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TM 11-490-5
electrical,  thermostatic,  or  electromechanical  in
covered by dust or other material deposited from the air.
operation. A switch is a device that can abruptly change
damaged insulators, excessive voltage, loose tie wires
its electrical impedance from zero to infinity or from
or connectors to the powerlines, insulation breakdown in
infinity to zero. Transients throughout the circuit. which
transformers or capacitor banks, intermittent contact
result in generation of broadband emissions, may cause
between any two metallic objects on power poles, poor
interference.  A relay is a switch whose contacts are
ground connections between a neutral line and earth,
opened and closed mechanically by an actuator
and any sharp points projecting from powerlines.
controlled by the electrical field of a coil when an
(b ) Fluorescent lights.  Transients may be
(electromechanically actuated). In the case of manually
generated by the normal ionization of fluorescent lights
actuated switches, the emission produced is generally of
within the tube or by defective starter and ballast units.
relatively short duration unless there are capacitors or
The interference from fluorescent lights may arrive at a
inductors in the circuit. Usually such emission is either
receiver by direct radiation of the steep wavefronts from
nonrepetitive or is repetitive at a slow or irregular rate.
the tube to the receiver antenna, by conduction from the
Switching interference is more severe when large
light fixture through the powerline, or by radiation or
current values or highly inductive circuits are involved
conduction from the tube to the powerline and
because electrical discharge across switch contacts
reradiation to a receiver antenna.
(without capacitor protection) when making or breaking
circuits greatly intensifies the interference in both level
(c) Arc welding equipment. Interference may be
and duration.  Electromechanically actuated switches
caused by the transients associated with a welding arc.
such as relays, vibrators, and buzzers create exactly the
The  interference  may  be  suppressed  by  proper
same type of broadband emissions as do manually
shielding, bonding, bypassing, and grounding. Welding
actuated switches, but usually at a faster rate.
equipment in general should be located in a position
Repetitive interrupters, such as vibrators, produce the
permitting the shortest possible grounding connection to
same broad spectrum emission on a continual basis.
earth. High capacity, reactance-type welders of the type
The steep wavefronts thus generated are possible
used in industry are usually equipped with adequate
sources of interference. The signals present in these
suppression components.  However, lack of proper
steep wavefronts may be radiated directly to a receiver
maintenance of these units may result in interference
antenna or they may be induced into the vibrator
radiation over a broad frequency range for long
solenoid and conducted through power and control lines.
distances.  When units of this type are found to be
creating interference, they should be inspected and any
(f ) Rotating machinery.
Interference is
loose connections, broken bonding straps, defective
generated in rotating machinery (e.g., motors or
filters, or defective bypass capacitors should be repaired
generators) due to the transients from brush bounce,
or replaced.
commutator surface irregularities, brush arcing, and
static discharge.  Any rotating machinery with sliding
(d) Ignition noise. The interference caused by
contacts may be regarded as a potential source of
transients in vehicular ignition systems is usually most
electrical interference. A motor with clean commutator
troublesome in the lower portion of the VHF frequency
contacts generates less interference than one with
range (30-150 MHz).  It is, however, detectable at
rough, dirty contacts. The switching and arcing process
frequencies of 0-500 MHz.
Instances have been
of commutation causes rapid current and voltage
recorded of interference being detectable and in some
changes which distribute interference energy throughout
cases troublesome at distances greater than 500 feet
a wide frequency range. Another source of interference
from its source. Arcing produced in the low-tension side
may be static discharges between a rotating shaft and
of the ignition system by the contact-breaker points and
bearing surface brought on by the insulation or dielectric
sparking in the high tension system by the spark plugs is
effect of the lubricating film.
usually responsible.  Additional sparking may occur if
the ignition harness is defective.  Steep wavefronts,
(g ) Radar pulse. The radar output signal (pulse
associated with the electromagnetic fields produced by
train), generated as it is by very rapid voltage changes
the arcing and sparking processes, contain many
and  radiated  as  periodically  occurring  bursts  of
potential interference frequencies which may radiate
electromagnetic energy, contains frequencies which
directly from the discharge, from lowor high tension
spread over a wide band. These frequencies consist of
wiring, or from any metallic object not properly bonded
a wide range of harmonics of the basic frequency of
or grounded.
occurrence of these pulses, which is the pulse-repetition
frequency, as well as at the harmonics
(e ) Switches and relays.  All switches cause
essentially the same types of interference, whether

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