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What Causes Sonar Interference on a Garmin Chartplotter or Fishfinder?

Causes of sonar interference

As seen on a fishfinder, chartplotter, or display capable of showing sonar images, sonar interference will appear as if the screen is a blank, black screen, and the flasher seems to stop working or distorts the image. Electromagnetic Interference (EMI) near a sonar device typically causes interference. The EMI sources can include but are not limited to: trolling motor, AC/DC power cables, bilge pumps, macerators, magnetic sources, stereos, radar, VHF Radios, and RF cables. EMI can affect all types of data cables: Garmin Marine Network cables, J1939 cables, NMEA 2000 cables, NMEA 0183, sonar transducer cables, and RF coaxial cables.


EMI and EMF Interference

The most common cause of sonar interference is when the transducer cable or the transducer itself is too close to a source of EMI. When using an electric trolling motor, the user depresses the throttle on an electric trolling motor. The electrical current surges through the power cables toward the electric trolling motor; this power surge creates an Electromagnetic Field (EMF), which will encapsulate or surround the sonar transducer cables. The effect of the EMF will distort the data signal presented in the chartplotter. As the electric trolling motor powers up and RPMs increase, it also produces a second EMF, encapsulating the transducer and resulting in a distorted sonar image. The resulting EMI changes the sonar signal, and the user will see a distorted sonar image on the chartplotter. When the throttle is released, which reduces the power requirement, the EMF will collapse, and the sonar image may clear up.


Multiple transducers operating same frequencies

Sonar interference may occur when two transducers use the same frequencies. Or when near another boat using the same sonar frequencies. The transducers receive the sonar signals from the other transducer, but their timing is out of phase. The sonar returns are processed, but they are erroneous, resulting in distorted sonar imagery or data that will not be correct. Stop running one of the transducers, or change the frequency of one of the transducers.




To prevent or eliminate the source of EMI

Inspect the sonar or data cables for connections at the chartplotter and the cable connector for corrosion

  • Install a ferrite bead or ferrite choke on the transducer cable just before the transducer connects to the chartplotter, sonar module, or fishfinder

  • Run the transducer cable away from the EMI source

Run the transducer or data cable away from any source of EMI

Typically, six inches of separation between a power cable, a transducer, or a data cable may be adequate on small boats, but this depends on the voltage and current traveling through the power cables. Testing a sonar system or data cable, explicitly looking for interference, is recommended before final installation. If installation must run the transducer or data cable over or under the AC/DC power cable, position the wires at a 90-degree angle. The angle will aid in keeping the transducer or data cable out of the electrical phase of the AC/DC power source.


Use EMI tape as a barrier to protecting the EMI source

Verify the grounding of the trolling motor to the vessels' common ground. If not, the interference levels may be higher. Refer to the trolling motor installation and operating manual for specific instructions or call the OEM.


Finding and isolating the source of EMI

  1. Isolate the fishfinder, chartplotter, or sonar module so that only it is powered

  2. Turn on the chartplotter, select a sonar presentation, and adjust the gain level to Medium values

  3. Systematically turn on each electrical device to identify the source of the EMI. When turning on a device displays a distorted sonar image, the last device turned on may be the source of EMI

  4. Follow the sonar, data cables, and electrical wires from the suspected device, and if it comes close to the sonar or data cables, move the wire and recheck the system for interference





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