RFI Suppression

RFI suppression requires locating the exact source of the RFI, sometimes it is multiple sources.  Before you do anything, locate the source of all this RFI and you can start troubleshooting the source of the RFI on your various boat electronics and communications equipment. This is actually a relatively simple method of locating the troublesome noise source. It is really a matter of logic and the use of systematic sequence of switching off of each item of equipment to locate the noise source. When the noise stops the cause is generally located. While it is a simple process of elimination, it isn’t always so easy. Most cases will involve the identification of two or more noise sources that are overlaid to create a cumulative noise effect. A proportion of RFI noise will be intermittent, and this includes static discharges and lightning pulses, which will not be easy to identify. 

What RFI Tools are Required?

The best is a cheap and small battery powered AM radio, if you can still get one that is This is a great little tool for tracking down and sniffing out those radiated sources on the boat causing troubles on your marine electronics. When the radio is passed close to the cables or equipment the radio will detect the noise. Local lightning is also detected.

Some noise is simply related to time of day. Remember that interference from solar activity and ionosphere factors on radios is one source that must always be factored in. These sources will also affect your GPS, SSB, HAM and marine radios and satellite communications all simultaneously, leading to a conclusion of a greater problem. As this usually occurs around dusk you may already have the answer. Wait until later and try your radios again. 

Basic RFI Troubleshooting

The first step in troubleshooting problems on your various electronics equipment is the disconnection of the antenna. If the noise continues then the noise is probably caused by the electrical system. If the RFI tends to increase when the GPS or antenna and aerials are reconnected, the cause may then be atmospheric or from some other emission source that is being picked up through the antenna feed cables. It is important to remind you that RFI may be coming from more than one source or path, so investigate all possibilities. RFI suppression can be very frustrating at times so be methodical and patient.

What is that ticking sound on the radio?

If the noise on your radios presents as a ticking noise that seems to vary with the speed of the engine, then you are probably picking up ignition noise or it is from the alternator. In practice this would usually affect the power wire to the equipment.  The most common sources for this noise are usually the distributor, ignition coil, spark plug wires and the spark plugs. If the ignition noise is identified, relocate the power wire as far as practicable away from any of these sources. If things don’t improve then a check will have to carried out on all of these components. It is prudent to verify that the spark plug wires and the spark plugs are RFI shielded. 

Which RFI suppression method is best?

There are several RFI suppression methods available that can be utilized to reduce or eliminate RFI interference on your various marine electronics. The first is the use of shielded cables along with proper grounding. In many cases suppressors such as a filter or capacitor connected close to the noise causing equipment is common. This effectively short circuits noise in the protected frequency range. Suppression filters can be in several types and include ferrites, MOV’s and suppression capacitors are often required on alternators. 

About RFI and Grounding

If you want optimum electronics performance then grounding must be carried our properly. It is a fact that the majority of RFI suppression problems that include the noises, hums, buzzes, interference or cross talk on your marine electronics are often caused by bad grounding practices. When a difference in voltage is created between the 2 different ground points, this will cause current flow and therefore problems. These currents that flow through the different grounds are called ground loops.  The ground in any GPS and marine electronics system is the reference voltage point for all signals and is nominally equal to 0 Volts. To prevent ground loops from arising, all of the signal grounds should be connected to one single common ground point. Cable shields are generally designed to protect against interference from unknown or unspecified sources. The effectiveness of cable shields is measured in terms of transfer impedance. This is a measure of effectiveness in capturing the interference field and preventing it from reaching the conductor pairs inside, in other words its protection efficiency.

About RFI and Shielding

Signal and data cables also have shields that are made from foil/polymer laminate tapes or they have layers of copper braiding. These may also have a drain wire installed to facilitate the termination of the screen to ground. In general the majority of equipment manufacturers will nominate the termination of shields to ground.  A simple rule, never ground the cable screen at both ends, only ground at one end. Typically this is usually nominated at the equipment end. It is actually very common to find that cable shields are not connected at all. Always check and connect them, as it may be a source of your elusive RFI suppression challenges.  More great information from fishing and boats