Jamming is suspected

TM 11-490-5

multiplex equipment are affected by the interference

(d ) Voice. Voice interference can occur, of

problem.

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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