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:-
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.
A low pass filter such as that supplied in the interference suppression kit or similar should
be fitted at the coil end of this wire. The earth connection of the filter should be short and
well-bonded to the coil body.
The charging system circuit, consisting of either generator or alternator and a regulator may
also be split into three parts:-
A low pass filter such as that supplied in the interference suppression kit or similar should be
fitted to the main battery lead at the alternator. The filter must be rated for the maximum
current available from the charging system. The earth lug of the filter should be attached to
the alternator body or the engine block.
This wire carries switching pulses that often contribute noise to the receiver. Suppression
using capacitors or filters must not be attempted since damage to the regulator may result.
Separate the wire from all other wiring, keep it as short as possible and, if longer than about
300mm it should be shielded as described above.
These are normally adequately suppressed using good low-inductance bypass capacitors.
To be effective, these capacitors must connect to the wires to be suppressed and to chassis
with very short leads. For this reason, the 'pigtail' style of suppressor capacitor often used
with mf broadcast receivers is generally ineffective at HF.
Ignition systems
Coil to battery wiring
Battery charging system
Alternator / generator to battery wiring
Alternator to regulator control wire (generator field wire)
Other regulator wires
BARRETT 950 HF TRANSCEIVER
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