Radio Frequency Interference

There can be a few reasons as to why Communications equipment cause interference to other communications or digital equipment as well as to why radio communications equipment receive intereference.

Sympton: For instance, the most common type of interference being caused to Televisions by Communications equipment. Different types of lines on the TV also indicate different types of interference. One main reason of interference is the distance between the your transmitting aerial system and the television reception aerial.

Cure: One cure can be to place a 'high-pass' filter in series with the television co-axial cable going into the television. I wont go into detail but the arrangement of Capacitors and inductors in the filter pass higher frequencies while attenuating lower frequency signals.

Symptom: Audio equipment intereference, such as HI-FI systems.

Cure: Capacitors used to pass only audio frequencies but not allowing higher frequencies to pass wired into the audio system.

Cars are also a breeding ground for interference, but not generated by the CB or radio equipment onboard but received by it instead. The car ignition system generates noise co-inciding with the engine rpm because of the high voltages switching to fire the spark plugs in the engine. Alternator whine is another typical and annoying interference into the CB equipment. Again capacitors can be used to suppress alternator whine.

If you have problems with engine noise in a mobile installation, there are several things you can do to reduce or eliminate the problem.

Make sure you've completely installed your CB and the antenna is grounded properly. Check the amount of noise with and without the antenna connected. If the noise is greater when the antenna is connected, the source is probably the ignition system. If the noise remains unchanged after disconnecting the antenna, the source is probably in the CB's power supply.
For ignition noise, there are several parts of the vehicle that can act like RF noise transmitters if not properly grounded. Check (and connect to ground on the chassis/frame, if necessary) the engine block, hood, muffler, and exhaust pipe.
Ignition noise can be further reduced by installing "magnetic suppression" spark plug cables. (These are significantly more expensive than normal spark plug cables.)
For power supply noise, check if you have a capacitor attached to your ignition coil, alternator, and distributor. These can act as filters right at the source of the noise.
Ensure that your antenna feedline and power lines are as far as possible from any of the noise sources mentioned above, especially the engine block.
A shielded power cable can help as well. Make sure it is of sufficient guage to handle your radio's power consumption and that the shielding has a good, strong connection to ground on the vehicle chassis or frame.

Of course, the best performance can be obtained from a more expensive radio with better noise suppression circuits. These measures, however, will improve the engine noise levels for nearly any mobile radio.

Engine management systems can also cause interference because of their high switching rates in digital logic. However, engine management systems are also very susceptable to interference from radio transmitters but rarely fail to operate properly as a result. Avionics (Aviation electronics such as auto-pilot, and landing systems) do however suffer seriously from interference from digital equipment. Avionic systems are very sensetive.

One web page visitor by the name of Chris even mentioned this: "In one of your sections you mentioned interference with engine management systems. I've got a good one for you. A friend of mine had a CB in his car and whenever he keyed it the anti-lock brakes started to pulse. It only did it on a certain channels... ". How's that then?

Updated on Saturday, April 27, 2002