What’s the frequency?
  |  First Published: November 2014

Understanding the different frequencies your sounder is capable of using can make a great deal of difference to what you see on your screen.

The frequency that I’m referring to is the number that is shown on your screen in kilohertz (kHz). This is frequency of the sound pulses that are sent through the water from your transducer. When these sound pulses are sent and hit an object, they are returned to the transducer and an image is then displayed on the sounder screen.

There are a various frequencies out there that sounders can use. Generally most sounders with a basic off-the-shelf transducer will have two frequencies – 83kHz and 200kHz. For sounders that are designed for more offshore and deep water work, they have the option of the 50kHz and 200kHz transducer. There are also high frequencies that are used in Lowrance Structure Scan for giving us picture-like images, these come in 455kHz and 800kHz. If you aren’t already confused, we also have CHIRP, which is basically a variable range of frequencies that are sent (as opposed to a single number). I won’t go into too much detail about CHIRP just yet, it’s quite a complex topic and I think it deserves a whole article for itself!

So what’s the point of different frequencies? The best way to explain the different sonar frequencies is in Fig.4. The lower frequencies such as 83kHz and 50kHz will have a wider cone angle, we can see from the image that the fish A, B and C are picked up with this beam (in the green cone). Compare that to the 200kHz (pink cone) and we are only picking up fish B.

Generally the narrow cone of the 200kHz will give us better detail and better target separation. It also shows us what is directly under due to the smaller cone. The lower frequencies and wider cone of the 50kHz or 83kHz will give us more information as it covers a greater area. The 83kHz is great for shallow to medium depths and the 50kHz is great for deep water, as it is able to see down in over 200m depths.

Fig. 5 shows a top view of the cones, with the Structure Scan frequencies of 455kHz and 800kHz added. The red being the Down Scan and the yellow the Side Scan. The Side Scan will pick up fish that are wider than our conventional cones and the Down Scan will tell us what is directly beneath, but with much greater definition than the normal sonar cones. These are very narrow beams but give us amazing quality images due to such a high frequency.

When selecting frequencies for Structure Scan, use 800kHz when in shallow water (less than 5m deep) and then switch to 455kHz for anything deeper. When selecting between 83kHz and 200kHz go with the 200kHz in shallow water and move to the 83kHz when in water deeper than 30-40m. If you have the option to use 50kHz, this is best for water depths around 100m and beyond.

It can all vary depending on your own set up so it pays to play around with it and see which one works better for you and gives you an image that you can understand. As long as you understand the basics of how the sonar works and what the different frequencies are capable of, you will be able to correctly pick the right frequency for your fishing scenarios.

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