Equipment Susceptibility

TM 11-490-5

affected by radiation from the sun, some portions of the

regions may be more heavily ionized than other

portions.

This can disrupt long-distance HF radio

communications.

2-4. Equipment Susceptibility

a. General. Any equipment or device capable

of responding to electromagnetic fields or to electrical

signals

must

considered

susceptible

electromagnetic emissions. Whereas paragraphs 2-2

and 2-3 described the generation and transmission of

interference signals, this section deals with the reception

of, and reaction to, these signals by various equipments.

Two broad categories of equipment can be established:

those that are frequency selective and those which are

not frequency selective.

b. Entry Mechanism. In paragraph 2-3 it was

stated that undesired signals are transmitted by

conduction or radiation, or both. There are three means

by which an undesired signal may enter equipment: (1)

through a common interconnection, (2) through

capacitive and inductive coupling, and (3) through direct

impingement of electromagnetic energy. Figure 2-6

depicts examples of these entry mechanisms. In figure

2-6A, undesired signal current is conducted from its

source in equipment A to the receiving unit, equipment

B, through common input leads. The use of common

power supplies not properly isolated electrically

(decoupled), or improperly operating multiplexing

equipment are examples where this type of entry is

possible. Another means of entry through the common

interconnection is shown in figure 2-6B. Even though

the common connection point is ground, it must be

remembered that some impedance may be present

which is shared by the two signal paths and, therefore,

coupling between the circuits takes place. This type of

coupling is commonly encountered as a result of poor

equipment design, installation, or maintenance, and

Figure 2-6. Interference signal entry paths.

may be minimized by correct design of the ground

system. Figure 2-6C shows an example of the entry of

c. Equipment Susceptibility Characteristics.

undesired signals through capacitive and inductive

(1) Frequency Selective Equipment.

coupling between signal lines placed close together.

Frequency selective equipment includes all equipment

This type of coupling can occur between multiple signal

types which operate at fixed frequencies or which are

and control lines, other signal lines, adjacent power

tunable over a selected range of frequencies.

lines, and transmission lines. Direct impingement of

Communications receivers are subject to more

electromagnetic energy as a means of undesired signal

numerous, more complex interference situations than

entry is shown in figure 2-6D. Electromagnetic energy

other equipment types because of their highly sensitive

can also gain direct entry into equipments through

front end circuits and the fact that so many users must

ventilation holes, meter and other panel openings, and

share the RF bands. A discussion of typical interference

inadequate shielding, or indirect entry by causing

situations follows:

currents to be induced in equipment cabling, connectors,

(a)

Cochannel

and

adjacent

channel

and other wiring. The latter cases usually originate

interference.

Any transmission may interfere with

within a system or equipment and may be regarded as

another system if it is present simultaneously with a

intrasystem coupling.

desired RF signal. This interference can occur when

two transmitters have been assigned to operate on

2-12