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CHAPTER 4. DETECTION, RECOGNITION, ISOLATION, AND REMEDY OF EMC PROBLEMS
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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Recognition, Isolation, and Remedy of EMC Problems


TM 11-490-5
may first be reported by the subscriber, or be discovered
antenna; use of a more highly selective receiver, if
during routine circuit checks.
available; or use of a frequency-synthesized receiver
b. DCS (Fixed Installation ).
instead of the automatic frequency control type,
(1) In the case of a fixed DCS system, the
providing  the  distant  wanted  transmitter  is  also
equipment is normally installed in accordance with I)('A
frequency-synthesized).
standards and other installation criteria. Any deviation
c. Non-DCS (Transportable).
from these standards is corrected or a waiver is
(1) In the case of non-DCS transportable or
recorded when the system is accepted and placed in
mobile systems, each time the equipment is moved and
service. Daily logs concerning equipment performance
reinstalled,  new  problems  may  be  experienced.
are maintained and a history of any discrepancies and
Although general performance criteria are available for
corrective action taken are recorded.
Therefore,
both  equipments  and  systems,  performance  at
operating personnal at a DCS fixed site have both the
individual locations will vary.  Equipment logs can
standard  performance  criteria  for  the  particular
provide some insight into past performance of the
equipment and the past history of the performance of
equipment and are useful in establishing performance
the facility. If operating personel are cognizant of the
criteria.  However, the experience of the personnel
past performance of the system, as recorded in the
assigned to the system as to normal performance under
station log, they can readily recognize any deviation
similar geographic, soil (ground considerations), and
from normal performance.  The data also provide
distance conditions is the most significant factor in the
information on whether the problem is a recurring one
recognition and isolation of an EMC problem.
and what corrective action was previously taken. The
data may also assist in the isolation of the problem and
(2) Intrasite EMC problems at a non-DCS
a determination of whether it is an EMC problem or
transportable or mobile site are more prevalent than a
equipment malfunction.
fixed site, because equipments are subject to movement
and reinstallation, and therefore cables and connectors
(2) Intrasite EMC problems can often be
experience greater wear and breakage.  Each new
traced to a lack of adequate preventive maintenance
installation has a potential for problems with connectors,
which can result is partial or complete failure of
terminals, antennas, primary power, grounding, and
components.
either
causing
electromagnetic
interconnections.  The methodology for isolation and
interference (EMI) or increasing EMI vulnerability. The
location of these types of EMC problems is similar to the
usual cause of loose connections.
poor ground
fixed site, but the probability of occurrence is much
connections, corroded terminals. cold solder joints, and
greater.
faulty cables is lack of proper maintenance. It is worth
noting that the installation of new equipment with
(3) The  source  of  intersite  unintentional
accompanying cables and jumpers can be the cause of
interference at a non-DCS transportable or mobile site is
EMC problems not previously present. Existing cables
also more difficult to isolate and locate than at a fixed
not associated with the new installation can be relocated
site. Since the system is the new element in the area.
or damaged to the degree where interference occurs.
the number and location of potential interferers in the
All new installations or replacement work should be
area are unknown unless an extensive site survey and
checked for their effects on other circuits.
frequency monitoring have first been performed.
(3) Intersite unintentional interference at a
d. Unintentional EMC Versus Intentional ECM
DCS fixed site is generally caused by installation or
Problems.
operation  of  new  equipment,  new  frequency
(1) When outside interference is detected,
assignments, or weather conditions. This results in a
the operator must determine whether the problem is
site becoming more vulnerable because of physical
attributable to unintentional causes or whether jamming
changes at the site (such as grounding) or abnormal
or intentional intrusion is occurring. Where the latter is
propagation (such as ducting).  Frequently, when this
suspected, any action taken must be such as to deny
occurs, the interfering source may be identifiable from
the enemy any intelligence regarding the effectiveness
frequency assignments.  However, if the interfering
of his actions, or the fact that they have been detected.
frequency is assigned to the intefering station, and the
(2) The procedures for dealing with enemy
latter is operating within authorized bandwidth, mode,
action  are  prescribed  by  AR  105-3,  "Reporting
and emitted power limits, the solution will lie in obtaining
Meaconing, Intrusion, Jamming, and Interference (MIJI)
a different frequency assignment for one station or the
of Electromagnetic Systems;" AR 105-87, "Electronic
other. If this is not possible, steps must be taken to
Warfare;" and DCAC-310-70-6, "Jamming or Sabotage
minimize the degree of coupling (for example, slight
of DCS Telecommunications Facilities."
rotation of receive
4-2


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