Chapter 9 

 REFLECTION COEFFICIENTS 



9» REFLECTION COEFFICIENT MEASURE- 

 MENTS AT THE RADIATION 

 LABORATORY" 



DUKiNG THE LATTER PART of 1943 the S-baucl reflec- 

 tion coefficient measurements begun in tlie spring 

 and reported at tire July 1943 conference have lieen 

 carried on, and worli of a similar nature has been 

 started to determine X-band values. The interference 

 pattern was observed by recording field strength as a 

 function of distance with both receiver and trans- 

 mitter lieights held constant. One end of the path was 

 ground-based, while the other end was carried in an 

 airplane which flew over sea toward the land station 

 at a constant altitude and bearing. A one-way c-w path 

 was used, the transmitters, receivers, and recorders 

 being identical to those used previously. The time con- 

 stant of the receiver and recorder was 0.3 sec, corre- 

 sponding to 0.01 mile for the usual air speeds used. 



When appreciable specular reflection was obtained, 

 a regular succession of maxima and minima were 

 observed on the record. The product of the divergence 

 factor and reflection coefficient was found by deter- 

 mining the ratio of electric fleld strength at adjacent 

 maxima and minima. The geometrical expression for 

 the divergence factor was assumed correct and all 

 variations were lumped in the experimental value of 

 the reflection coefficient. It was required that a record 

 give a check on the positions of maxima and minima 

 for standard refraction and that the maxima obey 

 the l/W law (power) before the record would be 

 worked up. 



Flights over land made in 1943 at Orlando, Florida, 

 Riverhead, Long Island, and Cambridge, Massa- 

 chusetts, fail to show a regular interference pattern. 

 There is a move or less erratic variation of fleld in- 

 tensity with distance, but the magnitude of the varia- 

 tion is generally small, and the records obey the 

 l/R^ law. It is believed that the terrain is rough 

 enough to scatter all incident radiation of micro- 

 waves and that specular reflection will therefore not 

 be observed. There is considerable evidence, however, 

 that if a microwave transmitter is placed at a fairly 



low height over terrain as smooth as an airport run- 

 way, specular reflection will be observed. 



Observations over sea made late in 1943 on S band 

 with horizontal polarization have not agreed with 

 earlier results. A correlation has been foiind between 

 wind (and presumably wave) direction with respect 

 to the path and the magnitude of the reflection coeffi- 

 cient. The correlation suggests that low values ob- 

 served are due to back scattering. Figure 1 shows lines 

 drawn as a means of the values observed on the 4 days 

 when exceptionally good fliglits were made during the 

 winter. On November 25, the wind was blowing across 



n 1 1 — I / 



FLIGHT OF H/25/43 ^ 



(WIND BLOWING ACROSS PATH) 



FLIGHTS OF 12/29/43 AND 12/30/43 

 (WIND BLOWING OBIIOUELY WITH 

 RESPECT TO PATH) 



3 4 5 6 



GRAZING ANGLE IN DEGREES 



"By \V. .J. Fishback, Radiation Laboratory, MIT. 



Figure 1. Reflection coefficient, liorizontal polarization, 

 versus 'gi'azins angle. Sea water, ^^'avelength — S-band. 



tlie path, and high values of the reflection coefficient 

 were observed. On November 23, the wind was blow- 

 ing along the path, and low values were observed. 

 On December 29 and 30, the wind was blowing 

 obliquely with respect to the path and intermediate 

 values were observed. While the values on any given 

 flight lie fairly close to the lines, there is a consider- 

 able amount of scatter. This scatter is now believed 

 to be real and supports the results obtained by the 

 British. 



Figure 2 shows the results obtained this winter on 

 S band with vertical polarization. The values ob- 

 ser\'ed fall about the theoretical curve. If any corre- 

 lation witii wind direction exists, it is masked by the 

 variation within a single flight. 



Equipment difficulties have Just been overcome 

 and work is getting under way to determine X-band 

 values. One flight made on horizontal polarization 



137 



