I think this topic is sort of under the 'towertalk topic umbrella'. This is a very interesting article on propagation. Adcock's article on "Propagation of Long Radio Waves" at http://www.lwca.org/library/lfprop/adcock/lfprop6.htm or http://www.lwca.org/library/lfprop/adcock/lfprop1.htm (same article; different resolution on the figures) Tod Olson, KØTO