MEASUREMENT OF DIELECTRIC AND MAGNETIC PROPERTIES 439 

 5. INSTRUMENTATION 



Our measurement technique has been influenced by a number of prac- 

 tical considerations including the need for determining accurately and 

 quickly a figure of merit for a large number of different ferrite materials. 

 In particular, a variety of low loss materials has become available in 

 experimental quantities requiring a precise technique for measuring 

 small loss factors below resonance as a guide for further ferrite develop- 

 ment. Therefore we were faced with the problem to develop an instru- 

 mentation capable of measuring these small loss factors but simple 

 enough to be operated without detailed knowledge of microwave tech- 

 niques. Fortunately the use of thin discs permits us to introduce a fairly 

 large volume of ferrite into the cavity without violating the basic as- 

 sumption of a small perturbation. As a consequence frequency shifts of 

 the order of 10 mc are obtained at static magnetic fields just sufficient 

 to saturate the material. Thus the quantities Xm' and k' may be measured 

 without difficulty in the regions below and above resonance. 



The thickness of the disc should be chosen to attain an aspect ratio 

 (diameter/thickness) of 50 or larger. Discs of 0.005 to 0.007-inch thick- 

 ness were employed in actual measurements at 9200 mc. For a typical 

 measurement of a 0.005-inch disc at 9200 mc, a change of 1/Q of about 

 2 per cent corresponds to a loss term Xm" -f- /' = 2 X 10"*. Measure- 

 ment of Q with a reproducibility of 1 per cent has been accomplished 

 initially by careful work, and it has been our objective to maintain this 

 accuracy in routine measurements by semiskilled operators. This re- 

 quired the use of rather elaborate circuitry for the precise measurement 

 of the changes in l/Q of the cavity. Although most of these techniques 

 have been used before in the field of microwave spectroscopy we hope 

 that the description of this instrumentation will be of interest. 



Fig. 5 shows a block diagram of the circuit. A klystron Type V58 

 (Varian Associates) is swept through a frequency band of about 80 mc 

 at X-band. The resulting signal with a center frequency of 9,200 mc is 

 used to excite a TEm mode cylindrical cavity. Incident and reflected 

 signals are separated by means of directional couplers and displayed on 

 an oscilloscope. Both signals can be aligned with the aid of a shorting 

 gate and a precision attenuator in front of the cavity. The reflected signal 

 shows clearly the cavity resonance which splits into two if a ferrite disc 

 is placed against the endwall of the cavity and magnetized along the 

 cavity axis. 



One of the major problems of the measurement is the accurate deter- 

 mination of these new cavity resonance frequencies and of the line width 

 of the displayed resonance curves between half-power points. In the 



