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Figure 3-5. Examples of channel distribution for FDM and TDM.
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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DCS (Fixed Installation ).


TM 11-490-5
CHAPTER 4
DETECTION, RECOGNITION, ISOLATION, AND
REMEDY OF EMC PROBLEMS
assistance and to report promptly any problem outside
4-1. General
their experience.
The following list, although not
a.
EMC  problems  at  fixed  Defense
exhaustive, contains some of the "tools" available to
Communications System (DCS) installations or at fixed
operating personnel:
or transportable non-DCS facilities may be grouped into
two broad categories:
(1) Test equipment.
(1) Instrasite EMC problems associated with
(2) Testing and isolation procedures.
the equipments and installation which generally require
(3) Equipment alignment procedures.
some type of maintenance to resolve.
(4) Backup or spare equipment and modules.
(2) Intersite  EMC  problems  when  the
(5) Patching and rerouting techniques.
interference originates at some location other than the
(6) Interpretation of equipment meters and
communications site.
indicators.
b. A DCS station is a vital link in a chain of
(7) Antenna orientation.
communications which may provide service between
(8) Communication training.
local subscribers or between far-distant points and local
subscribers, and may interface with non-DCS facilities.
(9) Substitution.
The station .may handle clear-text or encrypted
(10) Signal tracing.
transmissions. The stations may be connected by radio
(12) Experience of senior operating personnel.
propagation media only, or in combination with cable
(13) Equipment history (log books and the
circuits.  However, with the exception of unattended
like).
relays, stations are manned by operating personnel who
are vital in maintaining successful communications
4-2. Discussion of EMC Problems
when interference is experienced.  At most stations,
a. Introduction.
some incidence of EMC problems may be anticipated.
(1) When a communications site is installed,
These problems may originate at some location not
intrasite conducted emission problems have normally
associated with the communications site, they may be
been reduced to acceptable levels by proper grounding,
the result of enemy actions, or they may originate within
equipment location, and similar installation procedures.
a station. When new equipment is procured, extensive
However, normal deterioration will occur over a period
EMC tests are made in an effort to eliminate EMC
of time, corrosion may form on grounds and other
problems.
However, these tests do not always
terminals, connections may develop higher resistances
reproduce all conditions in an actual installation. During
than normal, and components may degenerate or fail
acceptance tests of a new installation, it may not always
without causing an apparent equipment malfunction. All
be possible to make a complete test covering the entire
of these conditions may cause interference.  These
frequency range which is to be used in operation. As
interference problems can normally be avoided or
they age, components drift out of tolerance and
minimized
by
proper
preventive
maintenance
sometimes fail completely; when this happens, rapid
procedures.
action must be taken to isolate, locate, and repair or
(2) One major cause of operational EMC
replace the component. The human factor is thus of
problems is radiated signals from sources beyond the
vital importance in the solution of EMC problems.
local C-E site.  This type of intersite interference is
c. To facilitate rapid solution of EMC problems,
difficult to isolate or minimize. It may be unintentional
operation personnel must be familiar with all facets of
interference from friendly (or enemy) radio or radar
their station.  They must understand its scope and
transmitters, or it may be intentional jamming or
capabilities, be familiar with normal indicator readings
intrusion.
(as they usually provide the first indication of any
problems), and be conversant with the use of available
(3) Although interference may originate from
equipment.  Operating personnel must also learn to
an RF source, it usually manifests itself as a faulty
recognize  situations  which  require  more  skilled
operating condition in a subscriber's instrument. It
4-1


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