210 BELL SYSTEM TECHNICAL JOURNAL 



survey maps and correcting for earth curvature gives the profile map 

 of Fig. 8, where it is seen that the air Hne clears the intervening country 

 everywhere by 200 or more feet (61 meters). We were unable to 

 check this by direct optical observations as no sufficiently clear day 

 occurred during our tenure of the Lebanon site but we were able to 

 identify a neighboring hill of about the same altitude (Mt. Cushetunk) 

 and we have no doubt that an air-line path existed. 



MC CATHARn's 

 HILL 



beer's hill 



36 



Fig. 8 — Profile map. Beer's Hill to McCatharn's Hill. 



If we imagine a transmitting and receiving antenna pair located 

 above the earth's surface it is easy to see that the received radiation 

 will consist of a direct plus a reflected component. If now we com- 

 plicate matters by adding a pair of hills to support the antennas we 

 shall add a pair of reflections from the slopes of the two hills to the 

 initial two radiation components. A final random corrugation of the 

 earth and we have the actual Holmdel-Lebanon situation. The con- 

 ditions under which the first reflection occurs, practically grazing 

 incidence, with the earth irregularities very small compared with the 

 optical path length, make it very certain that this reflection will sub- 

 stantially survive the corrugation; the proximity of the hills to the 

 antennas themselves ensures the presence of the second pair of reflec- 

 tions. The actual transmission should thus consist of a direct com- 

 ponent plus a three-surface set of major reflection components, 

 together with a background of scattered and diffracted radiation arising 

 from the corrugations of the earth's surface. For an extreme path 

 length the lens effect of the earth's atmosphere, decreasing in density 

 upwards and thus refracting the entire radiation ensemble downwards, 

 will produce a path deviation which cannot be neglected.^ 



A verification of this radiation picture should be possible. The hill- 

 side reflection components can be demonstrated by separately raising 

 and lowering the two antennas. Inasmuch as the reflections occur 

 nearby, only a small movement of an antenna is required to vary the 

 path difference between the direct and reflected rays by half a wave- 

 length and thus vary the received signal intensity through a maximum- 

 to-minimum, or reversed, cycle. The earth reflection occurs sub- 



* Pedersen, "Propagation of Radio Waves," chap. X, p. 150. The importance of 

 this refraction effect has most recently been pointed out by Schelleng, Burrows, and 

 Ferrell, companion paper in this issue of Bell Sys. Tech. Jour. 



