1268 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1957 



Oont — 



1 1 



v; 



■'opt 



a/2 7>oi W - iY" 

 Aa V2 e 



y RJ 



a 



aoi 



J'oi 



VV^^iR 



(30) 

 (31) 



Av 



We note that 5opt does not depend on frequency. 



So far we have listed only the useful properties of the dielectric-coated 

 guide. There is, however, one serious disadvantage. Serpentine bends 

 caused by equally spaced discrete supports and the elasticity of the pipe 

 are inherently present in any waveguide line. At critical frequencies, when 

 the supporting distance I is a multiple of the beat wavelength \h between 

 TEoi and a particular coupled mode, an increase in TEoi attenuation 

 (23) and a spurious mode level (24) result from the mode conversion. 



We evaluate (23) and (24) for a dielectric-coated copper pipe of 2.000- 

 inch I.D. and 2.375-inch O.D. and a supporting distance I — 15 ft. The 



1.0 

 0.8 



0.6 

 0.4 



0.2 



0.1 

 0.08 



"^ 0.06 



an 



o 0.04 



O 



z 



If) 

 ir> 

 O 



0.02 



0.01 

 0.008 

 0.006 



0.004 

 0.002 



0.001 



!. 



\ 

 \ 



V- 



10 



20 



40 60 100 200 



BEND RADIUS IN FEET 



400 600 1000 



Fig. 6 — Bend convension loss in a dielectric-coated guide of optimum design 

 according to equation (28). 2a = 2 inches; 5 = 1.25%; € = 2.5; X = 5.4 mm. 



