WAVEGUIDE TRANSMISSION 



317 



This static situation is extremely temporary, for the tension momentarily 

 created in the lines of electric force soon forces the configuration as a whole 

 to move backward. As the wave front gets under way, the magnetic force 

 H increases in magnitude in accordance with the relation H = e(vxE). 



The fact that the wave front extends momentarily for a short distance 

 beyond the physical end of the fine and requires time to come to rest and 

 get into motion in the reverse direction implies inertia or momentum in the 

 wave front. This is the inertia referred to in the fourth principle mentioned 

 in Section 6.1. In this form of reflection, fringing is usually very evident, 

 and because of fringing we may have an apparent reflection point that is 

 considerably beyond the end of the wires. Thus the distance from the end 

 of the wires back to the first voltage minimum is much less than the quarter 

 wave that otherwise might be expected. 



^\\\\\\\\\\\\\v\\\ \\vy 



^\\V\S\\\V\VV\v. 



(a) 



(b) 



Fig. 6.2-7. (a) Transmission along a line open at the remote end. (b) Nature of reflection 



from open end. 



It is generally true that processes of reflection in which fringing takes place 

 are usually attended by considerable amounts of radiation. This suggests 

 that in the process of reflection some of this extended wavepower detaches 

 itself from the parent circuit and is lost. Experience shows that this lost 

 power may be greatly enhanced by separating the two wires or by flaring 

 their open ends. The so-called half-wave dipole, so familiar in ordinary radio, 

 is but a transmission line in which the last quarter-wave length of each wire 

 has been flared to an angle of 90 degrees. If we wish to minimize radiation, 

 we follow a reverse procedure and reduce the spacing between the two parallel 

 wires. This also reduces fringing, for we find that the measured distance from 

 the ends of the wires to the first voltage minimum is now more nearly a 

 quarter wave. 



It is of interest to compare reflections taking place at the open end of a 

 transmission line with those at a closed end. When a wave front becomes 

 incident upon a perfect conductor, the electric force vanishes. At the same 

 time, the lines of magnetic force, though effectively brought to rest, are 



