178 



DIELECTRIC CONSTANT. ABSORPTION AND SCATTERING 



i»*^ Rainfall Intensity 



Orographic lifting of the unstable moist tropical 

 air caused frequent 2- to 3-(lay periods of precipitation 

 having a wide range in intensity. On one occasion in- 

 tensities as high as 135 mm per hour were observed. 

 Due to the light winds associated with orographic 

 ]>recipitation 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 lioth 

 with time and in space, well-coordinated measuring 

 techniques having sufficient coverage detected periods 

 when tJie rate of fall along the path was uniform. 

 Since such jjeriods 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-jjen recorder at the receiver sta- 

 tion. In addition, foixr 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 for 



ATTENUATION PATH 



-6400' 



[-711' -4" 711'— jjl 



a 



X AUTOMATIC RAIN GAUGE GRAIN SHELTER 



FloURE 20. Layout of e.xperiiuental path and apparatus. 



receiving instruciions as well as siniultane(.>us 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 wei'e stabilized. The technique for a satisfactory 

 measurement involved a comprehensive check of the 

 "clear weather' values liefore and after any one rain- 

 fall. 



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. AVave-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-mc bandwidth. The second 

 ilctector output fed an audio amplifier and recorder. 



A signal generator was used to check the receiver 

 charactei'istic. This generator consisted of a 2Iv33 

 tube and two flap attenuators. Fixed pads were used 

 on either side of the flap attenuators to provide a flat 



TRANSMITTER 



SIGNAL GENERATOR 



RECEIVER 



MONITOR 

 THERMISTOR 



2-FT PARABOLOID 



2-FT PARABOLOID 



FLAP ATTENUATORS 



POWER 

 SUPPLY AND 

 MODULATOR 



Figure 21. Block diagram of K-band attenuati(jn measurement apparatus. 



