MICROWAVE REPEATER RESEARCH 185 



Ground Reflections 



Some of our first experiments with ultra-short-waves showed that regular 

 reflections were obtained locally from open, relatively flat fields. The 

 reflection coefficients were in good agreement with theory. Later, measure- 

 ments of propagation between a transmitter located on a hill top and a 

 receiver carried in an airplane^ showed that for near-grazing angles of in- 

 cidence, the irregular and wooded terrain, typical of the New Jersey country- 

 side, could give rise to regular reflections at wavelengths as short as four 

 meters. The depth of the minima in received signal strength, caused by 

 wave interference between the direct and ground reflected components, cor- 

 responded to a reflection coefficient of about 0.9. In 1939, unpublished 

 results obtained over the 39-mile Beer's Hill-Lebanon optical path (See map 

 of Fig. I-l) indicated that for a wavelength of 30 centimeters the reflection 

 coefficient was still large, about 0.8. 



More recently, microwave propagation studies have been made over the 

 same type of terrain at wavelengths of 3.25 centimeters and 1.25 centimeters 

 and the situation in regard to ground reflections seems to have changed 

 somewhat. Experiments were conducted over the 12.6 mile Beer's Hill-Deal 

 path in which the height of the transmitting terminal was varied and which 

 also made use of narrow-beam scanning antennas to separate the direct wave 

 from a possible ground reflected component. The results showed the 

 apparent reflection coefficient to be of the order of 0.2 at 3.25 centimeters 

 and to be even less at 1.25 centimeters. Figure 1-2 shows typical curves 

 of signal level versus transmitter heights for wavelengths of 3.25 and 1.25 

 centimeters. Actually, the shapes of the curves can be accounted for better 

 by diffraction, for which the hill about two miles from Deal is considered to 

 be a straight edge, than by reflection from an assumed average ground 

 plane. The true picture is probably a combination of reflection and 

 diffraction effects. 



In an effort to minimize ground reflection, over-water paths were avoided 

 in the layout of the New York-Boston microwave repeater circuit and as a 

 final check a number of variable antenna-height tests* were made in the 

 preliminary survey of all sites. A few curves obtained at a wavelength of 

 7 centimeters are reproduced in Fig. 1-3. Similar results were observed 

 during a survey of sites between Chicago and Milwaukee. 



It is concluded, therefore, that although in the wavelength range down to 

 30 centimeters, at least, the effects of ground reflection must be taken into 

 account in the choice of sites for an optical path radio circuit, in the lower 

 microwave range, below say 10 centimeters, scattering and absorption of the 

 reflected wave by rough terrain and vegetation usually results in sub- 

 stantially free-space propagation under normal conditions when the line of 



* F. F. Merriam was in charge of this work. 



