ULTRA-SHORT-WAVE TRANSMISSION 507 



index-height relation was an approximate exponential one without any 

 evident boundaries. These data will be referred to later. 



Theory 



The fading phenomenon was explicable in several ways. In our 

 previously cited work ^ we found that variable atmospheric refraction 

 was present, the airplane carried receiver being up where the refracted- 

 diffracted field strength was high and dominant. In general variable 

 refraction would be expected to be a slow phenomenon, operating in 

 hours, or even days, rather than in minutes, and much too slow to 

 explain five-cycle-per-minute oscillations, for example. 



Another explanation was air-mass boundary reflection (or refrac- 

 tion),^ such a boundary readily explaining the rate of signal variation. 

 No Kennelley-Heaviside layer reflection was in question ; this had been 

 quickly ruled out by the experimental data. When, therefore, we 

 elected to transmit the frequency modulated signal, already described, 

 and the oscillograph revealed a cyclic maximum-minimum frequency 

 characteristic of the other path itself, it was evident that there was no 

 possibility other than wave interference left — interference presumably 

 between a direct-diffracted and one or more boundary-reflected 

 components. 



These boundaries have apparently not been positively identified at 

 longer wave-lengths and for that reason we have tried to get some 

 further experimental contact with them. Attempts, since the closing 

 down of the Atlantic Highlands-East Moriches circuit, to demon- 

 strate an air-mass boundary, any boundary whatever, by high-angle 

 transmission, have failed. No reflected components have appeared. 

 Of course an illy defined, or diffuse, boundary will operate in this 

 manner since only for near grazing incidence can such a boundary give 

 the appearance of a discontinuity for the incident radiation. 



If we assume such a boundary a few kilometers up, and assign to it a 

 relatively small discontinuity in index of refraction, compared with 

 that of an earth or sea water boundary, then the four components of 

 Fig. 14 will be the only important boundary reflected ones for a radio 

 circuit such as ours. We now, fortunately, have theoretical formulae *• ^ 

 for computing the diffraction of an ultra-short-wave radiation around 

 the earth and the amplitude of the direct-diffracted component can 

 be calculated at once. 



That is, it can be calculated at once if the air mass has no refractive 

 bending effect upon the radiation trajectory. Since such a bending 

 effect is certainly present at times, and is equally certainly variable, 

 even if only slowly, it must be taken into account. 



