1258 THE BELL SYSTEM TECHNICAL JOURX.\L, NOVEMBER 1954 



Table IV — "Upper Limit" Time Delay for Reconverted Energy 

 Components in Solid Pipe Without Mode Filters 



are indicated for the 2" diameter line and may also be regarded as pessi- 

 mistic estimates by virtue of our experience with the 5" diameter line. 



It is to be emphasized that the time delays of Table IV apply to solid 

 round guide without mode filters. The addition of mode filters in the 

 solid pipe would increase the loss for the uimsed modes and appreciably 

 reduce the "upper-limit" time delays for reconverted wave energy. For 

 the improved forms of circular-electric waveguide the increased losses 

 for the unused modes would result in similarly reduced time delays; 

 typical factors of reduction might run between 100 and 1 ,000 for spaced- 

 ring and helical waveguides compared to the data for solid pipe given in 

 Table IV. 



Taking mto consideration both conversion-reconversion effects and 

 some equalization of delay-distortion due to waveguide cut-off, it would 

 appear likely that baseband widths on the order of 100 to 1000 mc could 

 be employed in a 2" diameter line. 



The character of the reconverted wave interference will be as dis- 

 cussed in connection with Figs. 15 and 18 if the modulating wave of the 

 signal carrier contains frequencies near the reciprocal of the "upper- 

 Umit" time delay for the particular waveguide used. 



The magnitude of the reconverted wave energy has been discussed in 

 connection with Fig. 21, and it has been observed that ratios of signal 

 power to reconverted-wave power on the order of 20 db may be expected 

 on a well engineered waveguide line having 60 db of heat loss for the 

 signal wave. Although the time relations were not specifically taken into 

 account in this discussion, it seems likely that the order of magnitude of 

 the reconverted-wave power will be unaltered by more precise analysis. 



One might therefore conclude that the modulation method to be used 

 in a waveguide system must be one which will tolerate large amounts of 

 signal interference. Pulse code modulation is one arrangement of signal- 

 ing which will tolerate such interference, and undoubtedly there are 

 others as well. 



