Article by Capt. Steve Katz
Waves of Disruption: Navigating the Interference
On a boat, Radio Frequency Interference (RFI) is typically the presence of unwanted signals or electrical energy that detrimentally impacts a radio communication system. Interference can also impact non-voice communication systems such as fish finders, radars, Satellite TV etc.
The most recognized interference is often in an audio system on a boat, such as stereo or marine VHF communication radio. Interference in the stereo is the most common and annoying. Sometime the interference noise is only present when the engine is operating, or other times it appears only when certain lights are turned on.
How exactly is this interference getting into our equipment?
Conductive Coupling
Conductive coupling happens when two wires touch or are linked by a direct connection. This can lead to unwanted electrical signals traveling along the wires, to the effected device, causing interference. One example is the marine engine alternator – it is connected to the battery system, which is connected to most electronic devices, like stereos, VHF radios and fishfinder. A faulty alternator creates RFI/EMI that originates at the alternator and travels along wires to any connected device.
Capacitive Coupling
Capacitive coupling occurs when an electrical field or RFI/EMI moves along two adjacent conductors, typically close in wavelength. It is typical to find two electrical wires separated by something that does not conduct electricity, like the air or plastic on a boat, such as a VHF antenna wire or depth transducer wire bundled with lots of other wires, running to the helm or electrical panel. When one of the wires carries electrical power or radio signal, it creates an invisible electric force field that can reach the other wire through the non-conductive stuff. This causes the second wire to carry EMI to an undesired location or device.
Radiated Coupling
Radiated Coupling occurs when the source of the RFI/EMI and unintended target are separated physically from each other, the EMI travels via air or vacuum to reach the target. The electromagnetic interference is radiated in this type of EMI coupling. Radiated EMI mostly comprises higher-frequency signals in the microwave frequency range. Radiated EMI can also be referred to as radio frequency interference (RFI). Looking at the topside or hardtop of any mid-size or larger sportfishing boat, with its variety of antennas near each other, it is not surprising that EMI can easily travel to another antenna.
Why is the method of coupling important? It helps us mitigate or reduce the RFI/EMI at the affected device.
For instance, if your marine stereo has a high-pitched wine noise through the speaker that varies in pitch with the engine RPM, it could be an issue with the engine’s alternator. Knowing that the engine alternator is connected to the battery and then in some fashion connected to the stereo, it is a good bet that the RFI is traveling directly along the wires from the alternator into the boat’s systems, a typical example of Conductive Coupling
Another example, If your fish finder display has unusual line or streaks across the screen at somewhat regular intervals and only occurs when the marine air conditioning is operating and the transducer wire is tightly wrapped in a bundle from the engine room that includes the air conditioning wiring – it is a good bet that radiated coupling is occurring withing the wire bundle.
Finally, If you have recently installed a Starlink satellite internet antenna on your boat and it is mounted near the satellite TV antenna and you notice a fading or non-existent satellite TV picture only when the Starlink internet system is in use, it would be good bet that radiated coupling is occurring between that antenna systems.
Once you troubleshoot the issue and have determined the coupling method for the interference, then you can proceed to mitigate the interference.
To reduce or eliminate conductive coupling, typically generic noise filter can be installed directly into the wiring of the device producing the interference or if not possible, the filter can be installed at the device effected. On some marine electronics you may notice ferrite beads on the cables, they are a passive electronic component that can suppress high frequency signals on a power supply line. They are normally placed around a power/ground line pair that is incoming to a particular device, such as the power cord for your chartplotter. These beads work according to Faraday’s Law: the magnetic core around a conductor induces a back EMF in the presence of a high frequency signal, essentially attenuating the ferrite frequency response.
When it comes to Capacitive coupling, sometimes, something as simple unbundling the wires and separating the wire runs fixes the problem. Other times a physical issue such as wire damage or even the wrong type of wire used for a given applicator can cause problems and a replacement wire is needed. This is where specialty shielded cabling is often used and could include twisted pair wiring.
Radiated coupling is harder to address on a boat, since we are limited by distance. Though if the radar transmission is causing the EMI, often a radar transmission “blanking” zone can be created withing the 360-degree revolution, telling the radar not to transmit in a given direction, such as towards the satellite TV antenna. Other ways to mitigate radiated coupling are to mount the devices on distinct horizontal levels of the boat or even a raised mounting platform or RF shielding where practical.
Electromagnetic interference is invisible to the naked eye, often inconsistent and produces varying symptoms, often making its discovery, troubleshooting and mitigation a lengthy process. Learning about your boats systems and understanding the potential interference paths can help with finding a solution instead of living with annoying interference. §
