Nikola Tesla Books
-2- discharge in a thunderstorm several hundred miles from the receiver. The radiation was propagated from source to receiver over the surface of the earth, rather than through the earth, although under the influence of the earth, see circular 573 [4] (copy enclosed) during propagation. The receiving system comprised a vertical antenna (elevated terminals in Tesla's language) and a ground terminal both connected to the receiver recorder. The waveform, Re E(t', d), was analyzed and the Fourier amplitude spectrum, Fx (w, d), (amplitude vs. frequency) was deduced (copy attached). I note the sharp crest of the spectrum occurs at approximately 6.4 kilocycles which value corresponds rather well to the value given by Tesla for the average wavelength of the standing waves. Whereas, it has been found that the crest of the spectrum of sferic waveforms is quite variable over the approximate range of values given by Tesla (4 to 12 kc), this sferic could be quite typical for radiation from cloud- ground discharges at comparatively short distances for a particular thunderstorm. It is, therefore, not surprising that Tesla observed this approximate wavelength.
However, it is also necessary to explain the standing wave mechanism, if such existed. It is quite possible that the apparent nulls were a result of certain systematic changes in the nature of the storm as a function of time and distance such that the intensity increased and decreased as the cumulo-nimbus cloud "discharged" and "re-charged", thus producing apparent maxima and nulls. Also, the crest of the spectrum could possibly shift systematically in frequency with change in distance and time such that maxima and nulls would be produced by the frequency selective network of Tesla's receiver-recorder.
A true standing wave could be produced by an efficient re-radiator in the vicinity of the receiver. A metal tower could be an efficient re- radiator, but the required vertical dimension for efficiency should at least begin to approach one quarter wavelength at say 6.4 kc, i.e., 3 (108)/4 x 6.4 (103) = 12,000 meters = 39,000 ft. The only structure in the vicinity of Colorado Springs which begins to approach such a vertical dimension would be the "Front Range" of mountains which rise abruptly from the eastern plains and in particular Pike's Peak (el. ~ 14,000 ft. above sea level, less 5,000 = 9,000 ft. above the plains). Thus, a re- radiated or scatter field from the discontinuity produced by the Front Range could be postulated. It is simpler to consider a point re-radiator, Fig. 3. If such were possible, the system of hyperbolic standing waves, Fig. 3, (not circular as mentioned by Tesla) would develope in a manner analogous
