Chapter 2 Reducing Noise Interference; Introduction; Noise Sources; Radiated Noise - Vertex Standard VXD series Installation Manual

Chapter 2 Reducing Noise Interference

2.1 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 may require
slight or heavy noise reduction, depending upon the relative strength of the radio signal, and upon the
ability of the radio to reject undesired noise. These requirements vary from one vehicle to another, de-
pending 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 logi-
cal, systematic method of elimination until the interference is eliminated or reduced to an acceptable
level.
DO NOT add interference elimination equipment to vehicles equipped with electronic ignition
systems before obtaining advice from the vehicle manufacturer. Addition of some noise sup-
pression components may interfere with proper operation of electronic ignition systems and
could seriously damage them.
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.
Consult a service manual for the vehicle to determine what noise reduction provisions the manufactur-
er 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 compo-
nents 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.

2.1.1 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.)

2.1.2 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 com-
mon 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 fre-
quency near or even identical to the desired radio signal. The standard receiver may be unable to dis-
tinguish between the two signals (desired and noise). Therefore they both enter the receiver, resulting
in degradation of the desired signal.
Chapter 2 Reducing Noise Interference
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