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Depth Finder, Sonar Interference

Ray's picture

By Ray - Posted on 27 December 2010

Hey guys here is a great article about sonar interference and ways to correct it. This is a good read if you currently have interference on your depth finder or not.

Ray's picture


Interference from Electric Trolling Motors

Whenever you use an electric trolling motor on the same boat as a depth finder, you are likely to see noise from the motor on the display of your sonar. Today's high-tech motors perform better than ever, but can be very unforgiving to other electronic devices in the area.


Conducted Interference

A Multi-Meter is a valuable tools for working with electrical systemsIf you disconnect your transducer from your depth finder and you continue to see noise when the trolling motor is running, then you are dealing with conducted interference coming in through the power connection. This can happen when the boat's electrical system does not have one common ground that connects to the water. Using a multi-meter, available at places like Radio Shack, check for proper grounding in your boat's electrical system. Set the meter to measure resistance or continuity. Then connect one lead to the negative battery post that connects to the trolling motor and the other lead to the negative post on your starting battery. The reading should show zero and the meter may beep. Also, check between the negative starting battery post and an unpainted portion of the outboard motor, like the prop shaft or trim tab. You should also measure a complete circuit here as well. If the ground connection is missing between the trolling motor battery and the starting battery, add a small gauge wire, about 18 AWG, with a 1 amp fuse. This will complete the ground connection here. If the ground to the outboard motor is missing you should take your boat into a service shop to see why the ground is missing. All outboards should have this ground. If yours is missing something is wrong. If all grounds are in place, but noise is still a problem, a Ferrite Core can solve the problem.


Electromagnetic Interference

Actual view of EMI signals from an electric trolling motorWhile you’re electric trolling motor is running, disconnect your transducer from the back of your unit. If the interference goes away, then you have EMI created by the rapidly switching voltage and powerful DC motor in the trolling motor's lower unit. Proper grounding and Ferrite Cores can be a good remedy for this type of interference. First, be sure your system is grounded properly as described above. Incorrect grounds are a common source of problems. Vexilar has even incorporated a very unique grounded puck transducer in our sonar systems that will insure the electrical system is grounded to the water at the point of the interference. This helps keep EMI under control, but may not eliminate it completely.

By knowing how to identify interference sources, you will be able to eliminate or greatly reduce the troublesome noise that can render your electronics worthless. It may take a little time and effort in some cases, but it will let you get the most out of your sonar in all conditions. 




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Soft plastic lures that are hollow inside the body and most often used on a jig head.

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