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TM 11-490-5
interference is present in the receiver output would be to
receiver output is now interference free, the receiver
first check to determine whether the assigned RF is
was at fault. If the signal is still not normal the receiver
interference free prior to conducting any further isolation
antenna system should be checked. If available, use
another receive antenna system on circuit to minimize
circuit downtime during testing.
(d ) Multiplex equipment.  In simple terms,
3.  The above procedure is closely related to
multiplex equipment combines multiple input signals
that described in paragraph 4-3b (1) (a ), "Determine
(normally discrete audio channels) into a composite
origin of the interference." Both are examples of similar
signal that is used to modulate a transmitter without any
isolating techniques. There are other variants.
interchannel interference and reverses the process at
the receiver by converting the composite received signal
(b) Transmitter.
Using  the  orderwire,
into multiple channel output signals.  To isolate an
operational personnel at the distant transmitter station
interference problem to the multiplex equipment, the
should be requested to check whether transmitter
output and input signals are checked to determine
operation is normal. Generally, transmitter checks are
whether they are normal. If the input is normal and the
limited to assuring that modulation is normal and at the
output has interference, the problem is in the multiplex
proper level and that all monitoring indicators and
equipment.  If the system uses double multiplexing
instruments are reading normally (for example, the drive
(group and supergroup), composite input/output signals
level is correct, the transmitter is on the correct antenna,
of each of the multiplex elements should be checked in
and VSWR readings are normal).  At some stations,
a similar fashion to that described above. Generally,
spectrum analyzers are available, permitting visual
grounding problems or direct external interference
observation of the output signal. A spare receiver or
affecting a multiplex equipment will interfere with all
broadband portable radio with proper input attenuation
output channels of the specific multiplex element.  If
to prevent receiver overload or damage can provide an
only one output channel has interference, the most
indication of proper transmitter output, to include the
probable cause is a component malfunction within the
transmitting antenna system. If the transmitter output is
equipment or a misadjusted drive level.  Individual
normal, the interference problem is associated with the
channel drive levels are normally very critical on the
antenna systems or the transfer media.
transmit multiplex. Check to be sure these drive levels
(c ) Antenna.
are  adjusted  to  specified  values.
In  most
1. Generally,  transmitting  antenna
communication systems some interference (noise and
problems will be indicated by changes in the readings of
tines) is usually present. In a well-designed system, it
the output and loading at the transmitter. Interference
would be usual for interfering noise to be lower in level
problems associated with receiver antennas generally
than the system's intrinsic noise and thus could be
will cause fluctuations in the receiver signal level.
ignored.  However, if noise levels increase above
However, interference in the incoming RF signal can
predetermined levels, action is necessary to determine
cause similar indications. Interference problems could
whether a maintenance problem or an interference
arise from nonlinearities in multichannel HF antenna
problem exists. Up to a point, isolation procedures for
isolation or multicoupler devices.
both types of problems are identical and along the lines
2. If antenna problems are suspected,
described above. Further information which will assist
another antenna can be substituted, or repair personnel
in isolating the problem is as follows:
can be requested to make a voltage standing wave ratio
1. Type of noise.
-(VSWR) or noise power ratio (NPR) test on the antenna
(a) Whether the noise is wideband or
3. If the distant transmitter output and
(b) The  frequency  range  of  the
the antenna systems are normal, the interference is
entering through the RF medium. A spare receiver or
(c) Whether a tone is involved and
battery operated broadband portable radio with some
whether the tone appears in more than one channel.
type of directional antenna is probably the most rapid
Also, whether the tone is a modulated frequency of a
means of isolating the source, direction, and type of
tone in the channel.
interference.  The experience and capability of the
operating  personnel  are  most  important  in  the
(d) Whether the interference is a signal
recognition of the type of interference and its probable
interference such as music, voice, on-off keying.
If  appropriate  monitoring  equipment  is
(e) Whether the noise seems to be
available, the logical procedure when
powerline related.
2. Levels.
(a) Whether the receive signal power
levels are normal.

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