MICROWAVE KADAR TESTING 453 



Frequency Coverage 



Increasing the length of a wavemeter is desirable because this gives a 

 larger mechanical displacement for a given frequency interval. The per- 

 missible increase is limited, however, by ambiguity with the next lower 

 mode in going toward the upper end of the frequency scale and with the 

 next higher mode in going toward the lower end. This means that, if 

 ambiguit}- is to be avoided, the ratio of top to bottom frequency cannot 

 exceed (;/ -f 2)/w for a coaxial line of n quartenvaves. For a cylindrical 

 cavity resonator the ratio for the 0,1, « mode must be less than n -f I)/n, 

 the exact limit depending on proximity to cutoff. 



Giiideposts 



The following guideposts are suggested for choosing the type of wave- 

 meter in the microwave range. WTiere limitations of size and Q permit, 

 the coaxial quarter-wave type should be used because of its greater linearity. 

 If this type is inapplicable, the cavity type with r£i,i,„ should be used 

 unless its Q is inadequate, in which case TEo,i,n should be employed. 



Couplings 



A loop, oriiice or probe may be used for coupling to a wavemeter. Cou- 

 pling to a coaxial wavemeter is generally effected by a loop placed near a 

 short-circuited end so as to be in the maximum magnetic field. For coupling 

 to a cylindrical cavity wavemeter, an orifice in or near the base of the cavity 

 is usually employed. The coupling to the wavemeter is kept small enough 

 to avoid serious reduction of loaded Q. 



Types of Detectors 



\'arious types of detectors may be'associated with a wavemeter, the most 

 commonly used being (1) a cr\'stal rectifier and microammeter, or (2) a 

 thermistor bolometer. Wlien a crystal rectifier is employed with a cavity 

 or coaxial wavemeter a circuit similar to that shown in Fig. 9 is used. Im- 

 portant items in such a circuit are the "RF by-pass" condenser, and the 

 "video" condenser. The latter, by providing a low-impedance path to the 

 video signals, improves rectification efficiency when the input signal is 

 pulsed. The quarter-wave stub shown in the figure is used when the input 

 or coupling circuit does not provide DC and video paths, \\lien the signal 

 is pulsed at a low duty cycle, high peak currents through the crystal are 

 obtained even though the average current through the meter is small, and 

 it is possible to impair or burn out the crystal unless extreme care is taken. 

 The use of a thermistor, which is self -protecting for large overloads, avoids 

 this danger. Another ex-pedient is to limit the crystal current to a small 

 value and employ a video amplifier and oscilloscope. 



