DIELECTRIC CONSTANT, ABSORPTION AND SCATTERING 295 
sity presents a difficult problem which is not yet well 
understood. 
The practical importance of the curvature shown in 
Figure 18 was emphasized. It was pointed out that 
the effects of a wide range of humidities had been in- 
vestigated, some very high compared to those ordi- 
narily encountered. In practical work most of the data 
would be obtained from the low end of the curves of 
Figure 18, where little ambiguity in numerical values 
would obtain. 
K-BAND ATTENUATION DUE TO 
RAINFALL? 
Introduction 
Tn order to- determine the attenuation of 1.25-cem 
wavelength radiation by rain, controlled radio and 
meteorological measurements were undertaken in an 
area providing adequate climatic conditions for the 
study. It was apparent that the attenuation measure- 
ments should be made in an area of maximum precipi- 
tation for expediency. Furthermore, the experiment 
demanded periods of varying rates of rainfall with fre- 
quent “clearing” for calibration purposes. Tropical 
orographic (mountainous) rain seemed to offer the 
greatest probability of fulfilling these conditions, 
A brief reconnaissance of the Hilo, Hawaii, area 
showed that a site near Kaumana was adequate, hav- 
ing a yearly fall in excess of 250 in., as compared with 
an annual rainfall of 10.10 in. in the San Diego area. 
A 1.21 statute mile link was chosen parallel to the 
mean trade wind vector, i.e., due east-west, and was 
located on a lava flow of 1881. The lava was covered 
with saw grass and low brush. The terrain had a gentle 
slope from the receiver at 2,500 ft to the transmitter 
at 2,800 ft above mean sea level. 
Rainfall Intensity 
Orographic lifting of the unstable moist tropical 
air caused frequent 2- to 3-day periods of precipitation 
having a wide range in intensity. On one occasion in- 
PBy L. J. Anderson, U. S. Navy Radio and Sound Lab- 
oratory. . 
TRANSMITTER 
MONITOR 
THERMISTOR 
2-FT PARABOLOID 
OIRECTIONAL 
GOUPLER 
SIGNAL GENERATOR 
SUPPLY AND 
MODULATOR 
tensities as high as 125 mm per hour were observed. 
Due to the light winds associated with orographic 
precipitation an essentially vertical trajectory of the 
raindrops was obtained ; and, therefore, representative 
sampling of the rain falling through the radiated 
energy path was accomplished by placing the gauges 
directly in line between the transmitter and receiver. 
Although the rainfall intensity varied widely both 
with time and in space, well-coordinated measuring 
techniques having sufficient coverage detected periods 
when the rate of fall along the path was uniform. 
Since such periods of uniformity seldom lasted longer 
than 60 sec, precise control and timing were vital. Two 
methods of determining the rate of precipitation were 
employed. Five Julien Friez tipping-bucket automatic 
recording rain gauges were evenly dispersed along the 
path and their signals were recorded on a single 
Esterline-Angus five-pen recorder at the receiver sta- 
tion. In: addition, four rain shelters employing the 
“funnel and graduate” technique were installed be- 
tween the automatic gauges as shown in Figure 20. 
The rain shelters were provided with field phones four 
ATTENUATION PATH 
T R 
% AUTOMATIC RAIN GAUGE ORAIN SHELTER 
Ficure 20. Layout of experimental path and apparatus. 
receiving instructions as well as simultaneous signals 
for taking graduate readings and exposing drop size 
blotters. During operations, signals for graduate read- 
ings were given every 30 sec. Blotters for drop size 
measurements were simultaneously exposed on an 
average of every 5 min. 
Radio Equipment 
The equipment used for the attenuation measure- 
ments is shown in Figure 21. It was relatively simple 
and required little attention once the initial warm-up 
drifts were stabilized. The technique for a satisfactory 
measurement involved a comprehensive check of the 
“clear weather” values before and after any one rain- 
fall. 
(/ PARABOLOID 
: TF 
l AMPLIFIER 
RECEIVER 
POWER 
Figure 21. Block diagram of K-band attenuation measurement apparatus. 
