296 ; RADIO WAVE PROPAGATION EXPERIMENTS 
The transmitter was housed in a small elevated 
shack and the antenna and guide were protected from 
the rain by a back-sloping shutter flap. 
A 2K33 tube, modulated with 800 c, was used as the 
transmitter. Wave-guide feed was employed on a 2-ft 
paraboloid antenna (beam width 1,7°). A thermistor 
with a directional coupler was used as a power 
monitor. 
A 2-ft paraboloid collected energy at the receiving 
end and fed the receiver through a wave guide. A 
superheterodyne utilizing a 2K33 local oscillator drove 
a 30-mc i-f amplifier with 6-me bandwidth. The second 
detector output fed an audio amplifier and recorder. 
A signal generator was used to check the receiver 
characteristic. This generator consisted of a 2K33 
tube and two flap attenuators. Fixed pads were used 
on either side of the flap attenuators to provide a flat 
line. The characteristics of the flap attenuators were 
checked every few hours, using a K-band thermistor. 
Each flap was calibrated and used over a 12-db range. 
Resettability was approximately +0.1 db. A small 
nozzle was used to direct the output of the signal gen- 
erator upon the receiving paraboloid. Calibrations 
were made before, during, and after rainfalls and were 
within +1.0 db over the 5- and 6-hour measuring 
periods. 
Analysis 
The primary attenuation curve of Figure 22, shown 
with solid dots, was obtained by choosing periods when 
the rainfall at all stations, including the automatic 
gauges, was essentially uniform. Six such periods of 
° 10 20 30 40 50 60 70 80 
MM PER HOUR 
FicurE 22. Primary attenuation curve of K-band 
radiation in rain. K-band attenuation versus rainfall 
intensity. (Note: Decibels per nautical mile.) 
uniform fall along the path were selected covering the 
important range of 0 to 41 mm per hour. Figure 23 
is a rainfall intensity profile of the highest uniform 
fall recorded. By Humphreys’ classification of rain® 
the intensities covered by the primary curve are more 
91 mm per hour, light rain; 4 mm per hour, moderate rain; 
15 mm per hour, heavy rain; 48 mm per hour, excessive rain. 
See reference 24. 
60, 
4 oe, 
B40 By ce el 
pe 
Fd 
gz AVG 41.2 
a ATT 21,0 
T 
DISTANCE ALONG PATH 
Figure 23. Profile of rain intensity along the path. 
Time 223645. 
than adequate for normal rates of precipitation en- 
countered in nature. 
Using the primary attenuation curve thus obtained, 
it was possible to extend the curve for extremely high 
rates of fall (cloudbursts) in the following manner. 
Figure 24 is a rain intensity profile at an interval of 
pee TOTAL ATTENUATION 22.3 
+ KNOWN PARTIAL ATT 7.4 
100, 
& 
g 
RAIN INTENSITY IN MM PER HR 
& 
OISTANCE ALONG PATH 
Ficure 24. Intensity profile during uneven precipita- 
tion. Time 171115. 
nonuniform rainfall distribution. The area under the 
curve is divided into sections as shown. These sections 
cover the portions of the path where the intensity was 
below 41 mm per hr. Hence with the primary attenua- 
tion curve and a planimeter it is possible to assign the 
contribution that each section makes to the total ob- 
served attenuation (assuming that the attenuation in 
decibels is linear with distance). 
After subtracting out the part of the attenuation 
already known, the high intensity central portion is 
left to account for the residual attenuation. Dividing 
the residual attenuation by the fraction of a mile cov- 
ered by this part of the path and plotting this value 
against the average intensity in the interval gives a 
point at 78 mm per hour. As a check on the method, 
similar profiles were worked up which gave points 
below 40 mm per hour. These are plotted as open cir- 
cles, as shown in Figure 22. It will be seen that the 
open circles agree quite well with the solid ones, 
and hence considerable confidence in the high inten- 
sity points is justified. 
Discussion 
The total observation time of the experiment was 
roughly 3 hours, about half of this total time being 
represented by the figures presented with the paper. 
It was necessary to employ a large number of precipi- 
tation measuring stations along the path in order to 
obtain an accurate precipitation profile. Some inci- 
