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.