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. This month we take a closer look at EMI.
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.
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.
Noise varies The noise that you may get from a failing alternator is quite different from that of an unshielded ethernet wire. EMI is electricity and has the same characteristics as other electricity, like voltage level and frequency. When an alternator fails, it can send 100-volt spikes across the ship’s grounding system and cause havoc with electronics. An ethernet wire may emit just one volt of noise, but at a radio frequency. This type of noise is more likely to interfere with other similar frequencies like video or radios. EMI can be elusive.
Start with the source All the rules of troubleshooting apply here. Be scientific, vary just one variable at a time. Duplicate the problem before you start. Understand how it should work, so you can tell if it does work. Regardless of the situation, we always follow the basics. Identifying the source of EMI is where we start. The process of elimination applies here. Duplicate the problem, then turn things off one by one until the noise disappears. Conversely, turn things on one by one until the problem shows up, which confirms the culprit. This method usually identifies the offending piece.
Once we know the source of the EMI, the next step is to identify how it gets to its victim. It may be radiated directly to the unit, it may be conducted to the unit through its cabling, or it may be radiated to the cabling and subsequently conducted to the unit. Identifying the mode of transmission is an important step to understanding and resolving noise problems. This is where things get tough. Let’s say your VHF radio has a noise problem and you unplug the antenna and the problem disappears. One could assume a bad antenna or poorly made coax connection and be right, but it could also be a wire routing problem with the new dimmer switch and LED lighting in the vicinity of the antenna or its wiring.
Sometimes experience is the only solution to noise problems. From defective batteries to spinning shafts, noise can come from many places. The AMEI (Advanced Marine Electronics Installer) class from the NMEA covers EMI. One thing that helps with noise is a properly installed device. Most products that run on 12VDC have a connection to the positive and negative battery supply plus a connection to RF ground. The RF ground connection is the wing nut on the back the unit or the shield from the unit’s power cord. The purpose of this connection is to provide a low impedance path to the water for noise dissipation. If these connections are not made or if the boat lacks an RF ground, it may be impossible to solve certain problems. The NMEA MEI class teaches a whole section on grounds and grounding.
Solutions So how do we solve EMI problems? As mentioned above, improving the quality of the installation, ensuring proper grounding is number one. Further mitigation is possible using ferric rings or ferrites. These circular magnets are constructed to change the impedance of the cables going through them, shunting frequencies above a certain level to ground. Placing a ferrite on an antenna line or transducer line would be wrong, since these cables contain our desirable electromagnetic energy. Ferrites on power cables can prevent high-frequency (RF) noise from being conducted into a noise victim.
Ground loops are a common thing. When we ground things, natural loop pathways for electricity are inevitably formed. Not all ground loops are created equal, and sometimes a ground loop can cause noise problems. Ground loops have an impedance and noise has a frequency. If they match, we get gain, and noise can manifest itself. Since grounding and ground loops are beyond the scope of this article, suffice it to say that sometimes removing the RF ground connection may help.
Fishfinders susceptible Noise manifests itself most obviously in audio devices, causing distortion and buzzing and it shows up in video displays, causing lines or smearing. Fishfinders are particularly susceptible. Noise can also cause intermittent data problems or other seemingly mysterious symptoms. Using a portable AM radio to sniff for noise can be quite effective. An oscilloscope or spectrum analyzer are great tools to see noise. If you know where the noise is coming from, always mitigate the source as opposed to protecting the victim. Removing the offending unit is a good idea. Following best practices with tight connections and proper grounding and using quality marine grade equipment are the best trick all!
