Noise (radio)

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Atmospheric noise as a function of frequency in the LF, MF, and HF radio spectrum according CCIR 322. The vertical axis is in decibels above the thermal noise floor. It can be seen that as frequency drops atmospheric noise dominates other sources.

In radio reception, noise is the superposition of white noise and other disturbing influences on the signal, caused either by thermal noise and other electronic noise from receiver input circuits or by interference from radiated electromagnetic noise picked up by the receiver's antenna. If no noise were picked up with radio signals, even weak transmissions could be received at virtually any distance by making a radio receiver that was sensitive enough. In practice, this doesn't work, and a point is reached where the only way to extend the range of a transmission is to increase the transmitter power.

Thermal noise can be made lower by cooling the circuits, but this is only usually worthwhile on radio telescopes. In other applications the limiting noise source depends on the frequency range in use. At low frequencies (longwave or mediumwave) and at high frequencies (shortwave), interference caused by lightning or by nearby electrical impulses in electrical switches, motors, vehicle ignition circuits, computers, and other man-made sources tends to swamp transmissions with thermal noise. These noises are often referred to as static. Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. At very high frequency and ultra high frequency these sources can still be important, but at a much lower level, such that thermal noise is usually the limiting factor. Cosmic background noise is experienced at frequencies above about 15 MHz when highly directional antennas are pointed toward the sun or to certain other regions of the sky such as the center of the Milky Way Galaxy.

Electromagnetic noise can interfere with electronic equipment in general, causing malfunction, and in recent years standards have been laid down for the levels of electromagnetic radiation that electronic equipment is permitted to radiate. These standards are aimed at ensuring what is referred to as electromagnetic compatibility, or EMC.

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