320 BELL SYSTEM TECHNICAL JOURNAL 



One may picture a portion of the Faraday tubes of force turned back at the 

 interface while the remainder continue into the second medium. If one were 

 to reverse the direction of transmission and consider wavepower transmitted 

 from the second medium back into the first, a similar partial reflection would 

 be noted. In both cases the part turned back and returned to the source may 

 be regarded as a reactive component since no energy is really lost. In a similar 

 way, the transmitted component, since it is not returned to the source, may 

 be regarded as a resistive or dissipative component. 



If the medium into which wavepower is transmitted is a perfect insulator, 

 the transmitted wave will continue indefinitely except as attenuated by the 



Fig. 6.2-8. Reflection and transmission of lines of force incidental to a change of medium 



along a transmission line. 



wires along which it is guided. Its wavelength, X, in the dielectric will be 

 less than the wavelength, Xo , in air as expressed by the relation 



If the second medium is somewhat conducting, the wave will be further 

 attenuated, the rate of attenuation being related in a rather complicated 

 way not only to the conductivity of the second medium but to its dielectric 

 constant and permeability as well. Thus far in microwave practice, little 

 practical use has been made of materials having permeabilities very different 

 from unity. However, considerable use has been made of materials having 

 various dielectric constants, e^, and conductivities, g. Sometimes these take 

 the form of plates placed across a waveguide transmission line. Examples 

 will appear in Section 9.8. 



If a thin sheet of insulating material having a dielectric constant, €r, 

 and conductivity of zero is placed across a two-wire transmission line, the 

 percentage of power reflected is given approximately by 



qw = ^ (cr - 1) (6.2-5) 



Xo 



A thin sheet of this kind is approximated when wires carrying very high 

 frequencies pass through the glass walls of a vacuum tube. If the glass 



