iO-11] TYPES OF TRANSMISSION LINES, MODES OF PROPAGATION 



539 



higher-order mode and has a cutoff wavelength one-half that of the TEw 

 mode. Thus there is a one-octave frequency range between cutoff points for 

 the two modes. The recommended operating range is appreciably less than 

 this, however, as illustrated by Fig. 10-13, which shows cutoff frequencies 

 and guide wavelengths for representative X-band rectangular waveguides. 

 Typical recommended bandwidth for rectangular waveguide operating in 

 the dominant mode is about 40 per cent, and over this range loss decreases 

 as frequency increases. 



Ridge Waveguides. For very wide band or multiband operation, 

 ridge waveguide has particular utility. Cross sections of two types of ridge 

 waveguide are shown in Fig. 10-14. High-power ridge waveguides are 





il r 



(a) (b) 



Fig. 10-14 Cross Sections of (a) Double-Ridge and (b) Single-Ridge Wideband 



Waveguides. 



available covering a 2 or 3-to-l frequency range, although frequency ranges 

 of 4 to 1 or more are possible in ridge lines having reduced power-handling 

 capacity. A representative commercial ridge waveguide covers the fre- 

 quency range from 4700 to 11,000 Mc, a range which could be covered with 

 a minimum of three standard rectangular waveguides. 



Circular and Square Waveguides. Being axially symmetrical, 

 circular waveguides are able to play certain unique roles. There are three 

 circular waveguide modes of particular practical importance. These are 

 the TEii "linearly" polarized dominant mode, the TMoi circular magnetic 

 mode, and the TEqx circular electric mode (Fig. 10-lle, f, g). The TMoi 

 mode finds its principal application in rotary joints. The TEqi mode is 

 unique in that for a given waveguide size, loss decreases without limit as 

 frequency is increased, and an oversize guide can provide very low loss over 

 appreciable transmission distances. 



Circular waveguide finds its main application in the TEn mode. The 

 polarization of a circularly-symmetrical antenna feed can be controlled 

 simply by orienting the TEn mode in the connecting circular waveguide, 

 or it can be excited with either sense of circular polarization. A similar feed 

 with its radiating end deflected slightly from the antenna axis can be rotated 

 about the antenna axis to generate a nutated conical scanning action with 

 unvarying yet controllable polarization. Mode multiplexing is feasible 

 using two TEii modes which are orthogonally polarized in circular wave- 

 guide. 



