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Rediscovering the Zenneck Surface Wave

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From Feed Line No. 4


In 1916 while speaking of his system for global transmission, Nikola Tesla cited the analysis of mathematician Arnold N. Sommerfeld as verification of his explanations of observed radio phenomena. Tesla was referring to his system in which 90% to 95% of the electrical energy was manifested at the transmitters output as "current waves" with the remainder directed to the antenna structure, resulting in dissipating EM radiation. In 1909 another investigator by the name of Johann Zenneck, while working to explain Marconi's trans-oceanic results, showed that a unique type of surface wave could travel along the interface between the ground and the air. In the words of Dr. James Corum,

"The distinguishing feature of the Zenneck wave was that the propagating energy didn't spread like radiation, but was concentrated near the guiding surface. Sommerfeld had shown that an electromagnetic wave could be guided along a wire of finite conductivity, and Zenneck conceived that the earth's surface would perform in a manner similar to a single conducting wire."

In commenting on Sommerfeld's analysis of the surface wave, Dr. James R. Wait states that "The field amplitude varies inversely as the square root of the horizontal distance from the source..." It is interesting to note that Sommerfeld made a point of distinguishing between the "electrodynamic" surface wave and its Hertzian counterpart the space wave, believing that both components could be present in varying proportion in the wave complex. It was Tesla's assertion that the exact composition of the emissions was dependent upon the design of the transmitter.

As the study of radio propagation progressed and certain mathematical analyses excluded it, some question as to the existence of Zenneck surface waves began to develop. In 1937 limited support was given to these doubts after tests showed simple antennas driven at 150 mHz produced 100 times lower field strength than predicted. More recent investigations show evidence that Zenneck waves can, indeed, be generated. The lower the frequency, the lower are the propagation losses. It is also apparent that they are not a major contributor to the field produced by an electric dipole or quarter wave radiator, however they can be strongly excited by a quarter wave resonator! Once again to quote Dr. Corum,

"the resulting wave is a surface guided (single conductor) transmission line mode which attenuates exponentially along the guide... There is no inverse square spreading or diffraction, as with Hertzian waves... With appropriate constitutive parameters, a pure Zenneck wave would seem to hold out the promise of guided propagation with no radiation field to waste energy."

Plots of field strength vs. frequency indicate that a Zenneck wave propagates best at ELF and VLF frequencies and would lose much of its advantage as frequency rose above 1 mHz.

[This piece is derived from Appendix II of the paper entitled "Nikola Tesla, Lightning Observations and Stationary Waves" presented at the 1994 Colorado Springs Tesla Symposium. It is available from PV Scientific Instruments, 309 Second St., Ithaca, NY 14850 (607) 272-4295, pvsci@arcsandsparks.com. Visit the " Tesla Reprint Page" on their website for additional information]

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