Sleuthing out electromagnetic interference
4/26/2021

Unwanted lines or blurry images on your fishfinder or maybe some type of distortion on the VHF may point to a common occurrence called EMI. Marine electronics dealer and NMEA technical instructor John Barry wrote a primer for MEJ that lays out some basics of identifying the source and eliminating the problem. Below are excerpts.

By John Barry

Electromagnetic Interference or EMI is defined as electromagnetic energy that causes interference. Since electromagnetic energy is the foundation of all that we do, we must distinguish between wanted and unwanted electricity. When unwanted electrical energy interferes with the proper operation of equipment we call it noise or EMI. Problems with EMI are still a factor on many boats.

Two years ago, a Notice to Mariners was issued by the US Coast Guard to warn about light dimmers causing interference in VHF radios To detect the EMI, this notice recommended setting the VHF on the edge of breaking squelch and then turning on the lights. If squelch broke, this indicates an EMI problem with the lights. This rudimentary method can be done by novices and serves as a good warning. Light dimmers are often the culprit for this type of EMI noise problem which gets worse with more dimming.

Noise has many sources

A deeper look at noise problems allows for troubleshooting this difficult problem. There are many types of noise and many ways that noise manifests itself on boats. EMI noise has certain characteristics depending on where it comes from. Boats have different characteristics that may improve or worsen the environment for noise to spread. Basically, noise is unavoidable and can be conducted or radiated or both. Decreasing the noise to a level that makes it unharmful is the objective.

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