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Noise Suppression
Noise generated by motor or electrical accessories on the vehicle may cause
objectionable interference to the received signal. This noise enters the receiver
either by means of the battery leads or the antenna system. Providing that the
recommendations concerning battery wiring given earlier in this book are followed,
noise injected via the battery lead is unlikely to be significant. Most noise problems
result from pick-up by the antenna. Practical cures involve either preventing the
noise from being generated or minimising it from being radiated by the wiring
connected to the noise source.
Interference Suppression Kit (Barrett P/N BCA90017) is available to assist in
noise suppression and contains filters, suppressing capacitors, earth straps and
fitting instructions.
The techniques involved in noise suppression include re-routing of wiring, screening
and the use of filters. It is also necessary to maintain all electrical equipment in
good working order as worn brushes, loose connections and the like, will increase
the amount of noise generated.
Before attempting to cure a noise problem, the source (or sources) of noise must be
identified. Ideally, there should be no difference between background noise in the
receiver with motor and accessories on and that with motor and accessories off.
If a detectable difference does exist, turn off all accessories one by one until a
change in noise results. Continue, noting each contributing unit until there is no
detectable difference from the "all off" noise level. (For accessories such as
alternator, motors, instruments etc. a wire or drive belt may have to be temporarily
removed for this assessment). After identifying each noise source, they can be
worked on one at a time until an acceptable level of suppression is achieved.
Another approach to this problem is to remove or disconnect all possible sources of
noise then replace and suppress them in turn.
Some suggestions for suppressing particular noise sources follow:-
Ignition Systems
All high tension wiring from the ignition coil through to the spark plugs should be
kept as short as practicable, clean, and as close to the engine block as possible.
The cable should be an impregnated neoprene resistive type and the coil must be
either mounted on, or immediately adjacent to, the engine block. The low tension
wire from the coil to the distributor contact breaker points must be as short as
possible, and not included with other wires in a harness or loom. This wire must be
shielded if more than 300mm long. Twin flex or 'figure eight' cable provides a
suitable shield when connected in lieu of the original wire. This method is useful for
shielding other wires suspected of radiating noise. Do not ignore the wire to an
electric tachometer if one is fitted.
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