WAVEGUIDE TESTING AVITH MILLIMICROSECOND PULSES 61 



{from the point of the discontiimity down to the piston at the far end, 



land then back to the discontinuity. If the discontinuity is at the far 



lend, this time difference becomes zero, and a single pulse is seen. By 



i [making a measurement of the pulse spacing, the location of a single 



i icon version point can be determined. 



[ In the arrangement illustrated in Fig. 17, two isolated sources of 



j conversion existed. They were spaced far enough apart so that they 



\ were resolved by this equipment, and all three pulses were observed. 



The two outside pulses were due to the first conversion point. The center 



pulse was caused by the other squeeze, which was right at the reflecting 



|:)iston. If this conversion point had been located back some distance 



rom the piston, it would have produced two conversion pulses whose 



'spacing could be used to determine the location of the conversion point. 



I The coupled-line transducers are calibrated for coupling loss by send- 



ng the pulse through a directional coupler into the branch normally 



ised for the output to the receiver. This gives a return loss from the 



lirectional coupler equal to twice the transducer loss plus the round 



rip line loss. 



1. MEASURING DISTRIBUTED MODE CONVERSION IN LONG WAVEGUIDES 



; Measurements of mode conversion from TEoi to a number of other 

 nodes have been made with 5" diameter guides using this equipment, 

 rhe arrangement of Fig. 18 was set up for this purpose. This is the same 

 IS Fig. 17, except that a long taper was used at the input end of the 5" 

 waveguide, and a movable piston installed at the remote end. 



One of the converted modes studied with this apparatus arrange- 

 uent was the TMu mode, which is produced by bends in the guide, 

 rhis mode has the same velocity in the waveguide as the TEoi mode. 

 Therefore energy components converted at different points in the line 

 tay in phase with the injected TEoi mode from which they are converted, 

 rhere is never any time separation between these modes, and a single 



TO RECEIVER 



■ I ■■'■ '■■■ ^ 



^^i 



Si 



TEro— *TE^, COUPLED LINE -^^p^P, 5„ 0,^^^ MOVABLE 



TRANSDUCER TRANSDUCER HOLMDEL LINE PISTON 



FOR THE MODE 

 BEING MEASURED 



Fig. 18 — Arrangement used for measuring mode conversion in the 5" diameter 

 aveguides at Holmdel. 



