Radio Interference

electromagnetic radiation that acts upon the circuits of a receiver in such a way as to impede correct reception of a signal. Electrical processes in the circuits that inhibit correct reception and are not associated with the signal by a known functional relationship are also sometimes referred to as interference, as are distortions of the signal that occur during the propagation of the radio waves.

Interference causes random, or unpredictable, distortions of the shape of the received signal. The distortions can take the form of, for example, extraneous sounds in a loudspeaker during the reception of transmissions containing speech or music, misprints in the received text of telegrams, or distortions of the shape of a received image on the screen of a picture tube. Radio interference is the principal factor limiting the quality of the reproduction of the transmitted signal by the receiver for a given transmitter power and a given distance to the transmitter. In other words, interference limits the distance of signal transmission for a given quality of reproduction.

The following types of interference are distinguished according to the origin of the disturbance: cosmic interference, atmospheric interference, industrial interference, jamming, interference from other radio stations, interference owing to the propagation characteristics of the radio waves, and the receiver noises.

Depending on how the interference affects the signal, a distinction is made between additive and multiplicative (nonadditive) interference. Additive interference appears independently of the signal and the signal and additive interference are summed. Multiplicative interference arises only in the presence of a signal and results in irregular variation of the signal level. In the case of multiplicative interference, an increase in the amplitude of the received signal does not improve the quality of reproduction of the signal. The opposite is true of additive interference. The internal noise of the receiver is an example of additive interference; fading is an example of multiplicative interference. In most cases radio interference can be regarded as independent random processes with different probabilistic characteristics. As a rule, these characteristics differ from the characteristics of the signal. Various methods of suppressing interference are used to reduce interference effects.