Case studies of beyond-the-horizon propagation in tropospheric ducting environments

Several case studies of propagation in offshore San Diego tropospheric ducting environments are examined via waveguide formalism. Both ground-based and earth-detached ducts produced by elevated layers at about 600 and 2000 ft respectively are considered. The model assumes a trilinear refractivity profile and the layer to be stationary and horizontally homogeneous. The ground-based duct study includes cases ranging from single-mode propagation (65 MHz) to cases where more than a hundred modes have been evaluated (3300 MHz). The earth-detached duct studies are for the single frequency of 3087.7 MHz with the transmitter at 68 ft. Approximate agreement has been found between measurements and calculations in the case of the ground-based duct. Calculations for earth-detached ducts dramatically overestimate the observed path losses for beyond the horizon fields. This discrepancy is probably due to layer fluctuations and/or inhomogeneity, along with the possible existence of an unaccounted for, persistent, ground-based duct produced by a low level layer. The evaporation duct for which duct heights are generally less than 100 ft would be such a possibility.