Clear reception of radio signals depends upon the ability to exclude unwanted radio frequency energy that interferes with desired transmissions. Interfering signals include those from another transmitting station using the same frequency in another part of the world, or else one whose assigned frequency is so close that the harmonic content of the unwanted signal overrides or beats with the wanted wave. A further type of interference may result from cross-modulation with a more powerful wave from another station. (This problem should be distinguished from that of radio noise or atmospherics which may also interfere with radio reception).
Such interference may also be caused by freak weather: atmospheric thunderstorms and tropospheric conditions (causing reception of normally distant and non-interfering stations).
Other sources of interference include industrial, medical scientific and radio control transmitters such as garage door openers, microwave ovens, radio controlled models, radio and television receiver oscillators, induction heating machinery, wireless intercom and diathermy and other medical apparatus. Radio frequency transmitters include power tools, automobile ignitions, appliances, fluorescent and neon lights, microcomputer, power transformers and touch control lamps.
The victims of radio frequency interference include TV sets, am and fm radio receivers, amateur radio receivers, medical equipment, motor vehicle anti-skidding devices, hi-fi audio systems, intercom systems, blasting devices, smoke detectors, video cassette recorders and automobile cruise controls.
It has been estimated that at one stage the USSR spent more resources on jamming the BBC World Service to the USSR than the BBC spent in producing it. Use of electronic equipment (mobile phones, disk players, laptop computers) is increasingly restricted on airplanes because such equipment may affect the sensitive on-board navigational and communications systems.
Man-made intentional interference adds to the congestion in telecommunication frequencies. For political reasons, some countries are jamming frequencies on which foreign broadcasters are beaming messages to their homelands.