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It seems like the manufacturers of electronics want to keep us all in the dark. They have so many models with so many features and no way to compare apples to apples. They may tell you how many kilohertz that they operate, but what good is that if you don't know what that means? They may give you the cone angle, but don't tell you how to measure the angle, so you have no idea how much area is shown under that transducer. They may tell you the difference between a narrow and wide cone area, but fail to mention what degree they consider to be wide or narrow. They tell you how many watts of power the unit puts out, but they don't tell you how many watts you need. What about fish arcs? The picture on the boxes always have nice fish arcs, but most units fail to produce anything resembling what is on the box. What about resolution? How many pixels is enough? Is it worth paying double for twice the pixels? What about mapping and GPS/WAAS? Will it help to catch more fish? What about features? Go to Lowrance's website and you will see words like FlashGraf™, COLORLINE™, HyperScroll™, FasTrack™ ASP™, Advanced Fish Symbol I.D.™, and FishTrack™ just to name a few. Who has time to learn all those definitions? Who really cares who can create the most intelligent vocabulary?

Without getting technical, here is my understanding of how depth finders work. A depth finder powers a transducer that is located either on the back of the boat, the trolling motor, or it shoots through the hull. The transducer transmits sonar (SOund, NAvigation, and Ranging) waves in the shape of a cone. The waves get spaced out more as they go deeper. When the waves strike an object (fish, structure, bottom, etc.), they are reflected back to a receiver in the transducer. The signal is then sent to the unit and displayed for us to see. These waves are sent at high speeds so that they are displayed several times per second. Most depth finders will display each echo as a pixel on the right side of the screen and the chart will move to the left. The right side will show what is currently in the cone. What you see on the left has already passed through the cone, so it is just historical information.

Most units come in either high frequency (192-200kHz), low frequency (50kHz), or dual frequency (50/200kHz). This can be very confusing because cone angles can be considerably different at the same frequency. Narrow cones are better suited for deep water applications while wider cones are better for shallow water. If you are fishing for bass, you will probably want a high frequency unit with about a 60 degree cone. If you fish deep salt water, you may consider the low frequency or dual frequency unit.
With a 60 degree cone angle, you will see approximately 1.15 times the depth. For example, at 10 ft deep the cone will spread out to reach 11.5 feet across. At 20 ft deep, it would span a diameter of 23 ft across. A narrower cone angle of 30 degrees would show approximately 1/2 times the depth. At 20 ft, it would span a diameter of 10 ft. Conversely, a wider a cone angle of 90 degrees would show 2 times the depth. That would span a diameter of 40 ft at 20 ft deep. If you turn down the sensitivity, it will narrow the cone angle.

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