A good explanation of what Schumann resonance are, can be found at the Stanford University web-site:
The Earth-ionosphere cavity
In first order approximation, the Earth-atmosphere system can be seen from an electromagnetic point of view as a radial shell of three layers of conductivity. The Earth and the ionosphere in about 100-150 km height appear as a perfect conductor with the air of negligible conductivity in between. They form a spherical shell of conductivty, denoted Earth-ionosphere cavity, in which electromagnetic radiation is trapped. Lightning strikes within the troposphere radiate energy into this system and the waves are travelling around the Earth. In the case of constructive interference, Earth-ionosphere cavity resonances are excited in the frequency range of 6-60 Hz.
Spectrum of the Earth-ionosphere cavity resonances
Average amplitude spectra from 2 to 100 Hz for Arrival Heights, Antarctica (AH), Sondrestromfjord, Greenland
(SS), and Stanford, California (SU) for the interval January to March 1990. The spectrums are dominated by
sharp spectral features of the power supply networks at 50 and 60 Hz. At the Stanford site, modulation
frequencies at 30 and 90 Hz occur, which result from non-linearities of the power transmission system. Every
spectrum exhibits a carrier frequency of 82 Hz with a modulation of 0.2 Hz that is produced by a Russian
submarine communication system. A variety of other spectral peaks of unknown origin occur occasionally and
distort the frequency range under consideration. Superimposed on the 1/f natural background, eight
Earth-ionosphere cavity resonances can be distinguished.

That is the theory, this is my personal esperience:

As showed in "Below 150Hz", the reception of the signal under 300 Hz is hided by overload of the strong spherics signal around 7 kHz on the input circuits (SoundBlaster or preamplifier). This effects is more biggest in the vertical polarization because spherics are more strong in vertical electric field. Filtering the input with a low-pass filter, the noise in electric vertical field came down to very low level, revealing (if the antenna is adeguate) the presence of Schumann resonance.

To add the low-pass filter function at the Marconi antenna (see picture in About me and my station), has been replaced the 2,7nF capacity with 100uF and 6,8nF capacity with 2,2uF.
In a horizontal loop has been added a resistance of 5,6Kohm in series (between 10 ohm resistance and mic input) and a 470nF capacity in parallel to mic input.


The results of low-pass filter are evident in this picture. At left Marconi vertical antenna, at right horizontal loop. Both signals are without statics (vertical band in horizontal loop are caused by main power noise). In vertical field weak horizontal band are visible, starting from 7.8Hz, 14Hz ... etc.. These are the Schumann resonance, the background natural noise at this frequencies.


Results are also evident in the spectrum. Red markers are placed at 8Hz and 14 Hz. Yellow line show the vertical electric field, and blu line the horizontal (receive with a loop). At 85 Hz the loop receive the monitor PC signal. The first four resonaces are loud but clear.

About 60 min. of electric vertical field. Five schumann resonances, normally indicated as:
7.8, 13.8, 19.7, 25.7 and 31.7 Hz are identifiable.

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