The first Schumann resonance is not at (300000/40000)=7.5 Hz, but
higher, usually around 7.7-7.8 Hz.
I was astounded when I observed it for the first time about 12 years
ago and found that the frequency was "not right" !:-)
The reason is simple: our Earth is quite transparent at very low
frequencies, so the average circumference of the ionosphere-Earth
waveguide is shorter than 40000 km.
In other words, the center of the first Schumann resonance is below
your feet, at a deptht of approximately 300 km.
The second one is a little bit higher than 15 Hz, and progressively
the other come closer to harmonics of a 40000 km wavelenght; but
since the Q is low, it is dfficult to extimate the exact frequency.
Have a look at http://www.vlf.it/Schumann/schumann.htm.
Years ago there was a website from Stanford University reporting the
measured frequency of Schumann resonances vs. time. The frequency
changes because of day/night ionospheric height variations, and
beacuse of SIDs and other disturbances. Fascinating !
To receive them you need a very large coil, or a simple E field
receiver and a quiet location; feed the signal to an audio interface,
and run a spectrogram in average mode. Quite easy to say, but it
takes some ingenuity and work to extract an useable signal.
Renato IK1QFK has a live spectrogram covering from 0.1 Hz up, see
http://www.vlf.it/cumiana/livedata.html - today we have thunderstorms
in the area, and the signal is completely wiped out.
Vy 73 - Marco IK1ODO
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