http://www.frankgermano.net/nikolatesla2.htm - (Lost source)
It has been proposed that there are two forms of terrestrial resonance, Schumann resonance and (for lack of a better term) Earth resonance, both related to the diameter of the earth. Schumann resonance is studied by observing the effects of lightning discharges upon the electrical condition of the earth, specifically the naturally occurring electric and magnetic fields. Earth’s electrical condition can also be modified by artificial means, as did Tesla, by use of a properly tuned and configured electrical oscillator. The world’s AC power distribution systems do this more or less continuously, to a limited degree.
Some harmonic of the fundamental earth resonant frequency—up to approximately 25 – 35 kHz—is used for the oscillator frequency. In this frequency range the around-the-world propagation efficiency is in the general area between 93 to 87%. As the frequency is increased above this point, efficiency further decreases. At 160 kHz the efficiency falls below 10%. [Corum, K. L. and J. F. Corum, “Nikola Tesla, Lightning Observations and Stationary Waves," Proceedings of the 1994 Colorado Springs Tesla Symposium, 1994, Appendix II, "The Zenneck Surface Wave"] The oscillatory transformer provides the high voltage alternating current needed to periodically charge the transmitter’s elevated terminal. A monochromatic subcarrier signal is then added. This is in the form of an abrupt lower frequency electrical impulse applied at a rate approaching the fundamental earth resonance frequency. Higher frequency impulses, above the oscillator frequency are also added at every harmonic of the low frequency subcarrier impulse, be it the fundamental earth resonance frequency or some lower harmonic of same. [Corum & Corum] While the following quote refers to the use of an RF alternator in conjunction with a resonance transformer to produce continuous waves, it appears to be applicable to the excitation of earth resonance modes as well.
"I reduced the number of poles, I think, in 1901. But then I reduced it for the purpose of generating currents of higher frequency. If I had a great number of poles, I could not realize my idea, because these poles would come in quick succession and not produce a rate of change comparable to the rate of change which is obtainable by the discharge of a condenser owing to a sudden break of the dielectric. That is to say, a blow. It has to be a blow, you see. I had to place my poles comparatively far apart, then run them at excessive speed and generate comparatively few impulses, but each of those impulses are of such tremendous intensity that the dynamo is practically short-circuited. That gave me a blow which replaced the arc. And then, of course, there remained to be perfected a scheme enabling me to get the energy of the alternator in the most economical manner, in high harmonics. That is not known, at least I have not seen anything of that kind in literature, and I believe that if anybody would attempt it without the devices which I have invented, he could not get much of the energy in high harmonics".
The system would be composed of two or more type-one transmitter/receiver signal-generating facilities, each contributing energy to the entire network to a greater or lesser degree. The resulting wave complex would be the combination of multiple electrical oscillations ranging from the fundamental earth resonance frequency or some low harmonic thereof, the oscillator frequency, and higher frequency impulses extending, possibly, to the upper limit of the radio frequency spectrum. The elevated terminal acting as an antenna might be capable of directly launching the higher frequency components of the wave complex as electromagnetic radiation, providing localized radio-frequency signal coverage. If true, this might provide an opportunity for locally originated programming that would be associated only with a local source or generator, to the exclusion of other signal-generating facilities operating on the same frequencies.
At Wardenclyffe Tesla operated at frequencies from 1,000 Hz to 100 kHz. He found the frequency range up to 30 – 35 kHz, “to be most economical.” [Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power, pp. 143, 155] In operation, the system would have generated and sustained a wave complex . . .
Based upon an analysis of the Colorado Springs Notes and other sources [including Corum & Corum] it appears a basic World System oscillator would develop a wave complex with an extremely low frequency (ELF) component in the 6 – 1000 Hz range [obtained with an alternator, NTAC, p. 155] plus a very low frequency (VLF) component around 25 – 35 kHz. Tesla specified a frequency of 925 Hz for power transmission in the patents “System of Transmission of Electrical Energy” U.S. Patent No. 645,576, dated March 20, 1900 and “Apparatus for Transmission of Electrical Energy” U.S. Patent No. 649,621, dated May 15, 1900.
For such a low frequency, to which I shall resort only when it is indispensable to operate motors of the ordinary kind under the conditions above assumed, I would use a secondary of fifty miles in length.