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1.0

Introduction

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 vehicle to another, depending upon the 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 25-50MHz band and reduce with increasing

frequency.

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:

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.

2.0

Noise Sources

The three major noise sources affecting mobile radio systems are 1) radiated noise, 2) conducted

noise, and 3) induced noise. (See Figure 2-1 for typical vehicular noise sources.)

2.1

Radiated Noise

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 build-up, ignition systems, or electrical motors. Radiated noise is the

most common cause of mobile radio interference.

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.

Reducing Noise Interference in Mobile Two-Way Radio Installations

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.

Introduction

2-1