85
The SGC Building, 13737 S.E. 26th St. Bellevue, Washington 98005 USA
P.O. Box 3526, 98009 Tel: (425) 746-6310 Fax: (425) 746-6384
© 1997 SGC Inc.
If you can't beat it, avoid it. Some types of interference are centered around particular frequencies or bands.
For example, fluorescent light noise is often worst at the lower frequencies. If you can move up into the 10 MHz
(and above) region, this type of interference will often drop significantly. Of course, these types of interference can
vary, so you will have to check the bands and determine which frequencies are best. Of course, your ability to
change frequencies will depend on what frequencies you are allocated to operate on.
In vehicles with traditional gas-fueled engines, ignition noise is one of the most damaging forms of
interference. Without some sort of ignition noise suppression, it will be difficult to receive, and thus to make any
contacts on the HF bands.
Quite a bit of engine noise can be eliminated if various parts of the vehicle are isolated and bypassed. Just
purchase some 0.01- to 0.1-microfarad 100-volt ceramic disk capacitors from an electronics parts house (such as
Radio Shack). Then install one capacitor inline with each lead from the battery and from the alternator. Make sure
that all solder joints are solid, clean, and sealed. If the ignition system is noisy, add a
capacitor in line with the primary side of the ignition. Don't put capacitors in any other
location around the ignition or you might reduce and delay the voltage to the ignition. If this
is the case, you will have countered the effect of the ignition and the car might not work!
Another method to reduce noise is to place resistance in line with the spark plugs. The resistances can be added
near the spark plugs, in ignition cable, or in the spark plugs themselves. When resistors are added near the spark
plugs, they are often installed at the distributor towers or spark plug terminals, moulded into the distributor rotor,
towers, or the center contact button.
The most commonly used ignition noise suppressor is resistance ignition cable. This cable is available in either
low-resistance types (3000 to 7000 ohms) or high-resistance types (6000 to 12000 ohms).
The last method is to use noise-suppressing "resistor" spark plugs. These special spark plugs contain resistance
elements to reduce radio-frequency radiation from the ignition coil and to virtually eliminate the high-frequency
part of the spark.
CHANGE OF FREQUENCY
Engine noise can
be isolated and
bypassed.
ISOLATION AND BYPASSING
NOISE REMEDIES AT THE NOISE SOURCE
To Next Page