Quantcast Chapter 18. RADIO INTERFERENCE AND SUPPRESSION

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TM 9-8000
CHAPTER 18
RADIO INTERFERENCE AND SUPPRESSION
Section I. INTERFERENCE
such sparks will disclose the location of the vehicle to
18-1.  Automotive Radio.  Although the supply and
sensitive electrical detectors. Because the units of the
maintenance of radio equipment is a responsibility of the
electrical equipment are connected by a wire or a series
Signal Corps, its use by motor vehicle units for
of wires, as in an automotive ignition system, the wiring
coordinating convoy movements in isolated areas must
acts as an antenna to transmit the interference created
be considered. Transmitting and receiving equipment of
by the spark into the air. The captive effect of the wires
this type depends upon the electrical system of the
and the spark-producing unit causes the radiated energy
vehicle in which it is installed for its source of power.
to affect a wide band of frequencies on a radio receiver,
with pronounced effects on certain frequencies.
a. Installation. Installation of these units varies in
different types and makes of equipment.  In general,
18-2. Ignition Noises.
units of radio equipment should be mounted on brackets,
panels, or metal members that are attached to the body
a.  Cause.  When distributor breaker points are
or frame by welding or riveting. All paints, lacquers, or
opened and closed by operation of the engine, the
primers should be removed from all mounting surfaces
ignition coil produces a high-voltage current that flows
that come in direct contact with the equipment, and the
across the gap in the spark plug to cause ignition. The
surfaces should be tinned in order to ensure the best
sparks at the plugs and those at the breaker points
possible ground. The units should be located so that all
cause violent surges of current to flow in all wires of the
switches or controls are within easy access of the
circuit (fig.  18-1).  A magnetic field builds up around
operator.  All flexible control cables should be free of
each wire and collapses with each make and break of the
sharp bends. So far as possible, installation or removal
circuit.  The rapidity with which these changes in the
of this type of equipment should be done by specialists of
magnetic field are repeated is determined by the engine
the Signal Corps who are trained in this type of work.
speed.
b. Power Requirements. Radio units require from 4
b. Recognition. The resultant noise in the receiver
to 5 amperes for receivers and 12 to 16 amperes for
from breaker points, distributors, or spark plugs is
large units and transmitters. In many instances, it will be
recognized by clicking sounds that vary in rapidity and
found necessary to equip the vehicle with a larger
intensity with the speed of the engine.
generator with a regulator device to supply the additional
current.  All power leads from the vehicle electrical
system should be of sufficient size to meet the current
18-3. Generator Noises.
requirements and should be equipped with fuses or other
overload protection devices. All leads should be as short
a. Causes. With the generator in operation, there
as possible.  High-voltage direct current sometimes is
is some sparking between the brushes and commutator
necessary, in which case a motor generator is required.
segments.  Generators in good mechanical condition
may exhibit some sparking, but this usually is not severe
c.
Interference.
Any sparks created by the
enough to cause radio interference.
This type of
operation of electrical equipment (such as spark plugs,
sparking is increased by any of the mechanical defects
circuit breakers, coils, generators, regulators, magnetos,
listed below.
or distributor assemblies), by loose or dirty connections,
or by chafing of metal to metal, may cause interference
(1) Brushes do not fit commutator.
with radio reception of nearby radio receivers.
In
addition,
(2) Brushes are worn more than one-half their
original length.
18-1


 


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