WAVEGUIDE AS A COMMUNICATION' MKDIUM 1231 



out on the lime scale I'atluT than ai)])(':ir as a sinj^lc wcll-ddiiicd distor- 

 tion pulse. 



2. Because some of the unused modes of propajiation haxc <2;r()Ui) 

 velocities greater than the grou]) \-elocitA' of the circulai- electric wave, 

 the recouverted euerg}' pulses may reach the receiving end of the trans- 

 mission system before the signal pulse itself ai)i)ears. 



3. The reconverted energy will, in general, be out of phase with the 

 signal from which it was derived. When the differential time of travel in 

 the unused mode is short, the principal effect of the conversion-recon- 

 version process will be to distort the signal wave. When the differential 

 time of traN'el in the umised mode becomes as large as the r(M'i])rocal of 

 the modulation frec]uencies iiu'olved, the reconverted energy will appear 

 more like an echo. In a time-division system such echo pulses would 

 interfere with the pulses representing other signal components. Because 

 of the large number of conversions contributing at random time delays, 

 this "echo-interference" may be unintelligible. In this sense the inter- 

 ference may be thought of as a noise effect, just as multi-channel cross- 

 talk due to amplitude non-linearities in a single sideband AM system 

 may be thought of as noise. 



4. The general case of a signal pulse, both preceded and succeeded by 

 a series of reconverted energy pulses, is sketched in Fig. 18. It is quite 

 apparent that if the reconverted signal pulses are allowed to become of 

 the same order as the signal pulse itself, even a pulse code modulation 

 system will be rendered inoperative. Other types of modulation will 

 experience difficulty at appreciably smaller magnitudes of reconverted 

 energy. 



5. The level of the reconverted energy relative to the signal is deter- 

 mined by the transmission medium. It is not possible to avoid this inter- 

 ference by using more power at the sending end of the transmission link, 

 for the interference rises with the signal. The need for low-noise receivers 

 is just as acute as in other transmission systems, because better noise 

 figure means that correspondingly less power is required from the trans- 

 mitter. 



6. If the loss to the mode TX is very large in the region between suc- 

 cessive waveguide imperfections, the TX pulse can be attenuated to a 

 negligibly small value before reaching the second deformation, thus pre- 

 venting any significant reconversion back to the signal mode. 



7. Limits can be placed on the time delay between the signal energy 

 and the reconverted energy returned to the signal mode. The lower limit 

 on this time delay is obviously zero, corresponding to a series of imi)er- 

 fections very close together. The upper limit can be taken as the differ 



