Choosing a Transducer Beam Angle

Beam angle has a large effect on the performance of your depth finder. There is more to it than simply area of coverage. Most of ourFL-8 seunits are sold with a wide beam, 19 degree, transducer. There are several reasons for this, but it does not mean that this angle is the best for you. Most fisherman, and the stores they buy from, believe that wider is better. If you have more area of coverage then you should be able to see more fish, right? The answer is a definite maybe.

The correct beam angle to use depends entirely on what you are trying to see with your sonar. If you are fishing for suspended Crappies then you probably would be very pleased with the performance of the wide beam. However, if you were going after Walleye that are hanging right on the bottom along a steep drop-off you may be disappointed.

DEAD ZONE – Dead Zone is the area within the transducers cone of sound that is blind to you. The wider the beam angle the greater the possible dead zone. The sonar will mark bottom as the nearest distance it sees. If you are fishing over a slope it may see the high side of the slope, at the edge of the cone, and mark that as bottom. The fish that are hanging on the bottom in the center of the cone will be invisible to you because they are actually within the bottom signal on your depth finder. A narrower beam angle will reduce this effect.

POWER – Your depth finder puts out a constant amount of power. It does not matter where you have the gain level set. Gain simply controls how much you amplify the signal that is bounced off of the bottom. Therefore, a narrow beam transducer will appear to be much more powerful than a wide beam transducer. This is because you are putting that same amount of power into a smaller area. This can be an advantage if you are fishing in deep water or a detriment if you are fishing shallow. A narrow beam transducer can be overpowering in shallow water.Switching to Low Power (LP), or the use of anS-Cableon an FL-8se, will solve this problem, though.

This chart shows the difference in area of coverage for our various transducers. It is meant to give you a rough idea of what the diameter of the circle, in feet, on the bottom you are seeing at a specific depth. You must keep in mind that these are transducer specifications and, unless you have your gain set at the maximum level, you will actually be looking at a smaller area. This brings up another interesting point. Your gain control acts much like a variable cone angle control. The drawback is that when you turn your gain up high everything in the middle of the cone gets blown up to the point where you can’t see what’s on the edge anyway.

 

Area of Coverage

DEPTH 8 9 12 19 20
10 1.4 1.6 2.2 3.4 3.5
20 2.8 3.2 4.3 6.7 6.9
30 4.2 4.7 6.3 10.0 10.6
40 5.6 6.3 8.4 13.4 14.1
50 7.0 7.9 10.6 16.7 17.6
60 8.4 9.4 12.6 20.0 21.2
70 9.4 11.0 14.7 23.4 24.7
80 11.2 12.6 16.8 26.8 28.2
90 12.6 14.2 20.0 30.1 31.7
100 14.0 15.7 21.0 33.5 35.3
120 16.8 18.9 25.2 40.2 42.3
150 21.0 23.6 31.5 50.2 52.9

ACTUAL vs. SPECIFIED CONE ANGLE

Most people picture the cone of sound to be triangle shaped. This is true only for the specified cone angle. The actual cone of sound is shaped much like the drawing. As you can see there is a lot of area outside of the specified cone. You may or may not be able to see a target in this area. It depends on how well the target reflects the signal back to the depth finder. Good fisherman understand this and can actually identify schools of fish that are way off to the side of them. They look at the depth finder in the area beyond the bottom. If this area is normally clear, but suddenly a group signal appear, then its a good bet that there is something out there. Also, notice the side lobes of the actual cone of sound. This area is generally considered undesirable and a good transducer has minimal side lobes.

Choosing the correct beam angle is a difficult decision. That is why we have Dual Beam (9/19) and Tri-Beam (8 / 12 / 20)transducers. These have multiple cones in one transducer. There is a switch box built into the cord so that you can switch between the angles. This is a great transducer for the fisherman who covering a wide range of depths and fishing for multiple species.