Understanding Sonar Specifications


A depth finder / fish finder has three main parts; the transmitter, the receiver, and the display. Power output is only one of the factors in the operation of the unit. It measures how much “punch” a sonar transmitter has. Other factors, such as receiver sensitivity, sonar resolution, and display resolution have just as big of an effect on the performance of the unit. Many times these other factors are not specified so it makes it a little tough on you when your comparing different units on the store shelf.

Receiver sensitivity is the units ability to listen for the transmit bursts return to the transducer. The Vexilar FL series flashers, such as the FL-12 shown, listens for this return and coverts it into the pattern you see on the display. The strongest returns are shown as a red color. The weakest returns are displayed as green. Orange covers the area in-between.

Our finders have always been designed with moderate output power and very high receiver sensitivity. This gives us a good clean signal throughout the units depth ranges. Other manufacturers may use a different design which uses higher output power and lower receiver sensitivity. The end result is that both units have about the same performance at the same depth range.

Sonar resolution, or target separation, is the units ability to separate one target from another. For example, separating a fish from the bottom or one fish from another one close by. This is a function of the transmitters pulse width, or how long the transmit burst lasts. The shorter the burst the closer the sonar can separate two targets. The FL-12 series can separate targets within 3 inches or so on the 20′ depth range. The deeper the range the longer the transmit burst, and thus, the separation increases.

Display resolution defines how small of a line the units display can draw. This specification is the one that probably affects the user the most. The better the display resolution the more detail the display will have. For example, the FL-20 can draw 525 individual lines around the display. When the range is selected to the 20 foot scale each line represents less than 1/2 inch. As the range increases the display resolution decreases.

Don’t buy a unit based on its power output specification alone. Compare all specifications and look for the features that will affect you in the real world. Also, go more by what other people say about their units and how they like them and not by what the salesmen tell you. Most of our units are sold on the lake, not in the store.