WAVEGUIDE TESTING WITH MILLIMICROSECOND PULSES 63 



rises until the pattern shown in the lower trace is seen. As the piston 

 is moved farther in the same direction the trace gradually changes to 

 have the appearance of the upper photo again. Moving the far-end 

 piston changes the phase of energy on the return trip, and thus it can 

 be made to add to, or nearly cancel out, conversion components that 

 originated ahead of the piston. When the time separation becomes 

 great enough to prevent overlapping in the pulse ^^^dth, phasing effects 

 cannot take place, therefore, the beginning and end of the spread-out 

 received pulse are not affected by moving the piston. Energy converted 

 at the sending end of the guide travels the full round trip to the piston 

 and back in the slower TE31 mode, and thus appears at the latest time, 

 which is at the right-hand end of the received pulse. Conversion at the 

 piston end returns at the center of the pulse, and conversion on the 

 return trip comes at earlier times, at the left-hand part of the pulse. 

 The TEoi mode has less loss in the guide than the TE31 mode. Since the 

 energy in the earlier part of the received pulse spent a greater part of 

 the trip in the lower loss TEoi mode before conversion, the output is 

 higher here, and slopes off toward the right, where the later returning 

 energy has gone for a longer distance in the higher loss mode. The pulse 

 height at the maximum shows the converted energy from that part of 

 the line to be between 30 and 35 db below the incident TEoi energy 

 level over the measured band\\ddth. 



Measurements of mode conversion from TEoi to TE21 in these wave- 

 guides show these same effects, and also a phasing effect as a function 

 of frequency. The TE21 mode has a group velocity 2.4 per cent faster 

 than the TEoi mode. For a full round trip in the guides, this is a time 

 separation of about four pulse mdths between the modes. At one fre- 

 quency and one far-end piston position, the TE21 response shown as the 

 top trace of Fig. 20 was obtained. Moving the far-end piston gradually 

 changed this to the second trace from the top, and further piston mo- 

 tion changed it back again. This is the same kind of piston phasing effect 

 observed in the TE31 mode conversion studies. The irregular top of this 

 broadened pulse indicates fewer conversion points than for the TE31 

 mode, or phasing effects along the guide length. Since the TE21 mode 

 has a higher group velocity' than the TEoi mode, energy converted at 

 the beginning of the guide returns at the earlier or left-hand part of the 

 pulse, and conversions on the return trip, having traveled longer in 

 the slower TEoi mode, are on the right-hand side of the pulse. This is 

 just the reverse of the situation for the TE31 mode. Since the loss in the 

 TE21 mode is higher than in the TEoi mode, the right side of this broad- 

 ened pulse is higher than the left side, as the energy in the left side has 



