322 



BELL SYSTEM TECIIXICAL JOURXAL 



When a two-wire transmission line assumes the coaxial form, the lines 

 of electric force are radial and lines of magnetic force are coaxial circles. 

 The directions of these two components obey the right-hand rule. (See Fig. 

 6.2-10.) Since the wave configuration is completely enclosed except for a 

 small exposure at each end, radiation from this type of line can be made very 

 small. 



K//////////////////////////^^^ 



• 1 • 







LINES OF ELECTRIC FORCE LINES OF MAGNETIC FORCE 



Fig. C.2-10. Arrangement of lines of electric and magnetic force associated with transmission 

 along a coaxial arrangement of conductors. 



6.3 Radiation 



Electromagnetic waves, including both light and radio waves, are not 

 unlike the waves that are guided along wire lines. Their difference is largely 

 a matter of environment. In one case they are attached to wires w^hile in 

 the other they have presumably detached themselves from some configura- 

 tion of conductors and are spreading indefinitely into surrounding space. 

 We shall present in this section one of several possible pictures of the launch- 

 ing of radio waves from a transmission line. Like other verbal pictures drawn 

 in this chapter, it should be regarded as highly qualitative. 



-Assume a two-wire line with one end flared as shown in Fig. 6.3-1. If at 

 some point to the left there is a source of wavepower, there will flow from 

 left to right along the line a sinusoidal distribution of lines of electric and 

 magnetic force not unlike that shown in Fig. 6.2-7. In order to simplify our 

 illustration, we shall single out for examination two representative lines of 

 electric force a-h and c-d located a half wave apart. It is understood, of 

 course, that there are present many other lines both before and behind those 

 represented. Also there are lines of magnetic force at right angles to the 

 electric force. As time progresses each element of length of the line of force 

 a-h moves laterally with the velocity of light. In the region where the wires 

 are parallel, it remains straight but, upon reaching the flared section, its 

 two ends fall behind the central section, thereby forming a curve as shown 

 in Fig. 6.3-1 (c). As this line of force moves to the end of the flared section 

 [Fig. 6.3-1 (d)], its successor c-d follows one-half wavelength behind. 



