When you listen to an AM radio during an electrical storm, you may notice a lot of noise. What you are hearing is white noise generated by far off lightning flashes.

RFI stands for Radio Frequency Interference, it is undesirable noise which interferes with transmissions in the radio freqency spectrum. In order to understand RFI, you need to understand that everything that has alternating current running through it radiates RFI, but most create very little.

Predicting noise patterns is a science unto itself. Every alternating current signal that isn't a pure sine wave (which would be pretty much any signal) generates an infinite number of side frequencies (smaller signals above and below the main, or "fundamental" frequency}. Each side frequency may be smaller than, or equal to the fundamental in magnitude, depending on the signal shape, frequency, magnitude, and duty cycle.

Enter lightning. Lightning can be described as a pulse train, much like a square wave, with a frequency of zero, and a duty cycle of zero

Regular Pulse Train:
Mag
 |                          frequency=0.1
1|..      ....      ....    duty cycle=40%
 | .      .  .      .  .
 | .      .  .      .  .
0+-----------------------t

Lighting
Mag
 |                          frequency=0
1|        .                 duty cycle=0
 |        .             
 |        .             
0+-----------------------t

To calculate the magnitude of the side frequencies for a pulse train, we use the sinc (cardinal sine) function.

Mag=sinc(Duty Cycle X π)

For a regular pulse train, the magnitude of our side frequencies will go up and down, decreasing slightly each time until they reach zero. Lightning, however, is a special case. The side frequencies will be 0 hz apart, and equal to the fundamental in magnitude! What does this mean? Lightning is an AC signal of 0 hertz, with side frequencies stretching off into infinity, all with the same magnitude as the original signal, i.e. White Noise, omnipresent on all frequencies at the same time.

How do you tell if there's a thunderstorm nearby? Just turn on the AM radio to any station, and you should be able to hear blips, coming from flashes of far off lightning. AM radio works better than FM radio as a lightning observatory, as it is more susceptible to noise than FM.

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