452 BELL SYSTEM TECHNICAL JOURNAL 



A^ 1 



Qo = 0.0937 X 10 

 Qo = 0.2762 X 10 



^^ 1 + 5'( 0.826 - + 0.295 ' ^^^ 



\ n 



10 ^ L 



v'/ 1 + ^^(2.4393 (5) 



with .4 — -^ , 52 = A^ — 1, and/ is in cycles per second. 



A 



The value of Q which determines accuracy is not the basic Q, but the 

 loaded or working Q, herein designated Ql. 



The resolving power of a wavemeter used for measuring a single frequency 

 can be made considerably better than f/Qi- With a sensitive meter it is 

 readily possible to detect differences less than 1 db, which corresponds to a 

 frequency interval oIJ/IQl. 



The required accuracy in a wavemeter is generally absolute rather than a 

 percentage. Hence increasingly large values of Qi are required at the higher 

 frequencies. Thus for a resolution of 1 mc, assuming 1 db discrimination, 

 the values of Q^ required for different frequencies are: 



An unnecessarily high value of Ql has the disadvantage of making it more 

 difficult to find the desired frequency. 



Linearity 



The displacement of the coaxial plunger of a coaxial type wavemeter for 

 resonance is substantially a direct linear function of free space wavelength 

 and if an ordinary centimeter micrometer drive is used it is possible to read 

 wavelength differentials directly. Over bandwidths less than 20 per cent, 

 displacement vs. frequency is also quite linear which is of considerable ad- 

 vantage for some uses. 



For the cavity type wavemeter the displacement is a variable function 

 of free space wavelength and becomes very non-linear as the cutoff fre- 

 quency of the guide or cavity is approached. This is evident from the 

 relation between wavelength in the guide, X„, wavelength in free space, \, 

 and cutoff wavelength, X<-: 



A cam or mechanical linkage may be employed to obtain a linear scale. 





