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Stations and systems affected.
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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RF signal level


TM 11-490-5
identification of cable grounding problems consists of
(b) Initiate action for possible sector
disconnecting  the  cable  between  the  sites  and
blanking of the radar signal or other reduction in the
measuring the resistance between pairs and the
radiation of this signal.
resistance to ground for each cable pair.  A suitable
(c) Relocate either the communications
ohmmeter, impedance bridge, or megger can be used
site or the radar, if practicable.  In some cases, the
for this test. Interpair resistance indications of less than
solution could be as simple as a relocation of receive
10,000 ohms per loop mile or resistance readings to
ground of less than 50,000 ohms is normally indicative
(3) Office equipment.  Qualified repairmen
of a faulty cable or cable pair. Coaxial cables are very
must be consulted to eliminate problems from these
susceptible to damage caused by heat, compression, or
machines, including the assurance that the internal
abrasion.
When these occur, the results can be
ground
connections
are properly
maintained.
mismatches, loss of desired signal, or introduction of
Procedures follow:
interfering signals. A floating shield caused by damage
(a) Identify specific equipment causing
to the cable may result in unwanted signals being
problems. A 3-wire power cord properly grounding the
introduced into or radiated from the equipment or cable.
equipment may solve the problem. If it does not,
(c) Power line.  If a battery operated,
(b) Report interference.
multiband radio is available, use it to trace the source of
(c) Initiate request for shielding or
powerline problems, particularly if a defective insulator
relocation.
or transformer is suspected as the source.  Usually,
(4) Power lines.
such interference will increase in intensity as the source
is approached with the radio. Poor ground connections
(a) Isolate the cause of interference to
at the power poles can result in a power frequency hum
the most likely source, such as a power transformer or
in the communications equipment as a result of the
insulator, with procedures given in paragraph 43c(3)(c).
common impedance in the ground loop. A rapid means
(b ) Report  this  information  to  the
of isolation, if this is suspected.  particularly in dry,
responsible authority, such as the local facility engineer.
sandy soil, is to thoroughly wet the ground around power
(5) Cables.
Damaged cables must be
poles close to the station with a saline solution. If the
replaced, or spare cables used. When the cause of the
problem .is powerline grounding, the interference will be
cable damage has been determined, appropriate steps
reduced or eliminated. In the case of a broken insulator.
can be taken to ensure that it does not happen again,
hitting the suspected pole with a sledgehammer will
including the erection of signs and barriers to prevent
cause a variation in the interference.
vehicular traffic over the cables. Corroded and loose
d. Remedial Procedures.  The procedures which
connections  can  be  corrected  by  cleaning  and
operating personnel can use in the correction of
tightening.  If this is not possible, reroute to another
interference problems may be limited by available test
cable pair, if available.
equipment and maintenance level authorization. The
(6) Circuit components.
Problems which
following procedures, where authorized. may be used
have been traced to faulty circuit components, such as
for the correction of the corresponding interference
line amplifier modules.
will generally require the
sources given in paragraph 4-3b (2) (b ):
services of a repairman to test at the appropriate
(1) Ignition noise.
monitoring jack and replace or adjust as necessary.
(a) Identify the source of the ignition
Patching may be required to bypass the particular
noise to the specific source such as privately-owned
equipment involved while this maintenance is being
vehicles, lawn mowers, or similar gasoline-powered
performed.
engines.
(7) Equipment grounding.
All electrically
(b) Report this information to the level
operated equipment within the site must be maintained
where remedial action can be taken. Such action may
at ground potential by interconnection with the common
include restriction of the ignition noise sources from the
station ground. At DCA facilities, a station ground must
communication site, installation of noise suppresser
be maintained at the DCS standard and periodically
devices, or restricting their use to periods of low
measured by the supporting maintenance or engineering
communications usage.
facility. Non-DCS transportable or mobile installations
(2) Radar.  Radars are sometimes located
also require good grounds. but have special grounding
where they will sweep the area of a communications
problems associated with their temporary deployment.
site. In the event that radar interference is encountered,
Grounding rods furnished with the equipment may not
the following procedures should be followed.
suffice in some areas. In sandy soils. use of buried
copper plates to increase the contact area with the soil
(a) Report  interference  to  proper
may be necessary.  Also, all grounds may require
authorities.
regular attention in accordance
4-14


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