating distinct portions of the water column, for each of the available techniques. Low-frequency SONAR is typically directed straight down, allowing anglers to find structure and fish beneath the boat. Higher-frequency beams can be transmitted in a similar downward direction to provide higher-resolution images of objects below the boat, or these beams can be transmitted laterally to the sides of the boat, dramatically expanding the amount of information provided by the fishfinder by investigating lateral areas that down-looking SONAR cannot.
In addition to their different transmission directions, each SONAR beam also has a characteristic coverage area. For low-frequency SONAR, it’s useful to think about the beam covering a portion of the water column and the bottom that expands as the water gets deeper. This coverage area is similar in shape to an upside-down ice cream cone, one that is small toward the surface and much wider at the bottom. Low-frequency SONAR coverage areas are often described by their cone angles, where a larger angle covers more water and bottom than a smaller one.
In general terms, cone angles decrease as frequency increases; thus, an 83 kHz beam might have a cone angle of 60 degrees, while a 200 kHz beam often has a cone angle of 20 degrees. The wider of these two beams covers a circular area that is roughly equal in diameter to the water’s depth, while the narrow beam interrogates a circular area with a diameter close to one-third of the water’s depth. For example, in 30 feet of water, the 83 kHz beam covers approximately 30 feet of the bottom, while the 200 kHz beam investigates a narrower, 10-foot-wide portion.
High-frequency techniques based on 455, 800 and 1200 kHz SONAR are not transmitted with expanding cone-shaped beams. Rather, these SONAR waves propagate in beams that are wide from side-to-side but narrow from front-to-back. In this way, high-frequency SONAR beam shapes are similar to a slice of bread, rather than an ice cream cone, as those beams cut through the water.
Modern fishfinders are indeed powerful tools for pinpointing the locations of structure and fish. Select the best sonar frequency and coverage area for your particular fishing situation, and you’ll be well on your way to learning the secrets your fishfinder is trying to teach you.