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The Effect of Propagation on Electromagnetic Fields Radiated by Lightning,
MISSION RESEARCH CORP ALBUQUERQUE NM
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It is of great interest to deduce the properties of lightning, such as the current profile as a function of time, from remote measurements of the radiated fields. For this reason, distortion of the signal by propagation path effects must be understood quantitatively. The study of the distortion of electromagnetic transient fields as they propagate over the earths surface has a long history Sommerfield considered the propagation of radio waves over an imperfectly conducting earth in 1909, and a number of recent texts review more recent developments. In general, the signals are attenuated, the attenuation increasing with frequency and decreasing as the surface conductivity increases. This presupposes smooth surfaces corrugated surfaces have also been studied in connection with propagation over a sea surface covered by waves. When the ocean waves have lengths small compared to the radio wavelength, a trapping of the surface wave is possible which results in an increase in signal, while at higher frequencies an increased attenuation results. The attenuation does not increase monotonically with frequency, as Barrick notes, but rather a saturation sets in, with the greates losses for frequencies in the range 10-15 MHz for typical ocean wave spectra.
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