How to minimize radio interference

Radio interference is the result of stray magnetic fields created by various pieces of electrical equipment in the car. It is most obvious when tuning into a weak station and when you have to turn the volume up high to make the broadcast audible.

The ignition circuit is the main cause of interference, followed by the dynamo or alternator; electric motors, flashing indicators, the voltage regulator and loose contacts anywhere in the car’s electrical system may also cause problems. Finally there is electrostatic interference: the result of static build-up in the wheels which discharges to the car bodywork.

To check how much of the noise coming through the radio is interference and how much is really part of the broadcast, tune to a point between stations. Have the volume control at various settings, rev the engine and go for a short drive. Try to operate all the car’s electrical equipment in turn – indicators, heater, wipers etc – to establish what is causing the interference. Then disconnect the aerial and carry out the same checks again, noting the results. If the noises occur both when the aerial is connected and when it is disconnected, the interference is coming into the radio through its supply lead; if the noise disappears when the aerial is disconnected, IT is coming in through the aerial; if it occurs when the car is travelling at a reasonable speed, the noise is likely to be caused by static build up. You can check this by applying the brakes, when the noise should stop; this is because the static build-up is earthed to the car chassis by the brake linings or brake pads.


There are several methods of cutting down interference. You can fit resistors, capacitors or a radio frequency choke or, in the case of a car with a glass fibre body, you can set up a screen between the source of the interference and the radio or aerial feed wires.

RESISTORS These damp down the oscillations which cause interference without restricting the current flow. They are particularly useful for suppressing high tension leads and plug caps. Modern cars have these already fitted; if you have an older car, you can buy a set of suppressed HT leads and simply fit these in place of the old ones, or you can insert line suppressors in your existing leads.

In each end of a line suppressor there is a screw with its point facing outwards. To fit, cut the HT lead as near to the spark plug as possible and screw the ends of the lead into the line suppressor. It is also advisable to fit a line suppressor in the HT lead between the distributor and the coil. Capacitors These do much the same thing as resistors, but in a different way. Instead of being put into the circuit, they lead from a point in the circuit to earth. A capacitor will not allow direct current to flow, but will allow the interference causing alternating current to short out to earth.

A capacitor has a bracket with an elongated slot fixed to its body and a single lead with a spade terminal. To fit a capacitor, bolt the bracket to a suitable earth point and slide the terminal over the existing terminal on the component you wish to suppress.

It is usually sufficient to fit capacitors to the dynamo/alternator and to the coil; but in certain situations you may also have to fit them to other electrical components. For example, if the aerial lead runs close by the wiper motor or if the radio is very near the flasher unit, interference will result when these are in use, so you must fit capacitors.

Capacitors are measured in microfarads -units of electrical capacity. The following is a brief guide to the size of capacitor for various components and where it should be fitted. Recommendations can only be general, however, and it may be that a manufacturer has special reasons why a capacitor should not exceed a certain value; so always check with your car handbook or a specialist dealer.


If your car has such special equipment as electronic ignition, capacitors must not be used in the ignition circuits. This is not a disadvantage from the interference point of view, however, since these units are always suppressed by the manufacturer.

Radio frequency choke This offers the opposite effect to that of a capacitor: a coil of fine wire allows direct current to pass,, but prevents the passage of alternating currents, particularly those of high frequency. The radio frequency choke is fitted in the supply lead to the radio; simply cut the lead and insert the choke with suitable connectors. SPRING-LOADED CONTACT KIT This is fitted to the brake drum and earths wheel static to the chassis. It is not necessary to fit contact kits to disc brakes since the static is automatically earthed by the disc pad rubbing against the disc. The positioning of these kits varies with the make of brake and fitting is best left to your garage.

SCREENING Cars with glass fibre bodies often require more than the standard suppression equipment. The usual answer is to line the inside of the engine compartment with aluminium foil or perforated zinc sheet; this prevents the passage of magnetic radiation as long as it is well earthed. This is achieved by soldering one end of a wire to the screening material and taking the other end to a suitable earthing point on the metal chassis. To be completely effective, the aerial earth connection must also be connected to the screen.

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