REDUCING NOISE INTERFERENCE IN
MOBILE TWO-WAY RADIO INSTALLATIONS


1. INTRODUCTION

1.1 Electrical noise generated by the electrical system of a vehicle, as well as local ambient noise, can interfere with normal operation of mobile radios. Satisfactory operation of a mobile radio in a given vehicle may require slight or heavy noise reduction, depending both upon the relative signal strength of the desired radio signal and upon the ability of the radio to reject the undesired noise. These requirements vary from one vehicle to another, depending upon vehicle type and required coverage area. When operating in a strong signal area, a certain amount of noise interference can be tolerated. In weak signal areas, however, noise reduction becomes very important. As a rule of thumb, noise levels create greatest interference in the 25-50 MHz band and reduce with increasing frequency.

1.2 Before attempting any noise reduction procedures, determine the noise source(s). Then, follow a logical, systematic method of elimination until the interference is eliminated or reduced to an acceptable level.

CAUTION

Do not add interference elimination equipment to vehicles equipped with electronic ignition systems before obtaining advice from the vehicle manufacturer. Addition of some noise suppression components may interfere with proper operation of electronic ignition systems and could seriously damage them.

1.3 Care and patience must be exercised in locating and eliminating noise sources. There may be several sources of noise, each slightly stronger or weaker than the other. Elimination of one source may seem ineffective because another noise source remains active at a barely discernible difference in level. It is suggested that a service manual for the vehicle in question be consulted to determine what noise reduction provisions the manufacturer applies when AM, AM/FM, or CB radios are installed as original equipment. These radios are also subject to electrical noise interference and the manufacturer may install noise suppression components only upon those vehicles which require radio equipment at the time of manufacture. These noise suppression components should be added in any first attempt to suppress noise.

2. NOISE SOURCES

The three major noise sources affecting mobile radio systems are 1) radiated noise, 2) conducted noise, and 3) induced noise. (See Figure 1 for typical vehicular noise sources.)

2.1 RADIATED NOISE

2.1.1 Radiated noise enters the radio through the antenna along with the desired signal and can block or degrade desired communication. It can be generated by power lines, fluorescent lights, or by electrical discharges from static buildup, ignition systems, or electrical motors. Radiated noise is the most common cause of mobile radio interference.

2.1.2 When a spark discharge or arc takes place through air, energy is radiated at frequencies from a few kilohertz to hundreds of megahertz. This spurious radiation may have some energy radiated at a frequency near or even identical to the desired radio signal. The standard receiver may be unable to distinguish between the two signals (desired and noise) and so they both enter the receiver with a resulting degradation of the desired signal.

2.1.3 It is impractical to prevent all arcing in the standard vehicle electrical system. In an 8-cylinder engine running at 2,000 RPM, arcing occurs across the spark plugs at a rate of 8,000 sparks per minute or 133 sparks per second. Electrical motors and generators also produce arcs.

2.2 CONDUCTED NOISE

Conducted noise enters [he radio through lhe points where the radio is attached to the vehicle's electrical system such as battery cables, ignition switch, chassis ground, etc. It can be generated by electrical transients, electrical motors, poor grounding points, or inadequate electrical system filtering (from alternators, generators, voltage regulators, or weak batteries). Conducted noise can degrade both transmit and receive performance of a mobile radio.

2.3 INDUCED NOISE

Induced noise enters the radio through the proximity of radio wiring to other wiring in the vehicle. Electrical currents through the standard vehicular wiring can induce undesirable noise signals into the radio cabling (and degrade communication) simply because the wiring provides a transformer-type coupling action without any actual physical connection. Induced noise can degrade both transmit and receive performance of a mobile radio.

3. OPERATION OF A CONVENTIONAL IGNITION SYSTEM

3.1 To effectively reduce ignition interference in a vehicle, it is well to understand the operation of an automobile ignition system.

3.2 Ignition is necessary in a gasoline engine to ignite the gasoline vapor and the air mixture in its cylinders. The system is made up of the battery, distributor, breaker points, coil, condenser, and spark plugs. The battery is the only electrical source of power in an automobile, so the lower battery voltage must be stepped up to the high voltage necessary to arc across the spark plug electrodes. This arc ignites the gas mixture.

3.3 In the conventional ignition system (Figures 2 and 3) a mechanical circuit breaker (the cam and points in the distributor) opens the primary circuit of the ignition coil, and high voltage is developed at the secondary. This high voltage is synchronized and applied to each spark plug by the distributor.

3.4 The battery is connected to the primary winding of the coil through the ignition switch. The primary circuit is returned to the battery through the breaker points, which are bypassed by the condenser. The points are normally closed. As the cam shaft is rotated by the engine, its lobes or corners open and close the points in proper synchronization with the piston in each cylinder.

3.5 With the ignition switch on and the breaker points closed, coil primary current builds up at a rate determined by the coil inductance. 

3.6 When the breaker points open, primary current decreases and, by self-induction, an electromotive force is induced in the primary which is many times greater than the battery voltage. The high voltage induced in the coil secondary causes a spark across the distributor rotor-to-spark plug wire gap and then across the spark plug gap for a short interval of time when the breaker points open. The condenser reduces arcing of the points.

3.7 The secondary circuit of the ignition coil, including the distributor rotor gap and the spark gap, is the main source of ignition interference. The lead inductance and stray capacitance provide a tuned circuit. Because the discharge of the circuit is through a low resistance (ionized spark gap), the circuit tends to oscillate. The frequency and amplitude of oscillation vary as current changes in the spark gap.

4. DETECTION OF NOISE SOURCES

4.1 Detection of offending noise sources is the heart of noise suppression because, after the noise source is identified, the solution becomes obvious. A logical, methodical procedure is basic to the effective noise suppression.

4.2 Use your available equipment to best advantage. A pickup loop, about one inch in diameter, can be connected to a portable CB radio or a mobile radio being operated at a frequency similar to the installed radio frequency but from an isolated supply voltage. The pickup loop can be moved throughout the vehicle with the radio used as a radiated noise detector. Be sure the pickup loop has sufficient insulation to prevent the input of the radio from coming into direct contact with high voltage points in the vehicle ignition system.

4.3 A non-polarized bypass capacitor, which has alligator clips firmly attached, can be used on a trial-and-error basis to locate wiring which needs additional filtering. Keep capacitor lead lengths short for best suppression. Ceramic disc capacitors are not as suitable or effective as automotive coaxial capacitors. (See Figures 4 and 5 for part numbers.) 

4.4 Some interference is due to geographic location and need not be eliminated with mobile radios since it is possible for the vehicle to be moved away from the interfering noise source. Noise from power lines, fluorescent lights, and other vehicles (emitting high levels of radiated noise) are examples of noise which depends upon location. Make certain that you are not attempting to noise suppress a vehicle while it is in a noisy location. If you 'suspect that your location is noisy, simply turn off everything in the vehicle except the radio and listen to the remaining ambient noise. If the noise level is objectionable, you may have to noise suppress the vehicle during a less noisy time of day or in a different, quieter location.

4.5 Check the installed radio for conducted and induced noise by using an unmodulated signal generator to supply a clean RF signal through a coaxial cable directly to the antenna connector on the radio set. This should prevent radiated noise from entering the receiver and masking conducted and induced noise. Make certain that the signal generator is not microphonic and is placed away from the noise of the engine and exhaust. Operate the controls for fans, blowers, power windows, headlamps, turn signals, windshield wipers, etc. and listen for the presence of noise in the received radio signal. This will allow the source of the interference to be determined. Some noise sources cannot be turned on and off at will. These sources will need to be attacked piecemeal on a trial and .error basis...alternator whine, voltage regulator, electric fuel pump, etc. Remember that induced noise can result from radio wiring being too close to other vehicle wiring. This problem is most easily solved by proper cable routing at the time of installation.

4.6 Radiated noise must be attacked after conducted and induced noise has been satisfactorily suppressed. The vehicle ignition system is usually the primary source of this interference. Antenna placement can be critical in some installations. Remember that static discharge generates static or radiated noise and probably will only be generated when the vehicle is in motion.

4.7 Since radiated noise interference is most noticeable in weak signal areas, it is suggested that suppression be finalized while listening to a weak "on frequency" signal. (The squelch should be "open" so that even marginal signals can be heard.)

5. NOISE REDUCTION TECHNIQUES

5.1 GENERAL

5.1.1 There are three basic ways to suppress noise. The first is the addition of resistance in circuits subject to ringing. This method is used for the ignition coil HV cable, spark plug wires, and spark plugs. The second is to filter noise from low voltage wiring using coaxial bypass capacitors. The third is to control static charge buildup using wipers for movable parts such as automobile hoods and trunk lids, or flexible bonding leads for fixed members. Static in wheels may also be controlled using collector rings. Applications of each of these techniques are discussed below.

5.1.2 The vehicle manufacturer's service manual also may provide noise suppression information which is valuable in any 1st attempt at noise suppression.

5.2 IGNITION SYSTEM INTERFERENCE

5.2.1 Engine Maintenance and Tune-Up

The most important step in reducing ignition noise is insuring that the engine is in proper tune. Pay particular attention to the following points if the noise interference from the ignition system is severe:

a. Be sure the spark plugs, distributor points, and
condenser are in good condition.

b. Be sure ignition timing is properly adjusted.

c. Be sure the distributor cap and rotor are in good
condition. They should be replaced at least every
30,000 miles.

d. Be sure that spark plug wires make good, solid contact at each end and are routed as far as possible from low voltage leads.

e. Many late-model automobiles are equipped with a
shield over the distributor points. Check to see
that this shield is properly positioned and securely
attached.

5.2.2 Available Noise Reduction Kits

5.2.2.1 Motorola offers two noise reduction kits; Model TLN8845A for alternator-equipped vehicles and Model TLN6252A for generator-equipped vehicles. These kits are for suppression of noise in the primary and secondary of the ignition coil and for bleeding of static charges on the hood. The TLN6252A also is used to suppress noise from the generator and voltage regulator. Figures 4 and 5 show the contents of the kits and their usage.

5.2.2.2 Motorola also offers wheel static collectors (Motorola Part No. 1K534254-regular type and 1K563173-small type. Use of these items are described in paragraph 5.8.

5.2.2.3 Most auto parts dealers can supply .05 uF bypass capacitors (condensers), resistive ignition wires, and resistor spark plugs. The type of spark suppression used in noise reduction should always be in accordance with the recommendations of the vehicle manufacturer, that is, if spark plugs are to be replaced with built-in suppressor types, they should be in the proper heat ranges and thread sizes for the particular engine. In each ignition system, there is a maximum amount of resistance that may be connected between the distributor and a spark plug. If resistance-type ignition wire is used, the length of each wire must be limited so that its resistance does not exceed the allowable maximum.

5.2.3 Ignition Coil Interference

This type of interference is characterized by a popping sound which is most noticeable when the engine is running slowly. To suppress the noise, connect a 0.1 uF coaxial capacitor (Motorola Part No. 8C82571B02) from the battery side of the ignition coil to vehicle ground (see Figure 6). This keeps the distributor noise from being conducted through the battery lead into the electrical system of the vehicle. Note that some electronic ignition systems do not route the battery lead to the ignition coil and can have proper operation impaired if the "input" terminal is bypassed.

5.2.4 Distributor Interference

This type of interference is characterized by popping sounds which are present at all engine speeds. It is caused by sparking between the rotor and the distributor cap inserts as the rotor turns. To suppress this noise, use resistance ignition wire to connect the ignition coil to the distributor cap.

5.3 BATTERY CONNECTIONS

The radio set power cable can pick up noise generated in the vehicle. This can be minimized by connecting the power cable directly to the battery instead of the fuse block. The battery acts like a large capacitor (about one Farad for a 50 amp/hr battery), which bypasses induced noise. The battery ground lead should be securely bonded to the vehicle frame. Undesirable parallel ground currents can be minimized by using the vehicle frame as a common ground point. If ignition switch control of the radio is desired, the radio power leads may be connected to the battery through a relay which is controlled by the ignition switch.

5.4 ALTERNATOR/GENERATOR WHINE
(See Figures 7 and 8)

This type of interference 'is characterized by a high-pitched whine which varies with engine speed. A 0.5 uF coaxial capacitor (Motorola Part No. 8C82571B01) can be used to bypass the whine. For generators, the capacitor is connected in the armature lead. Never use a capacitor in the field lead. Use the field suppressor assembly supplied in the TLN6252A Noise Reduction Kit. For alternators, the capacitor is connected in the lead to the battery post.

5.5 VOLTAGE REGULATOR NOISE

This type of interference is characterized by erratic popping noises which change only slightly with changes in engine speed. The noise is generated by arcing in the vibrating breaker contacts of the voltage regulator. It can be suppressed by connecting a 0.5 uF coaxial capacitor (Motorola Part No. 8C82571B01) in the battery and armature leads of the voltage regulator (see Figure 9).

CAUTION

Disconnect the battery ground terminal before attempting to connect components to the voltage regulator.

5.6 HOOD AND TRUNK LID NOISE

This type of noise is characterized by irregular popping sounds. It is caused by friction in poorly bonded automobile hoods or trunk lids, which causes static electricity to build up until arcing occurs. This type of noise is suppressed using a Motorola Hood Wipers and Mounting Hardware Kit (part of TLN8845A or TLN6252A Noise Reduction Kits) which provides electrical bonding of the hood or trunk to the vehicle body while still permitting them to be opened.

5.7 OTHER ELECTRICAL NOISES

Other elements of the electrical system which can generate noise are listed below. These can be suppressed by connecting a 0.5 uF bypass capacitor between the noise source and ground.

a. Ammeter-to-battery lead

b. Gauges (oil, fuel, temperature)

c. Ignition switch

d. Lamp bulbs (headlamps, tail lamps, dome lamps, etc.)

e. Accessory wiring (electrical fuel pump, electric windshield wipers, heater fan motor, window openers, etc.)

5.8 WHEEL STATIC

Road friction can cause static buildup on the front wheels. Suppress noise from this source using wheel static collector rings (Motorola Part No. 1K534254 regular type -- 1K653173 small type).

5.9 GROUND BONDING

Static charges can build up on various parts of a vehicle if not properly bonded and cause noise through arcing. This type of noise is suppressed by bonding the part using one-inch wide ground straps (keep them as short as possible). Some common points where bonding may be helpful are listed below. See Figure 10.

a. From engine block to the firewall.

b. From engine block to vehicle frame at points
where the engine is shock-mounted.

c. From the battery ground terminal to the vehicle body.

d. From top of front wheel "A" frames to chassis,
particularly in cases where rubber-mounted members are used.

5.10 EXTENDER RADIOS

Impulse noise (principally ignition noise) is heard as annoying "pops" along with a received message. Each noise impulse excites the highly selective receiver tuned circuits which "ring" after the noise impulse disappears. This "ringing" effectively extends the duration of the noise impulse as it passes through the receiver. Motorola radios with the "Extender" feature have additional circuitry which "blanks" the receiver for the duration of the undesired noise pulse so that annoying "pops" are not heard. The "Extender" receiver effectively chops out the small portion of the message which contains "pops". This type of receiver is effective in areas where other noisy vehicle ignition systems interfere with received messages. However, even "Extender" receivers cannot cope with extremely noisy environments because too much chopping can also destroy message intelligibility.


Copyright 1978. Motorola Corp.