Motorola GM1200E Installation Manual page 30

Detection of Noise Sources
Check the installed radio for conducted and inducted noise by using an unmodulated signal
generator to supply a clean RF signal through a coaxial cable directly to the antenna connector on
the radio set. This should prevent radiated noise from entering the receiver and masking conducted
and induced noise. Make certain that the signal generator is not microphonic and is placed away
from the noise of the engine and exhaust.
Operate the controls for fans, blowers, power windows, headlamps, turn signals, windshield wipers,
etc. and listen for the presence of noise in the received radio signal. This will allow the source of the
interference to be determined. Some noise sources cannot be turned on and off at will. These
sources will need to be attacked piecemeal on a trial and error basis. . . alternator whine, voltage
regulators, electric fuel pump etc. Remember that induced noise can result from radio wiring being
too close to other vehicle wiring. This problem is most easily solved by proper cable routing at the
time of installation.
Radiated noise must be attacked after conducted and induced noise has been satisfactorily
suppressed. The vehicle ignition system is usually the primary source of this interference. Antenna
placement can be critical in some installations. Remember that static discharge generates static or
radiated noise and probably will only be generated when the vehicle is in motion.
Since radiated noise interference is most noticeable in weak signal areas, it is suggested that
suppression be finalised while listening to a weak "on frequency" signal. (The squelch should be
"open" so that even marginal signals can be heard.)
2-6 Reducing Noise Interference in Mobile Two-Way Radio Installations