Knock out radio noise

Paul Franson tells how to suppress interference at its sources

Does your depthsounder jump around more than the depths on the chart? Does your knotmeter show changes in speed that aren't there? Do radio direction finder (RDF) signals get lost in a snowstorm or VHF broadcasts sound like a convention of krispies getting carried away with "snap, crackle, and pop"? If any of these conditions sound familiar, your problem could easily be radio frequency interference.

Most interference is simply a nuisance, but it can be a danger when it interferes with navigation and important communication. Some interference (such as atmospheric disturbances, static electricity, and overbearing signals from nearby radio transmitters) can originate outside your boat, but chances are that the biggest source of radio interference is your own electrical system.

Sparks cause the vast majority of radio frequency interference. Electricity jumping through air causes ionization of some of the atoms in the molecules it passes through. When these atoms "de-ionize" they release a tremendous number of radio waves. The frequencies are random and come and go in fractions of a second, but almost every electronic device that you have aboard can be affected by them to some degree.

By tracking down the sources of this "noise" and either suppressing it at its source or filtering it out before it reaches the instruments you are concerned with, you can minimize disruptive interference and go a long way toward assuring that your instruments and electronics function the way they should.

Photo 1: Capacitor installed on positive terminal of ignition coil (and grounded under mounting bolt on strap around coil) fights unwanted radio "noise"

Photo 2: Feed-through capacitors work well where currents are large. Here 40-amp capacitor is used on 35-amp alternator. Capacitor must have rating at least 10 percent greater than alternator (or generator)

Determining the type and source of noise affecting your electronic gear can be a simple task or it can prove to be incredibly frustrating. The basic method is to listen to the "noise" on a receiver of some sort. The "signatures" of various sources can often be seen in the distinctive sounds they produce (table, p. 108). In addition to recognizing the probable source by its sound, try to isolate the source by getting it to start or stop when you do something - usually turn something on or off. Eliminate the loudest noise first and then another masked by it may appear.

The best noise detector is a cheap AM portable radio. Tune it to a spot between stations that lets you hear as much of the "noise" aboard your boat as possible. Now follow the noise around the boat playing "hot" and "cold" as it gets louder and softer until you think you've zeroed in on the source. This isn't infallible, and you should be aware that noise can be generated in one place and then distributed around the boat by wiring or other metal, but tracking the noise with a poorly shielded, noise-susceptible AM receiver is a good place to begin.

Consider the most probable causes of noise first. The most likely villain is the engine, especially if it is a gasoline engine with its attendant ignition system. Although most manufacturers take some steps to suppress the noise produced by their ignition systems, it is often necessary to do additional noise-fighting once an engine is installed and operating.

Ignition systems produce sparks (and therefore radio frequency interference) at the coil where battery voltage is converted into high voltage that can jump across the gap in a spark plug, at the distributor where the high voltage is channeled to the proper plug, and at the spark plug itself where the spark ignites the controlled burning of the gasoline/air fuel mixture. Diesel engines compress fuel to ignite it in a process that produces negligible "noise."

The noise generated by a gasoline engine ignition system comes primarily from the high-voltage side of the circuit. If the whole system is defective or even wet, however, other arcs can occur creating hard-to-locate noise. You can often combat engine noise with a tune-up. Dirty points, worn spark plugs, and dirty ignition wiring all produce much more noise than new elements do. Replace points and plugs. Clean wires, then spray them with an antimoisture, anticorona chemical available from marine and electronic-parts suppliers.

The next step after a tune-up is to install either resistive spark plugs or suppression ignition wiring (but not both). Both contain internal impedance that dampens unneeded emissions from the ignition system. Automotive-type suppressors can't take the moisture on a boat, so using Magnetic Suppressor Wire (MSW) or its equivalent is your best bet. This new wire contains a relatively low-resistance wire over a ferrite core. It's very effective in suppressing engine "noise," and it doesn't affect engine performance.

Other steps include making sure the ignition coil is well-grounded to the engine block (or other metal connected to it). Also, install a quality 0.5 or 1 microfarad, waterproof, marine-quality bypass capacitor to the positive ignition coil terminal (Photo 1). Basically, capacitors allow alternating current to flow while not allowing direct current to pass. This has the effect of "attracting" noise (which is alternating current) and bleeding it to ground without interfering with the normal flow of direct, battery current through the system.

There are two principal types of capacitors in marine use. One is a bypass capacitor which consists of two wires, one leading to ground and the other connected to the "hot" side of the equipment being suppressed. Bypass capacitors are extremely compact which makes them suitable for use on instruments, small motors, etc.

Photo 3: Bypass capacitor (connected to battery - A+) works with resistor-capacitor (connected to field terminal - F) to suppress mechanical regulator. Connect resistor end of resistor/capacitor to terminal and capacitor end to ground

The second type of capacitor is a "feed-through" (also called coaxial) capacitor. These are connected in series and allow current to run directly through the capacitor. Thus they contain three wires: a positive, a negative, and a ground. These are bulkier than the bypass variety, but they are normally used in applications where they must handle a lot of current, such as on an ignition coil. There is no real difference in the suppressive capabilities of the two types of capacitors and choosing which to place where is a question of convenience and ease of installation. Feed-through capacitors, however, do have a better durability record in uses where there is a high level of current.

To install a coaxial capacitor on the coil, cut the existing lead close to the coil, then connect each of the cut leads to the end terminals of the capacitor. Screw the mounting lug firmly to the coil case after you clean paint from the case for good contact. A capacitor is already installed in the lead from the coil to the distributor, so installing another capacitor yourself is unnecessary.

If ignition noise is still a problem after taking these steps, try replacing a plastic-cased ignition coil with one shrouded in metal (which can be grounded to create a shield that keeps "noise" from radiating). The next step is to shield the whole ignition system. This is a tedious and costly process, but it's rarely necessary.

Many of the newest engines have electronic ignition systems as do most outboards. These reduce "noise" problems considerably because they eliminate arcing breaker points and deliver high voltage that tends to keep spark plugs clean. Should you have noise problems with these systems, however, the normal suppression procedures don't apply. Check with your supplier for instructions as to how to proceed with suppression.

After the engine itself, the most likely source of interference is the generator or alternator. Dc generators are much noisier than the ac alternators used in most modern installations. The loud rushing whine that they produce comes from the constant breaking of current as the brushes rub against the commutator. A bypass capacitor on the battery terminal usually cures the problem. Make sure, however, that the capacitor has an amperage rating at least 10 percent higher than that of the generator (or alternator). If the problem persists, it may be necessary to replace the brushes or have the commutator cleaned and repaired. Do not connect a capacitor to the field control terminal.

Although alternators produce less "noise" than generators (because no contact is broken as a rotor revolves), nonetheless they can produce a troublesome high-pitched whine. It can usually be eliminated by using a 0.1 to 0.5 microfarad feed-through capacitor (Photo 2).

Diesel engines do not have the noisy ignition systems that gas engines require, but they almost always have either a generator or an alternator, so don't assume that you have a noise-free engine just because it's a diesel. The combined noise from elements such as voltage regulators, battery chargers, ac/dc adapters, etc, can be considerable, and you're just as likely to have these components on a diesel-powered boat as on a gas-driven one.

One solution to regulator noise is to bypass the battery (and perhaps the generator terminal as well) with a 0.5 microfarad capacitor. Don't connect a conventional bypass capacitor to the field terminal; a resistor capacitor is required if you need to go that far (Photo 3). Perhaps a better solution than bypassing is to install a modern, solid-state regulator if one is available for your system. It eliminates the switch contacts, operates better, and should last much longer.

Although the auxiliary generator or alternator is likely to require suppression of some form, many of these systems generate 110 volts or even higher. This means that special techniques are required and that you should consult an expert. Battery chargers can be noisy but a bypass capacitor from output terminal to ground will usually help. Adapters that turn low-voltage direct current into 110 Vac can generate a whine. Check with the supplier to see if adding a bypass is a good idea.

Photo 4: For low "noise" you need good grounds. Installation of ground strap between alternator and engine block assures good ground continuity

Other common sources of noise that sailors sometimes overlook are the small dc motors used in gear such as bilge pumps, water pumps, anchor windlasses, fans, and refrigerators. Even home appliances containing ac/dc motors can cause a whine when they are used aboard at the dock. The "noise" is just like that from a dc generator and you cure it the same way - with a bypass capacitor.

Instruments, sensors, and automatic pilots can all contribute "noise" as well. Tachometers are perhaps the most guilty instruments. To stop the regular popping they can produce you should use a special bypass available from most marine suppliers. You might also have to replace the wire from the engine with a shielded cable if the bypassing proves insufficient.

Other instruments can also be baffling sources of noise. For instance, whenever an electrical circuit is broken, noise can occur. A fuel gauge sensor, for example, can cause an irregular crackling that is difficult to find unless you can wiggle the sensor arm in the fuel tank while you are listening for the noise. A bypass capacitor (at the tank, not the gauge) is sometimes needed, but often cleaning or replacing the contacts is the answer.

Autopilots, especially electromechanical ones, are also "noise" generators. Often they are well-suppressed, and, if there is a problem, locating it should prove to be quite straightforward. The hardest part is in recognizing your autopilot as a source of noise.

Many of the hardest-to-locate noises come from dirty electrical connections. Once they are found (the best way is with a transistor radio) dirty contacts should be cleaned with emery paper or bronze wool and then coated with a thin layer af petroleum jelly. Finally, refasten them as tightly as possible. Special marine or at least soldered connections are what to use when it's time for replacement. Defective switches can cause similar noise.

By routing wiring correctly you can avoid many noise problems. Keep antennas and other sensitive wiring away from power cables and wires (like depth sounder and tachometer leads) that carry noisy signals. A good technique is to create a separate electrical system for your most noise-sensitive equipment. You can do this by running a separate two-wire power cable directly from the battery to the radiotelephone and other sensitive gear. Make sure the wire is suitable for the power current drain, however.

Good grounds are required for low "noise." A good approach is to run a wide copper bonding strip the length of the boat and connect all electronic and electrical gear as well as the engine to it. If your boat has an external ground plate, connect that too. Ideally, all large metal on the boat should be fastened to the strip. However, be certain to keep the upper strip out of the bilge and to watch for electrolysis.

Two obscure sources of noise may come as a surprise even to knowledgeable sailors. The first is the interference created by the friction between lifelines and shrouds; the second is the "hash" created by the turning of the propeller shaft inside its stuffing box. Careful fastening of stays 'to life-lines wherever they cross is a solution to the first problem; installation of a grounding strap (most often a question - mark - shaped brass piece that rides over the shaft and is grounded to the boat's bonding system or engine) is a good way to fight the second.

Electronic gear provides modern sailors with capabilities and confidence that few sailors before them have enjoyed. However, making sure that the electronic gear aboard performs as accurately and as well as it can demands care and vigilance. Be alert to detect the presence of radio interference when it is at work, and think hard to discover which method of combating it that will work best for you. Although some problems require experience as well as information, you can learn a lot about your boat and her gear by fighting unwanted radio interference on your own.

SAIL September 1979