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Table 2-1. Propagation Characteristics of the RF Spectrum
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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Figure 2-7. Cochannel interference


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
2-4. Equipment Susceptibility
a. General. Any equipment or device capable
of responding to electromagnetic fields or to electrical
signals
must
be
considered
susceptible
to
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


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