VHF air-ground propagation far beyond the horizon and tropospheric stability

Results are presented of twilight-region propagation experiments on 128 Mc over the airline route from San Francisco, Calif., to Honolulu, Hawaii, including high-altitude jet flights. Greatly enhanced fields are observed on most of the flights, and these are correlated with a new meteorological parameter related to mean index gradient and atmospheric stability. It is also shown that minimum propagation conditions are some 20 db improved over other ground-to-ground or ground-to-air experiments. This improvement is attributed to greater antenna height at the ground terminal which lowers the lowest lobe of the vertical radiation pattern of the ground terminal. It is also pointed out that there is a high probability that minimum propagation conditions will not exist simultaneously for the plane-to-California and plane-to-Hawaii paths. Enhanced propagation conditions will therefore exist from the plane to one of the terminals for an operationally significant fraction of the time. It is pointed out that many existing VHF air-to-ground systems throughout the world can be extended to at least 500 miles through increased effective power and high antenna sites. The results obtained in the present paper are compared with the existing body of related propagation data beyond the horizon, and an interpretation is offered of the mechanism of the propagation.