WAVEGUIDE TESTING WITH MILLIMICROSECOND PULSES 



53 



I that there should be increasing amounts of delay distortion reading 

 ifrom top to bottom for the pulse bandwidth used in these experiments. 

 ;The effect of this delay distortion is to cause a broadening of the pulse. 

 Examination of the pulse pattern of Fig. 12 shows that the later pulses 

 corresponding in mode designation to the lower curves of Fig. 13 do in- 

 deed show a broadening due to the increased delay distortion. One 

 method of reducing the effect of delay distortion is to use frequency 

 division multiplex so that each signal uses a smaller bandwidth. Another 

 way, suggested by D. H. Ring, is to invert the band in a section of the 

 waveguide between one pair of repeaters compared with that between 

 an adjacent pair of repeaters so that the slope is, in effect, placed in the 

 opposite direction, and delay distortion tends to cancel out, to a first 

 order at least. 



The (luantitative magnitude of delay distortion has been expressed 

 by S. Darlington in terms of the modulating base-band frequency 

 needed to generate two side frequencies which suffer a relative phase 

 error of 180° in traversing the line. This would cause cancellation of a 

 single frequency AM signal, and severe distortion using any of the 



3 4 



FREQUENCY 



5 6 7 8 9 10 



IN KILOMEGACYCLES PER SECOND 



12 



1 Fig. 1.3 — Theoretical group velocity vs. frequency curves for the 3" diameter 

 ivaveguide used for the tests shown on Fig. 12. The vertical shaded area gives the 

 bandwidth for the millimicrosecond pulses employed in that arrangement. 



