Evaporation duct influences on beyond-the-horizon high altitude signals

A case study of beyond-the-horizon propagation previously examined in terms of an earth-detached duct produced by an elevated layer at 2000 ft is reexamined in terms of ducting produced by evaporation ducts (duct heights
$\lesssim$
100 ft). The case study applies to a frequency of 3087.7 MHz, a transmitter altitude of 68 ft and a receiver altitude of 3000 ft. It is concluded that relatively shallow surface ducts of the type examined can significantly affect signal level beyond the horizon at elevations which are an order of magnitude or more greater than the duct height and could conceivably account in large measure for the disparities previously reported.