MICROWAVE REPEATER RESEARCH 



187 



sight is well clear of intervening obstructions. In order to have a rule-of- 

 thumb as to the amount of path clearance desirable, we have suggested that 

 the first Fresnel region should be clear of all obstacles. The first Fresnel 

 region for a given transmitter and receiver is bounded by points for which the 

 length of the path, transmitter to point to receiver, is greater by one-half 



beer s hill 

 (receiver; 



353' 



^ +2 



-8 



BEER'S HILL— DEAL PROFILE 



4 5 6 7 8 



DISTANCE IN MILES 



DEAL 

 (TRANSMITTER) 



h 



i. 

 12 12.63 



110 120 130 140 150 160 170 180 190 

 TRANSMITTER HEIGHT IN FEET (ABOVE SEA LEVEL) 



200 



Fig. 1-2. — ^Variable antenna-height tests on Beer's Hill-Deal Path. 



wavelength than the direct path from transmitter to receiver; its cross- 

 section by any plane perpendicular to the direct path is the first Fresnel 

 zone in the sense used in optics. A wave can be transmitted with practically 

 no loss through an opening whose area is of the order of the first Fresnel 

 zone. Also, in the case of a smooth reflecting surface between transmitter 

 and receiver, the first Fresnel zone clearance provides a maximum in re- 



