Where does the radio static sound come from?

Question

Why do FM radios give a static sound when tuned to a blank station (and older TV sets when TV used to be analog)? Where does the static noise come from? Why would it just be silent if there is no signal present on a certain frequency?

Answer 1

In radio reception, noise is the superposition of white noise (also called "static") 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 was 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.

Answer 2

There is no such thing as total quiet, anywhere and at any frequency, there is what is called the noise floor. Besides noise from man made devices, the stars and sun make a lot of electrical noise. Anything above absolute zero emits noise. It's sort of like heat. This is why you can not measure beyond a very finite precision in a given amount of time / bandwidth.

So when you tune between stations the automatic gain control goes into max boost and brings up the noise floor. Normally, the noise is a small fraction of the station signal strength so the AGC backs off and you don't notice the noise. The radio circuits themselves generate noise, ie they have a noise floor of there own.

http://en.wikipedia.org/wiki/Noise_floor

http://en.wikipedia.org/wiki/Thermal_noise

http://en.wikipedia.org/wiki/Uncertainty_principle

http://en.wikipedia.org/wiki/Automatic_gain_control

Answer 3

if any of the wiring from the headlights runs on the brink of any speaker or radio cord, or perhaps the aerial cord you will get interference if the present is extreme sufficient. carry on with the wiring, if it runs next to any radio cord flow it away.