Circular Electric Wave Transmission in 

 a Dielectric-Coated Waveguide 



By H. G. UNGER 



(Manuscript received January 9, 1957) 



The mode conversion from the TEoi wave to the TMn wave which nor- 

 mally occurs in a curved round waveguide may he reduced hy applying a 

 uniform dielectric coat a few mils thick to the inner wall of the waveguide. 

 Such a dielectric coat changes the phase constant of the TMn wave without 

 affecting appreciably the phase and attenuation constants of the TEoi wave. 

 Thus, relatively sharp bends can be negotiated to change the direction of the 

 line or deviations from straightness can be tolerated. For each of these two 

 cases a different optimum thickness of the dielectric coat keeps the TEoi 

 loss at a minimum. At 5.4-mm wavelength a bending radius of 8 ft in a 

 l-inch pipe or 50 ft in a 2-inch pipe can he introduced with 0.2 db mode 

 conversion loss. Deviations from, straightness corresponding to an average 

 bending radius of 300 ft can he tolerated in a 2-inch pipe at o.Jf-mm wave- 

 length with an increase in TEoi attenuation of 5 per cent. Serpentine bends 

 caused by equally spaced supports of the pipe may, however, increase the 

 mode conversion and cause appreciable loss at certain discrete frequencies. 

 Compared with the plain waveguide the dielectric-coated guide behaves more 

 critically in such serpentine bends. The mode conversion from TEoi to the 

 TEo2 , TEo3 , • • • waves at transitions from a plain waveguide to the dielec- 

 tric-coated guide is usually very small. 



I. INTRODUCTION 



In a curved section of cylindrical waveguide the circular electric wave 

 couples to the TEu , TE12 , TE13 • • • modes and to the TMu mode.i 

 The coupling to the TMu mode presents the most serious problem since 

 the TEoi and TMu modes are degenerate, in that they have ec[ual phase 

 velocities in a perfectly conducting straight guide. In a bend all TEoi 

 power introduced at the beginning will be converted to the TIMn power 

 at odd multiples of a certain critical bending angle. One can reduce this 

 complete power transfer by removing the degeneracy between TEoi and 



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