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NMEA Boater Blog

Getting rid of EMI

Last April, The Mic ran an article by marine electronics dealer John Barry about EMI—Electromagnetic Interference. EMI is the culprit that is often responsible for otherwise unexplained distortion on your VHF or sounder. The article below follows up on that discussion with advice on what to look for and some simple solutions for eliminating it.

By John Barry

Despite tight manufacturing practices and high-tech circuitry, EMI continues to rear its ugly head—a recent spaceflight was cancelled because of EMI. Through the years, EMI problems have been solved by superior shielding, noise-tolerant devices and software error corrections. As man gets better at solving problems, the problems evolve. When it comes to EMI, there are some old problems and some new ones.

Of course, electricity hasn’t changed. The same rules hold for grounds and conductors. When it comes to noise (EMI) on boats, we need to get rid of the noise that exists or is created on board the vessel. We use the water to do this, sometimes called Earth Ground, but properly referred to as RF (Radio Frequency) Ground. The connection to the water, whether through an RF grounding plate or through the engine block, is our drain for noise.

We know that NMEA’s installer training teaches about several "grounds,” which we’ll address grounding in more detail in an upcoming article. For EMI mitigation, we provide a low impedance path to the water (RF Ground) for noise to dissipate. Low impedance means fat wire (8 gauge or copper strap). Trying to drain noise through an 18-gauge wire only creates an antenna for the noise!

Induction can be a problem

EMI is a natural phenomenon. When we connect electrical energy to a wire, current flows through the wire to the load. A magnetic field is the natural result of current flowing in a wire. The magnetic field around the wire can be picked up by nearby wiring or devices. This is called induction and it’s the characteristic of electricity that makes a transformer work. The primary and secondary windings in a transformer do not physically touch. Rather, a voltage is induced through a magnetic field and voltage develops on the secondary without touching. This same induction can cause wiring to cross talk, where a signal from one wire appears on another.

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