PART II 
MISCELLANEOUS EXPERIMENTS 
Chapter 4 
REFLECTION COEFFICIENTS 
REFLECTION COEFFICIENT MEASURE- 
MENTS AT THE RADIATION 
LABORATORY* 
De THE LATTER PART of 1943 the S-band reflec- 
tion coefficient measurements begun in the spring 
and reported at the July 1943 conference have been 
carried on, and work 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 heights 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 field 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 1/R? 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 more or less erratic variation of field in- 
tensity with distance, but the magnitude of the varia- 
tion is generally small, and the recotds obey the 
1/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 
*By W. J. Fishback, Radiation Laboratory, MIT. 
259 
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 found 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 flights were made during the 
winter. On November 25, the wind was blowing across 
FLIGHT OF 11/25/43 
(WIND BLOWING ACROSS PATH) 
FLIGHTS OF 12/29/43 AND 12/30/43 
(WIND BLOWING OBLIQUELY WITH 
RESPECT TO PATH) 
FLIGHTS OF 41/22/43 
(WIND BLOWING ALONG PATH) 
° ‘ 2 3 4 5 6 7 () 9 
GRAZING ANGLE IN DEGREES 
REFLECTION COEFFICIENT -HORIZONTAL POLARIZATION 
Fiaure 1. Reflection coefficient, horizontal polarization, 
versus grazing angle. Sea water. Wavelength—S-band. 
the path, and high values of the reflection coefficient 
were observed. On November 22, 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- 
served fall about the theoretical curve. If any corre- 
