WAVEGUIDE TESTING WITH MILLIMICROSECOND PULSES 57 



he pulse. This increased time of travel compensates for the shorter 

 ime of travel of the high frequency edge of the band in the 3" round 

 .vaveguide, so equalization takes place. Since this waveguide close to 

 cutoff introduces considerable delay distortion by itself, the taper effect 

 nust be made larger in order to secure the equalization. This can be 

 ilone by making the taper sufficiently gradual. This type of equalizer 

 ntroduces a rather high loss in the system. For this reason it might 

 le used to predistort the signal at an early level in a repeater system, 

 ilqualization by this method was suggested by J. R. Pierce. 



.0. MEASURING MODE CONVERSION FROM ISOLATED SOURCES 



I One of the important uses of this equipment has been for the meas- 

 irement of mode conversion. W. D. Warters has cooperated in develop- 

 ng techniques and carrying out such measurements. One of the prob- 

 ems in the design of mode filters used for suppressing all modes except 

 ;he circular electric ones in round multimode guides is mode conversion. 

 Since these mode filters have circular symmetry, conversion can take 

 alace only to circular electric modes of order higher than the TEoi mode. 

 This conversion is, however, a troublesome one, since these higher 

 Drder circular modes cannot be suppressed by the usual type of filter. 



An arrangement for measuring mode conversion at such mode filters 

 rom the TEoi to the TE02 mode is being used with the short pulse equip- 

 :nent. This employs a 400-foot long section of the b" diameter line. Be- 

 ause the coupled- line transducer available had too high a loss to TE02 , a 

 3ombined TEoi — TE02 transducer was assembled. It uses one-half of 

 :he round waveguide to couple to each mode. Fig. 15 shows this device. 



The use of this transducer and line is illustrated in Fig. 16. Pulses in 

 :he TEoi mode are sent into the waveguide by the upper section of the 

 transducer as shown. Some of the TEoi energy goes directly across to 

 ohe TE02 transducer and appears as the outgoing pulse with a level 

 down about 32 db. This is useful as a time reference in the system and 

 s shown as the outgoing pulse in the photo of the oscilloscope trace 

 ibove. The main energy in the TEoi mode propagates down the line as 

 hown by dashed line 2, which is the path of this wave. Most of 

 ohis energy goes all the way to the reflecting piston at the far end and 

 ohen returns to the TE02 transducer where it gives a pulse which is 

 narked TEoi round trip on the trace photograph above. Two thirds of 

 ;he way from the sending end to the piston, the mode filter being meas- 

 ired is inserted in the line. When the TEoi mode energy comes to this 

 node filter, a small amount of it is converted to the TE02 mode. This 



