440 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1957 



solution of this problem, more than ordinary emphasis was placed on 

 ease of operation and elimination of ambiguities in the frequency de- 

 termination. The resulting instrumentation uses as a stable reference 

 frequency a crystal-controlled oscillator and a frequency multiplier which 

 yields three reference frequencies; 55.8, 223.3 and 4,020 me. These are 

 mixed in a crj^stal harmonic generator and mixer leading to a line spec- 

 trum of numerous reference frequencies with a constant spacing of 55.8 

 mc. The same crystal mixer produces a beat frequency signal between 

 the incident signal from the kl3\stron and each of these reference fre- 

 quencies. A communication receiver with modified IF stage permits 

 selection of a frequency marker out of these beat frecjuency signals. This 

 marker appears as a blank spot on the traces of incident and reflected 

 signal, and can be moved to any desired point bj^ simply changing the 

 frequency setting of the receiver. Since the receiver dial cannot be read 

 with great accuracy on the high frequency ranges, a frequency counter 

 connected with the local oscillator of the receiver permits a reading of 

 the oscillator frequency to an accuracy of 1 kcps. Noting that the local 

 oscillator is 0.455 mc removed from the difference signal, one obtains 

 the frequency of the difference signal with more than sufficient accuracy. 



6. Results of Measurements 



Transmission- and reflection-type cylindrical cavities have been em- 

 ployed for measurements with linearly and circularly polarized excitation 

 in the 6,000- and 9,000-mc frequency bands. Circular polarization is pre- 

 ferable in the region below saturation where frequency shifts are small. 



It is not necessary to choose ferrite discs of relatively small diameter 

 for the purpose of staying within the region of circular polarization close 

 to the cavity axis. The derivation of Xm ± k is not restricted to circularly 

 polarized fields, and the result takes into account that the field becomes 

 more and more elliptically polarized as one approaches the edge of the 

 cavity. This can be seen l)y rewriting (19) and (20) as follows: 



X"/ ± k' = ^^^^^^^ [Acoi (/?! + R,) - Aco^ (i?i - 7?2)] 



xn," ± k" = ^^^ [A(l/Q)±(/?i + R-^ - Ml/QURr - i?2)] (26) 



where F = t\o-/{2U). 



The factor (/?i — 7? 2), which is zero for circular polarization, corrects 

 the values for x». ± k if the disc extends into the region of elliptical 



