(1) The two sources of interference are internal and external.
the interference or suspected jamming can be eliminated or substantially
reduced by grounding the radio equipment or disconnecting the receiver
antenna, the source of the disturbance is most likely external to the radio.
If the interference or suspected jamming remains after grounding or
disconnecting the antenna, the disturbance is most likely internal.
is, it is caused by a radio malfunction.
External interference must be
checked further for enemy jamming or unintentional interference.
(2) Unintentional interference may be caused by other radios, some
other type of electronic or electrical equipment, or atmospheric conditions.
The battlefield is crowded with radios and other electronic equipment.
Unintentional interference is virtually unavoidable. The static electricity
Unintentional interference normally travels only a short
distance. A search of the immediate area may reveal the source of this type
Moving the receiving antenna short distances may cause
noticeable variations in the strength of the interfering signal.
variations normally indicate unintentional interference.
Little or no
variation often indicates jamming.
b. Some jamming takes advantage of the design characteristics of
certain radio equipment, such as the AN/VRC-12 in the new squelch on mode.
To activate these radios, a 150-Hz tone must be transmitted to them along
with the transmitter carrier signal. These radios lock onto the strongest
signal they receive.
c. When an enemy transmits a strong signal to these equipment items,
they will lock onto the strong signal. If a friendly signal, along with the
150-Hz signal, is transmitted to these equipment items, they will not
receive it as long as the jamming signal is stronger than the friendly
This type of subtle jamming is called squelch capture.
operator cannot detect this method of jamming.
It may appear as if the
radio is faulty because the operator cannot receive any friendly signal.
Report suspected enemy jamming and unidentified interference that disrupts
This applies even if the effects of the jamming or
interference are overcome.
d. An enemy can use powerful unmodulated or noise-modulated signals.
You should note that modulated jamming signals are characterized by obvious
interference noises. The following procedures help operators determine
whether their radios are being jammed.
function control from the SQUELCH ON to the OFF
(a) Turn the
(b) A lack of noise may mean an unmodulated jamming signal is
jamming a radio.
Temporarily disconnect the antenna.
If normal static
noise returns when the antenna is disconnected, the radio is likely being
jammed by an unmodulated signal.