298 



BELL SYSTEM TECHNICAL JOURNAL 



Characteristic Impedance 



A second intrinsic property of wave guides is characteristic im- 

 pedance. It may be calculated by intet^ratinj^: the complex Poynting 

 vector over the cross-sectional area and dividing; the result by the 

 square of the eflfective current. Formulas for this purpose are in- 

 cluded in the companion mathematical paper referred to above. The 

 numerical results of such calculation are shown in Fig. 9 for a 4-inch 

 diameter hollow copper conductor for each of the four principal waves 

 mentioned above. 



Fit 



1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000 

 FREQUENCY IN MEGACYCLES PER SECOND 



9 — Calculated values of characteristic impedance of a 4-inch hollow copper 

 pipe for each of the four more common forms of waves. 



It w^ill be remembered that when an ordinary ware line is terminated 

 in its ow^n characteristic impedance there are no standing waves. 

 This condition leads to a maximum of power delivered to the receiver. 

 Such an impedance match is sometimes referred to simply as a termina- 

 tion. Terminations for wave guides are entirely similar in their be- 

 havior to those of wire lines and may be had by a variety of means. 

 One is a thin film of resistance material placed perpendicular to the 

 axis of the guide followed at a prescribed distance by a perfectly 

 conducting reflector. It is often convenient to provide the latter in 



