Far‐field, extremely‐low‐frequency propagation measurements, 14 March to 9 April 1971

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Abstract
From 14 March through 9 April 1971, the New London Laboratory of the Naval Underwater Systems Center conducted a far‐field, extremely‐low‐frequency (ELF) attenuation‐constant measurement test. Two sites located along the same great‐circle path were utilized. One site (3.9 Mm) was in St. John Island, Virgin Islands; the other (1.7 Mm) was near Swansboro, North Carolina. The horizontal magnetic‐field strengths were measured at a band of frequencies centered at 45 Hz and at 75 Hz to determine the attenuation rates for daytime and nighttime propagation conditions. The EW antenna of the US Navy ELF Wisconsin Test Facility, Clam Lake, Wisconsin, was the transmission source. The principal results obtained from these measurements were that (a) the daytime attenuation rate is higher than the nighttime rate at both 45 and 75 Hz and (b) the ionospheric excitation factors are quite different for daytime and nighttime propagation conditions. A comparison of these results with the data taken previously and concurrently shows excellent agreement.

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