WAVEGUIDE TESTING WITH MILLIMICROSECOND PULSES 59 



jihort dashes. This goes down through the waveguide to the far end 

 Ijiston and back, and is received by the TE02 transducer and shown as 

 [he pulse marked TE02 round trip. The pulse marked TEoi round trip 

 las a time separation from the outgoing pulse which is determined by 

 ,he group velocity of TEoi waves going one round trip in the guide. The 

 |rEo2 round trip pulse appears at a time corresponding to the group 

 /elocity of the TE02 mode going one round trip in the guide. Spacing the 

 node filter two-thirds of the way down produces the two conversion 

 :)ulses equally spaced between these two as shown in Fig. 16. The first 

 ponversion pulse appears at a time which is the sum of the time taken 

 or the TEoi to go down to the filter and the TE02 to go from the filter 

 uo the piston and back to the receiver. Because of the slower velocity 

 bf the TE02 , this appears at the time shown, since it was in the TE02 

 node for a longer time than it was in the TEoi mode. The second con- 

 [/ersion, which happened when TEoi came back to the mode filter, comes 

 jiarlier in time than the first conversion, since the path for this signal 

 ivas in the TEoi mode longer than it was in the TE02 mode. This arrange- 

 ,nent gives very good time separation, and makes possible a measure- 

 Inent of the amount of mode conversion taking place in the mode filters, 

 viode conversion from TEoi to TE02 as low as 50 to 55 db down, can be 

 neasured with this equipment. 



Randomly spaced single discontinuities in long waveguides can be 

 ocated by this technique if they are separated far enough to give in- 

 lividually resolved short pulses in the converted mode. Fig. 17 shows 



CONVERSION 



FIRST CONVERSION AT FAR END SECOND CONVERSION 



AT NEAR END SQUEEZED AT NEAR END 



SQUEEZED SECTION SECTION SQUEEZED SECTION 



TO RECEIVER 



TEJo — *- TEq, TE2, -• »- TE,o NEAR END 250 FT OF FAR END 



TRANSDUCER COUPLED LINE SQUEEZED 3"DIAM ROUND SQUEEZED 



TRANSDUCER SECTION GUIDE SECTION 



Fig. 17 — Arrangement used to explain the measurement and location of mode 

 onversion from isolated sources. A deliberately squeezed section was placed 

 t each end of the long waveguide, producing the pulses shown in the trace photo. 



