Fluorescent lights

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

electrical

signal

applied

the

coil

(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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