Radio noise

Radio noise is a combination of natural electromagnetic atmospheric noise ("spherics", static) created by electrical processes in the atmosphere like lightning; human-made radio frequency interference (RFI) from other electrical devices picked up by the receiver's antenna; and thermal noise present in the receiver input circuits, mostly caused by the random thermal motion of molecules inside resistors.

When this ratio is below one (0 dB) the noise is greater than the signal, requiring special processing to recover the information, if that is even possible.

At frequencies below about 20 MHz the ionosphere traps radio waves inside the atmosphere – the same phenomenon that enables continent-wide up to world-wide communication in the shortwaves.

Human-caused electromagnetic interference (EMI) can disrupt the operation of any electronic equipment in general, not just radios, causing malfunction.

Cosmic background noise is experienced at frequencies above about 15 MHz when highly directional antennas are pointed toward the Sun or to certain other radio-bright objects in the sky, such as the center of the Milky Way Galaxy, or the planet Jupiter.

Atmospheric noise and human-caused noise as a function of frequency in the LF, MF, and HF radio spectrum according to CCIR 322. [ 1 ] The vertical axis is in decibels above the thermal noise floor. The graph shows that as frequency drops atmospheric noise dominates other sources, and that as frequency rises, although all noise quiets, human-caused noise exceeds atmospheric noise.