Abstract
Measurements of the amplitude and phase of VLF radio waves over long paths are discussed. A system that permits the simultaneous measurement of phase and amplitude of VLF signals over complementary great circle paths is described. Phase and amplitude measurements of NAA VLF transmissions (18.6 kc/s) obtained simultaneously at San Diego over the two great circle paths are shown. A simple preliminary analysis of the data is used to show that the difference in the magnitudes of the amplitude over the two paths is in good agreement with that predicted by previously published attenuation rates. A change in long path amplitude during sunset at the transmitter is attributed to an excitation factor change, and it appears to be in good agreement with theory.
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