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TM 11-490-5
multiplex equipment are affected by the interference
(d ) Voice.  Voice interference can occur, of
course, as a result of crosstalk, or from other
transmissions; however, there is a possibility of the
(a ) Single  subscriber  affected.
If  the
interference being the result of a voice-modulated
interference affects a single subscriber, the trouble
jammer. Figure 4-3 shows the oscilloscope display as a
source is likely to be between the subscriber unit and the
result of such a jammer signal interjected with an AM
communications site or the appropriate patching facility
signal.  The sound of such a signal at the receiver
at the site. However, a single-channel fault could also
output would be that of two signals being heard at the
be the source of interference.  A typical isolation
same time. If the signal is strong and no known friendly
procedure is as follows:
station is nearby, jamming should be suspected. If the
1. If the link to the subscriber is by cable, first
interfering signal is weak, fades irregularly, or occurs
substitute spare subscriber equipment, including security
only at a certain time of day, it is more likely the result
devices. If interference is still present, check the cable
of ionospheric refraction from a distant transmitter (HF
in accordance with paragraph 4-3c (3) (b ).
and low end of VHF). Any interfering signal, digital or
2. If the subscriber is linked by radio, the output
voice modulated, should be compared with normal
of the subscriber's receiver must be checked to
traffic through the station.
Friendly interference
determine whether the problem originates within or
commonly encountered should also be compared.
external to the subscriber station. If the receiver output
(4) If jamming is suspected.
is normal (correct signal content and level) then
(a ) Determine frequency of the interference.
interconnecting cables and end-instruments may be
faulty.  Noise must be checked, substituting spare
Use a spare receiver connected to a separate antenna.
equipment, if available, to maintain service.  If the
or a battery powered multiband radio to determine
interference is introducted by RF means (para 43b (1))
whether the interference is tunable; that is, whether the
and no interfering station can be discerned, steps must
interference appears on the desired signal RF channel
be taken to determine whether a fault exists in the
only, on adjacent RF channels, or at an image
distant transmission.  Check in accordance  with the
frequency. To determine whether transmissions at an
procedures described in paragraph 4-3c (2).
image frequency are responsible, tune the radio above
(b ) Multiple subscribers affected.
If the
and below the desired signal tuned frequency by an
amount equal to twice the IF of the station receiver.
interference affects multiple subscribers, checks will
The characteristic sound of the interfering signal should
depend upon whether or not interference is present on
increase at one or the other of these settings if the
the incoming signal. Isolate as follows:
image frequency from another transmitter is involved. If
1.  Incoming signal interference free.  If a
the interference is found to be cochannel with the
problem affects various subscribers of the local station.
desired signal, a strong possibility exists that the
and the distant station has checked and confirmed that
interference is intentional.
their outgoing transmission is correct.  and no other
external interference can be detected, the interference
(b) Determine direction of interference. Use a
is local in origin, most likely occurring within the station
directional antenna in conjunction with the receiver or
demultiplexing equipment at the group level.
radio of step a above to identify the direction from which
determine, check the input to the particular group
interference is coming. Based on the results of steps a
demultiplexer in question in accordance with paragraph
and b, check with the appropriate authority to determine
4-3c (2) (d 1).  If the signal is clear, the problem
whether a friendly station would normally be operating
originates in the demultiplexer equipment.
If the
from that direction and on the frequency encountered.
interference is present.  check the preceding patches
c. Isolation Procedures. The first step in the
and terminals in accordance with paragraph 4-3c (3) and
isolation of an interference problem is to examine the
also check for the usual problems of loose connections
degree to which the interference has affected the
and corrosion.
communications system; that is, how many subscribers
2. Incoming signal contains interference. If a
are reporting problems.  Next, the problem must be
check of the incoming signal reveals that interference is
isolated to a major system, then further isolated to a
present,  and  if  the  interference  affects  multiple
major component within the system. These procedures
subscribers, the trouble most likely originates in the RF
are given in this order in the paragraphs that follow.
link with the preceding station or in that station's RF
(1) Isolation based on extent of interference.
equipment. Consider a typical example where a carrier
The following procedures are based on whether one or
system employs voice frequency telegraph over a 4-kHz
multiple subscribers being serviced by a particular
audio channel and

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