WAVEGUIDE AS A COMMUNICATION Mi;i)HM 



1257 



iiitci'lVrinn cITccts of the coiixci'sioii jjroccss, it is iiu])()il;iiit to lake iiilo 

 acfowiil the time relations liclwccii the siiiiial coinpoiiciils aii<l the rc- 

 conxcrUHl \va\e component.s. lii j^ciicial, il seems clear tiial rrconvcrlecl 

 energy returning to the signal mode within a lime inteixal a|)preeial)ly 

 less than the reeiproeal of the higliest mothilation ireciueiieies will \)v 

 less interfering than reconverted wave components separated from the 

 signal components by time intervals on the same order as the reciprocal 

 of the modulation frequencies. In practice, a distribution of reconversions 

 will be obtained from a line containing randomly disi)()sed imperfections. 

 As discussed in connection with ('([nation (1), an appi'oximate ui)])er 

 limit can be placed on the time separation between the signal (MUM'gy 

 and the reconverted wave energy. Table IV records the "upper-limit" 

 time delay for the 5" diameter experimental 9000 megacycle waveguide 



Fig. 30 — Proposed method of multiplexing several frequency bands in the 

 circuhxr electric waveguide. 



and for a 2" diameter pipe used at a frequency of 48,000 mc. The nega- 

 tive time delaj^s represent reconverted-wave components which precede 

 the signal at the receiving end of the system. These calculations suggest 

 that reconverted -wave energy might precede the signal by almost 3-^ 

 microsecond or lag the signal by approximately 1 micro.second in the 5" 

 diameter 9,000-mc line. Photographs of the observed pulse transmission 

 shown m Fig. 15 mdicate iieghgible reconverted-wave energy at more 

 than one-half these time intervals, and therefore it appears that the 

 method of estimating (described in connection with equation 1) is 

 pessimistic. However, the "upper limit" time delay does provide a con- 

 venient way of comparing the ex])erimental line with a proposed 2" 

 diameter line used at 48,000 mc. Observe in Table I\' that the 2" diam- 

 eter hne w^ill shoAV appreciably less delay for the reconverted wave 

 components than is to be expected from the 5" diameter line. This is due 

 to higher attenuation in the uiuised modes and to smaller differences 

 between the group velocities of the various modes in the 2" diameter 

 pipe. "Upper lunit" time delays in the range 1 to 100 millimicroseconds 



