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


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


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