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BELL SYSTEM TECHNICAL JOURNAL 



discontinuity. Open air wire lines have to be supported on poles which, 

 together with the insulators, constitute unavoidable discontinuities. The 

 beginning and the end of a line are always present. Usually these latter 

 discontinuities are simply unavoidable; but, in radio, at least one dis- 

 continuity, the antenna, is made to serve a useful purpose. It is clear 

 that the generator and the load connected by a two-wire line. Fig. S, are 

 dipoles which will generate spherical waves as well as the wave guided by 

 the transmission line. At low frequencies the length of the dipoles is so 

 small compared with the wavelength that the field does not reach out into 



Fig. 5 — Formation of spherical waves at the ends of a long pair of parallel wires. 



Fig. 6 — An antenna. 



the region where the radial capacitance becomes negligible and where the 

 spherical wave starts carrying off all the energy that gets there. Spherical 

 waves generated at the beginning and the end of the transmission line are 

 practically stationary waves and constitute merely local reactive reservoirs 

 of energy. The energy is withdrawn from the generator or the transmission 

 line during one half of the cycle only to be returned during the other half.^ 

 At low frequencies the energy thus exchanged back and forth is so small 

 that normally we don't even think about it. The antenna, Fig. 6, is designed 

 to be a more efficient transformer of the plane wave guided by the parallel 

 pair into the spherical wave which will carry ofif power to distant points. 

 Quite frequently discontinuities are introduced intentionally in order to 



