316 



BELL SYSTEM TECHNICAL JOURNAL 



containing the two wires will be the first to be reflected and they will behave 

 at reflection much like those already discussed, whereas those outside the 

 plane of the two wires will not be intercepted initially by the thin cross bar 

 but instead will advance for a short distance beyond the end of the line 

 before their forces of tension bring them to rest. These outlying lines of 

 force are represented by the lines designated as c in Fig. 6.2-6. After the 

 first lines of force have been reflected, lateral pressure will be removed from 

 those adjacent, with the result that they will close in and collapse on the 

 conductor at a slightly later time than their neighbors. One over-all result 

 of this process is to make the effective length of such a line slightly greater 

 than the true length. Effects of this kind are observed in practice and they 

 are referred to as fringing. Discrepancies between the wavelength as 

 measured in the last section of line where fringing may take place and that 

 measured between other minima along the same line are usually small but 



J'/ //// /////// 



/////, 



9^ ^ / ^ / / / / / /> /•////•/ 



(a) 



(b) 



Fig. 6.2-6. (a) Representative transmission line terminated by a conductor of finite 

 dimensions, (b) Nature of reflection by a finite conductor. 



they are nevertheless measurable It is also true that, as the wave front ap- 

 proaches a limited barrier of this kind, some of its energy continues on into 

 the space beyond and is lost as radiation. In general, the smaller the barrier, 

 the larger will be the losses. 



Consider next a line open at its remote end, as shown in Fig. 6.2-7 In 

 this case, none of the lines of force of the advancing wave is intercepted by 

 a conductor, with the result that a very considerable number momentarily 

 congregate near the end of the line and, because of inertia, they e.xtend into 

 the space beyond as suggested by Fig. 6.2-7 (b). This process continues until 

 forces of tension in the lines, still clinging fast to the ends of the wires, bring 

 the assemblage temporarily to rest. At this moment, there is no magnetic 

 component; for v, in the relation H = ((v x E), is zero while the correspond- 

 ing electric intensity is approximately 2£. The lines of electric force, being 

 momentarily at rest, represent energy stored in the electric form. 



