WAVEGUIDE TESTING WITH MILLIMICROSECOND PULSES 



45 



Fig. 8 shows the same test for a 3" diameter ahiminum waveguide 

 250 feet long. This line was mounted horizontally in the test building 

 with compression couplings used at the joints. The line expanded on 

 warm days hut the friction of the mounting supports was so great that 

 it pulled open at some of the joints when the temperature returned to 

 normal. These open joints produced reflected pulses from 40 to 50 db 

 down, which are shown here. They come at intervals equal to the length 

 of one section of pipe, about 12 feet. Some of these show polarization 

 effects where the crack was more open on one side than on the other, 

 but others are almost independent of polarization. These two photo- 

 graphs of the trace were taken with the polarization changed 90°. 



Fig. 9 shows the same test for a 3" diameter galvanized iron wave- 

 guide. This line had shown fairly high loss using CW for measure- 

 ments. The existence of a great many echoes from random distances 

 indicates a rough interior finish in the waveguide. Fig. 10 shows the 

 kind of inperfections in the zinc coating used for galvanizing which 

 caused these reflections. 



6. TESTING ANTENNA INSTALLATIONS 



The use of this equipment in testing waveguide and antenna installa- 

 tions for microwave radio repeater systems is shown in Fig. 11. This 

 particular work was done in cooperation wdth A. B. Crawford's antenna 

 research group at Holmdel, who designed the antenna system. A direc- 

 tional coupler was used to observe energy reflections from the system 

 under test. In this installation a 3" diameter round guide carrying the 

 TEu mode was used to feed the antenna. Two different waveguide 



TE,, IN 3"D1AM aluminum GUIDE (250 FT LONG) 



Fig. 8 — Reflections from several defective joints in a dominant (TEn) mode 

 waveguide. The two trace photos are for polarizations differing by 90 degrees. 



