This is episode 2 in our series of how to understand the technology and science behind sonars and fish finders. Be sure to subscribe and hit the notification bell so you will receive updates of new videos .
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In this video we are focusing on traditional, or 2D, sonar technology. This technology seems old school since we now have a lot of new tools to play with such as Down Imaging, Side Imaging, Livescope, Humminbird 360, and Livesight. However, traditional sonar provides us anglers a wealth of information that may not be seen on other scanning images.
Traditional sonar uses a radio wave, like all other sonars, to generate the image you see on your sonar display. This wave is emitted at an angle beneath the transducer, and the signal expands the further it travels in the water column. Therefore, the coverage area of traditional sonar looks like a cone, and is called the cone of coverage. The sonar sees less of an area near the transducer and a lot larger area on the bottom of the lake.
The lower the frequency you use on your traditional sonar, the large the coverage area. Most manufacturers, such as Garmin, Humminbird, Lowrance, and Ray Marine, use between 77-83 khz as their lower frequency on traditional sonar. Additionally, they typically emit this signal from the transducer at 60 degrees. If you know the beam angle and depth you are fishing you can use this formula to determine how wide across the coverage area is at the bottom of the lake. 2 x Depth x tan(Beam Angle/2). However, the rule of thumb for a 60 degree angle is the coverage area equals the depth you are fishing. Therefore, if you are fishing in 30 feet of water, the base of the sonar cone will be 30 feet across. That means an object right off the bottom on your sonar screen could be 15 feet away from the transducer in any direction.
Higher frequency traditional sonar is usually emitted at a smaller angler, typically around 20 degrees. The formula still applies but the rough estimate is your coverage area if you are using a 200 khz traditional sonar is your sonar sees 1/3 of your depth. So again, if you are fishing in 30 feet of water, then you sonar only sees a 10 foot wide circle on the bottom of the lake.
The benefits of using a lower frequency with traditional sonar is you can see a lot more of the water column. Fishing in shallow water this might be preferred. However, if you are fishing deeper then your sonar isn't as precise. Using the 30 feet example again, if you see a rock on the bottom in 30 feet of water with the 80 khz traditional sonar, that rock could be anywhere within a roughly 900 square foot circle. You will have to make a lot more casts to find that target.
With that said, you can see that higher frequency traditional 2D sonars are more precise since you don't see as much. Additionally, higher frequency sonars allow for more target separation on your screen and you can see more detail.
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You can run both sonars at the same time. This can help unlock the advantages of both. If you see an object on the lower frequency but not the higher then you know that object is between the two cones of coverage. You can regraph the area until you see the object on both sonars. When that happens you know that that object is pretty close to that transducer.
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Fish arches are created when a fish enters the cone of coverage. At this point the fish is the furthers away from the transducer. As the fish swims through the cone that radio wave has less distance to travel back to the transducer to that fish appears to rise slightly on your screen. As the fish exits the cone of coverage it is again the furthest away from the transducer.