MICROWAVE RADAR TESTING 451 



Coaxial Wavemeters 



A coaxial wavemeter is formed of a section of coaxial transmission line of 

 small enough diameter so that only the coaxial mode (in wave guide nota- 

 tion TMo. 0, n) can exist. Usually the line is short-circuited at one end and 

 open at the other, in which case resonance occurs at the odd quarter wave- 

 lengths X/4, 3X/4 etc.). The open circuit is obtained merely by terminating 

 the inner coaxial conductor, the continuing outer conductor acting as a wave 

 guide below cutoff. Sometimes the line is short-circuited at both ends, 

 giving resonance at the even quarter-wavelengths (X/2, X, etc.). 



Cylindrical Cavity Waremelers 



A cylindrical ca\-it_\' wavemeter is merely a section of cylindrical wave 

 guide transmission line'* whose length is varied. In order to avoid confusion 

 with other modes, it is preferable to use the dominant mode (described in 

 wave guide notation, r£i,i,„*) i.e. the mode with the lowest cutoflf frequency. 

 The cutoflf wavelength (Xc) of this mode is 1.706Z), where D is the diameter in 

 meters. For a higher Q it may be necessary to use the circular electric 

 m.ode TEo,i,n- The cutoff wavelength for this mode is .82D. No useful 

 purpose is served by using modes with / and m subscripts above unit}-. 

 TM modes are often used for fixed frequency cavities, but for variable 

 cavities TE modes are preferable since these have zero current at the inner 

 wall of the cylinder and thus obviate moving contact difificulty. If any 

 mode higher than the dominant one is used, suppression of unwanted modes 

 may be required. 



The accuracy of a wavemeter is dependent on its resolving power. This 

 in turn depends upon Q, which is an index of the decrement of the resonant 

 circuit, and is equal to //A/, where A/ is the distance between 3 db points 

 on the resonance curv^e. 



In a coaxial wavemeter, maximum Q for a given inner diameter is obtained 

 with a diameter ratio of about 3.6^ The basic Q of a. coaxial wavemeter, 

 assuming copper of standard conductivity, is roughly^ 



(3o - omiD Vf (3) 



This expression neglects end effects and hence gives somewhat too high a 

 value of Q. 



The basic Q's for TEi,i,„ and TEo.i.,, cylindrical cavity resonators em- 

 ploying copper of standard conductivity are, respectively, 



* TE and TM represent, respectively, transverse electric and transverse magnetic. 

 The subscripts /, w, n denote, respectively, the number of wavelengths around an>- 

 concentric circle in the cross section, the number of wavelengths across a diameter, anil 

 the number of half wavelengths along the length of the cj^linder. 



