Copied from the BBC:
High frequencies (shortwave), interference caused by nearby electrical impulses in electrical switches, motors, vehicle ignition circuits, computers, and other man-made sources tends to swamp transmissions with thermal noise. These noises are often referred to as static. In the marine industry Trolling Motors, Trim Motors, Bilge Pumps and other electrical devices can create static noise similar to the frequencies used by today’s electronic sonar systems with a broad range of listening abilities and frequencies.
Radio Frequency or Electrical Noise is a strange phenomenon and not everyone will get but there are solutions to reduce or filter the noise. I’ve personally seen 20 brand new boats built exactly alike and one had the phenomenon and the other 19 did not. Why did this one have it because the frequency was close enough the fishing system was able to detect the Radio Frequency Level.
First thing is to find the source of RF Noise by powering off all accessories except the sonar then beginning turning on accessories one at a time until the source can be located.
Here are some tips to help Prevent, Kill or Manage Radio Frequency/Electrical Noise.
1) Make sure the motor is properly grounded. Most trolling motor manufactures have internal grounding wire to help prevent RF Noise submission. If your motor is not properly grounded or the ground circuit is broken it will allow RF Noise into the system. On MinnKota Trolling Motors the late model motors have a fuse on the ground side in the trolling motor head. If this is damaged or broken this is the first step to a cure. Also make sure the foot is grounded to the head. If not a grounding wire can be run externally to the foot and secured to the foot. This can be done by using a hose clamp that mounts the transducer. Water acts as a natural grounding agent for a boat. Grounding from the foot to the negative side of the power supply may provide your solution to kill RF noise.
2) Adjust the Noise Filter on the unit. Most sonars have settings inside the unit to reduce noise or filter it from the readings it receives. In a Humminbird this in the Master Menu under Sonar Tab.
3) On some boats a wire between Battery where negative is hooked and negative side of the Cranking Battery may kill the noise. This provides a common ground for the network.
4) Never use the trolling motor battery to power sonar equipment this is an easy way to add RF Noise into your sonar system.
5) When running external transducer the transducer cable needs to be located away from the head unit and power cables as quickly as possible and Do Not run down the sheath of the steering cables and near the foot control.
6) Consult your Sonar Manufacturer they should have solutions to help keep RF Noise from entering the sonar system. Humminbird has taken a proactive approach and has Interference Kits available for those that have concerns with their product. Not everyone needs the kits but are available for those that experience RF Noise. The standard product packaging have incorporated many components like Nickel Plated Transducers, Power Cords with Ferrite Chokes built inline, Network Ferrite Chokes and more. If these components don’t solve your problem then the kit has additional components that are customized to your boat electrical set-up by equipment and motor type. Working with the Customer Service of your Sonar Manufacturer they should be able to provide solutions.
