Transfer Media

TM 11-490-5

are highly susceptible to interference, especially from

higher power circuits that share a common impedance.

of the radar's carrier frequency. If the radar antenna

The so-called ground loop, where undesired signals are

radiating this train of pulses sweeps in a circular arc, the

transferred from a highpower circuit to a low-power

intensity of the pulses will seem to vary in intensity as

circuit as a result of the impedance shared in a common

heard at a fixed radio station. Radar pulse interference

ground circuit resulting from a poorly designed ground

may be radiated from any metallic portion of a radar

system or poor ground connection, is one example of

system structure as a result of coupling from the circuit

this.

components which convey the pulses.

(2) Two conductors close together tend to

(h) Industrial, scientific, commercial, and medical

produce voltage variations within their circuits through

apparatus. Any apparatus using RF energy can be a

mutual capacitance. The capacitive reactance between

serious source of interference. Such apparatus may be

two conductors varies with the distance between the

diathermy equipments, X-ray machines, microwave

conductors, conductor sizes, and the frequency of the

ovens, or induction heaters.

Electromagnetic

signal being coupled. The effect of capacitive coupling

interference is radiated directly from the apparatus or

increases with increasing frequency. The use of a

conducted along the power and control lines. This type

shield (or shielded wires in the case of conductors in

of interference may travel hundreds of miles and often

proximity) will reduce the interference.

cannot be detected in the area between the source and

(3) The various circuit interconnecting

the receiver being disturbed.

equipments at a communications site cause closed

loops to exist with mutual inductance acting as the

2-3. Transfer Media

mechanism for interaction between the loops. This

a. General. If electromagnetic energy is to

interaction may be thought of as a transformer action

cause interference, it must be transferred from the point

between the interference source and the sensitive

of generation to the location of the susceptible device.

circuit. Circuits characterized by low impedances are

This transfer may occur over one or more paths by

particularly susceptible to inductive coupling. The effect

means of conduction, radiation, or combinations of

increases with increasing frequency and decreasing

these two modes of transfer.

distance separation. The use of a magnetic shield or

b. Conduction. Interference by conduction

the repositioning of equipments or wiring can reduce

occurs when signals are transferred unintentionally

inductive coupling.

between two circuits by an actual circuit element which

c. Radiation. Electromagnetic radiation of an

is shared by the two circuits (common impedance), or by

electrical signal involves propagation through a medium;

an effective common impedance (although no actual

therefore, the effects on the signal by the medium must

shared circuit element is present), caused by the

be considered. During transmission of an interfering

proximity of circuit wiring. A complete circuit must exist;

signal, losses are incurred which can render the signal

that is, there must be a signal path and a return path

harmless. If not enough loss is involved, however, the

between the affected circuits. Sometimes these paths

undesired signal will reach a receiver as interference.

and effective common impedances are not readily

When signals are radiated through the medium of the

apparent, but they can occur, for instance, by direct

earth's atmosphere, the losses incurred in transmission

wiring or use of a common ground. Coupling of

are dependent upon frequency, separation distance (the

undesired signals by means of common connections,

distance from the emitter of the signal to the receiver),

especially on the ground side, is a frequent occurrence.

terrain over which propagation occurs, and the

Further description of interference coupling by

electromagnetic properties of the transmission medium

conduction is contained in (1) through (3) below (see

(which change with day and night, seasons, weather,

also para 2-4, Equipment Susceptibility):

and ionization effects of the sun).

(1) Two circuits are said to be mutually coupled

(1) Table 2-1 shows the propagation

whenever voltages or currents in one circuit induce

characteristics of the RF spectrum. The three types of

corresponding voltages or currents in the other circuit.

radio wave propagation represented in this table are, in

The sharing of a wire or a junction point between two or

the order of increasing frequency, ground waves

more circuits (common interconnection) can result in

(propagated over the surface of the earth and influenced

common impedance coupling, whereby current in one

by the ground as well as the troposphere, the

circuit causes a voltage to appear in another circuit.

atmospheric region immediately above the earth),

The interference voltage level so produced is dependent

skywaves (propagated by refraction effects of the

upon the magnitude of the common impedance and the

ionosphere, the region above the troposphere), .and

current flow through the impedance.

Circuits

troposphere scatter waves (propagated by

characterized by high impedance (and low power levels)

2-10