MICROWAVE REPEATER RESEARCH 195 



ponents completely. This result suggested that the 1.25-centimeter scan- 

 ning antenna might be a very useful tool for investigating the fading mech- 

 anism at 7 centimeters wavelength. 



The 22.8-mile path between Crawford Hill and a hill on the Murray Hill 

 Laboratory property was chosen for study as a representative link in a 

 repeater circuit. (See Profile Map, Fig. 1-4.) Transmitters for the 1.25- 

 centimeter and 7 centimeter wavelengths were installed in the 100-foot tower 

 at Murray Hill. At the Crawford Hill receiving site were the narrow-beam 

 scanning antenna and a broad beam antenna for 1.25-centimeter operation; 

 also two broad beam antennas, spaced vertically 15 feet, for 7-centimeter 

 operation. In addition, a 1.25-centimeter radar could be operated with the 

 scanning antenna. A corner reflector target, 5J feet on a side, was located 

 at the Murray Hill tower. The signal reflected by this target was about 10 

 db stronger than the spurious reflections from other objects at the same 

 range. By making use of this target and ground echoes at intermediate 

 distances, the radar technique provided a considerable amount of useful 

 information concerning the transmission phenomena. 



Measurements were made on this path during the summer of 1946. As 

 had been hoped, the observations showed that transmission on 1.25 centi- 

 meters and 7 centimeters was often affected by the same conditions except, 

 of course, for atmospheric absorption effects at the 1.25-centimeter wave- 

 length. While it was not possible to arrive at explanations for all the fading 

 observed, the deep minima in the 7-centimeter signal, i.e., fades to levels 

 of 15 to 20 db or more below the free space field, usually were the result of 

 one of three types of propagation*: — 



Type 1. The 7-centimeter fading was of the rapid, large amplitude type 

 characteristic of wave interference. The 1.25-centimeter scanning records 

 showed the presence of multiple-path transmission in which two or more 

 readily separable wave paths were observed. " While the signals on both of 

 the vertically-spaced 7-centimeter antennas faded about the same in ampli- 

 tude, their signal minima did not occur simultaneously. A space diversity 

 system would be successful in reducing the effects of this type of fading. 



Type 2. The 7-centimeter fading was somewhat slower than in Type 1, 

 but still had the appearance of wave interference. The 1.25-centimeter 

 scanning records appeared to be of the single path variety. However, close 

 inspection showed that, in all probability, more than one transmission path 

 was involved but the 0.12 degree beam of the antenna was not sharp enough 

 to resolve them. The signals received on the vertically spaced 7-centimeter 

 antennas faded together so that space diversity would not be expected to be 

 successful unless an extremely large spacing of antennas were used. 



* Recently, on a different overland path having barely one Fresnel zone clearance, 

 an important fourth type has been observed when atmospheric refraction gives the ray 

 path a curvature opposite to that of the earth, thus effectively reducing the path clear- 

 ance. 



