WAVEGUIDE TRANSMISSION 321 



thickness, /, is small compared with the wavelength in air, Xc, the power 

 reflected by the glass envelope will likewise be small. 



Sometimes it is not feasible to reduce the wall thickness sufficiently to 

 avoid serious reflections. In these instances it may be possible to make the 

 thickness one-half wavelength as measured in glass whereupon the wave 

 reflected from one face of the plate will be approximately equal in amplitude 

 to that from the other face and, since they are separated by one-half wave- 

 length, they tend to cancel. 



Another case of practical interest is that in which the line is terminated 

 in a plate of very special dielectric constant e^, conductivity gi, and thick- 

 ness /. This is followed by a second plate of nearly infinite conductivity. 

 This arrangement is shown in longitudinal section in Fig. 6.2-9. By a proper 

 choice of constants, the combination may be made a good absorber of wave- 



Fig. 6.2-9. A transmission line terminated in a conductor coated with a special materia 

 such that all of the incident wave power is absorbed. 



power. It will therefore be substantially reflectionless It may be shown that 

 to satisfy this requirement 



and 



Xo = -f^ (6.2-6) 



' 607rgi(2« - 1) ^VTr{2n - 1) ^^"^ ^^ 



where n is any integer. One common example is that in which n = 0. The 

 plate is then a quarter wave thick as measured in the medium.^ A reflection- 

 less plate of this kind when placed at the end of a transmission line appears 

 to the source as though the line were terminated in its characteristic im- 

 pedance. Devices incorporating this principle are sometimes used as match 

 terminators for waveguides.* 



* A more complete discussion of this problem was published in 1938 by G. W. O. Howe, 

 "Reflection and Absorption of Electromagnetic Waves by Dielectric Strata." Wireless 

 Engr., Vol. 15, pp 593-595, November 1938. 



* Plates of this kind may be made very simply by mixing carbon with plaster in vary- 

 ing proportions until the right combination is reached. 



