How do I choose a sonar unit?
Response by Dave A. - Date unknown

       The scope of this article is to explain the various factors that are important when deciding on which sonar unit will best serve your needs. This article will not address concerns of how to use or interpret the sonar readings.

         The five main criteria that need to be considered when selecting the right sonar are:

  1. Cone angle
  2. Power
  3. Frequency
  4. Resolution
  5. Software and adjustments

Cone Angle
    The first concern, cone angle, is usually not a concern of most anglers. Few anglers I have talked to have really understood how cone angle impacts what they will (or wont) see. Cone angles on todays sonar units generally range from a low angle of 8 degrees to a high angle of 60 degrees. Some units have dual transducers each with a different cone angle, some use fancy computing to simulate a wider angle than the cone really has.

       What is really boils down to is this: The larger the cone angle, the more bottom area you will see. For sonars with an 8-10 degree cone angle you can compute the bottom area that is seen by the sonar by dividing the depth by 6. If you are in 60 feet of water, these sonars see a 10 foot area on the bottom and about 5 feet in the middle of the water column at 30 feet. For sonars with an 18-21 degree cone angle divide the depth by 3. If you are in 60 feet of water, these sonars see a 20 foot area on bottom and a 10 foot area in the middle of the water column at 30 feet. Wide angle sonars sporting a 45-60 degree cone angle see an area equal to the depth. In 60 feet of water, the sonar sees 60 feet of bottom area and 30 feet in the middle of the water column at 30 feet.

       When choosing which cone angle is right for you, it is important to consider the depth you spend most of your time in and what you will use your sonar for. If you spend most of your time trolling deep water, you will want a narrow cone angle. Wider angles will show you more information than you want to see. You might mark fish on your sonar, but they could be 50 feet or more away. If you spend most of your time in 20 feet of water or less, you should select a wide angle transducer or you will not see enough to easily understand the bottom contour or detect the presence of fish. With a 9 degree cone angle in 15 feet of water you will be viewing an area that is smaller in diameter than your boat is wide. If you fish a variety of depths you might consider a medium cone angle model as a compromise or you could select a model that offers multiple cone angles that can be selected from the unit.

    The amount of power your sonar unit has is another consideration. Power for a sonar unit is generally expressed as peak to peak power. Power refers to the strength with which the transducer sends the sonar ping. Sensitivity goes along with power. The transducer must be sensitive enough to hear the return ping echoing from objects it encounters while being able to handle the power being returned to it. Power can be determined by reading the box, sensitivity is usually a marketing claim. Thankfully, most modern sonars are just fine.

    Normally you dont get much of a choice of frequency when selecting a sonar. There are a few frequencies in use, but the most are near 200KHz or near 50Khz. We will refer to these as high and low frequency respectively. The lower the frequency, the longer the signal lasts. The higher the frequency, the more waves there are per second. To get an accurate reading while moving at speed, a high frequency is necessary. To detect very small objects a high frequency is also desireable. To probe deep depths accurately, a low frequency is desireable. A high power, high frequency sonar cannot see as deep as a low power, low frequency unit can. Now, lets make this whole business of power and frequency easy. Take whatever the manufacturer says the max depth for the sonar is and divide it in half. This is the depth at which you will be able to see gamefish. Take the max depth and divide it by 4. This is the max depth you will be able to see larger baitfish. The max depth a sonar is rated for is only meaningful for seeing bottom, not individual objects.

    Resolution refers to the number of pixels the units display has in both the vertical and horizontal dimensions. Deciding which sonar to buy is easy when it comes to resolution. If you are looking at 2 otherwise identical units, get the one with the highest resolution. If a unit can display 128 pixels vertically, that means if you are in 60 feet of water (720 inches)each pixel on the screen represents 6 inches of water(or fish). It also means that if a fish is less than 6 inches off the bottom, you cant see it on your display no matter how powerful and sensitive your unit is. It will be shown as bottom.

Software and adjustments
    So far we have discussed power, frequency, cone angle, sensitivity, and resolution. These make up the mechanics of the sonar unit. Behind the mechanical elements lies software. The software can make or break a sonar unit.

       Some sonar units have a feature that places a fish icon on the display whenever it detects a sonar signature it interprets as being a fish. This feature is indispensible if your sonars resolution is incapable of displaying the true sonar echo of the fish on the display. Most units are powerful and sensitive enough to detect fish that cannot be displayed in the limited room the screen offers. Suppose you are in 40 feet of water and a big fish is suspended 10 feet off the bottom. If you have a low resolution unit that displays 128 vertical pixels then each pixel is representing 4 inches. If the bass is 6 inches tall by 18 inches long, then only 3-4 pixels will be used to display the fish (horizontal resolution is usually less than or equal to vertical resolution). This is not nearly enough to put the distinctive fish arch on the screen. The fish will look like noise in the display. The problem gets worse as you get into deeper water. While this feature is wonderful in identifying fish once your screen resolutions limits have been reached, you will lose a lot of detail when this feature is on. You will not see baitfish, thermoclines etc.

       Another feature is the ability to select the water depth range you want to see on the display. You may be in 40 feet of water, but perhaps you notice that most of the fish are down 20-30 feet. Some units will allow you to omit displaying the area above and/or below this zone. This increases the number of pixels that can be used to display fish. In other words, you tell your sonar not to waste valuable pixels displaying empty water. Some units allow you to control the range that is displayed, most have factory presets that focus on the common fish holding ranges.

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