The Role of Elevated Ducting for Radio Service and Interference Fields

Harold T. Dougherty; Evan J. Dutton

Abstract: This report categorizes the manner in which atmospheric stratification can complicate the problems of frequency allocation and radio regulation by inhibiting service fields and enhancing interference fields. For the United States and its border regions, preliminary contour maps are presented for those parameters associated with the atmospheric layering (ducts) conducive to the propagation of unusually strong UHF and SHF fields over extremely long distances. The parameters of interest are: the percent occurrence of elevated ducts, a minimum trapping frequency, the modified refractivity lapse, the ducting-layer base height, the duct-base height, and the duct-top height. The role of these duct parameters in the prediction of potential interference fields is detailed by engineering formulas and illustrated by numerical examples. These predictions of duct characteristics from historical (radiosonde) data are necessarily preliminary because of present inadequacies of the data sample. Approaches for improving estimates of duct parameters are described. Appendices detail expressions for duct trajectories and map the variation of duct characteristics.

Keywords: SHF; anomalous propagation; atmospheric ducts; stratification; interference fields; ray trajectories; UHF

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