MEASUREMENT OF DIELECTRIC AND MAGNETIC PROPERTIES 441 



polarization. For relatively small discs {Ri = R2), one obtains 



Xm' ± k' = „„ Aa)± 



2^ 



oioFRi 



(27) 

 xJ' ± k" = ^A(l/Q)± 



A large number of measurements was made with a half-wave, reflec- 

 tion-type cavity and linearly polarized excitation. Some typical results 

 Avill be discussed below to demonstrate the applicability of the disc 

 technique. The frequency shift measurements and x»/ ± 1^' for a ferrite 

 material of 1,300 oersted saturation magnetization are shown on Fig. 6. 

 The ferrite disc has a thickness of 0.0063 inch and completely covers the 

 endwall of the ca^'ity. If the field in the cavity were circularly polarized 

 throughout, then the freciuency shift would vary as x»/ ± «'• However, 

 elliptical polarization causes a deviation of the measured curves for 

 Aco_t from Xm ± k'. Agreement between theoretical and experimental 

 values of Xm' ± k' is good in the regions below and above resonance. 

 Measurements in the resonance region are not possible with a disc of 

 this size because frequency shift and change in Q are so large that the 

 assumption of a small perturbation is violated. In order to establish 

 further that measurements of Xm' ± n' are independent of disc diameter 

 three discs of the same material (saturation magnetization 1300 oersted) 

 with diameters of 0.249, 0.400, and 1.050 inches were measured in the 

 aboA'e-mentioned cavity. Plots of x»/ and k' (Fig. 7) indicate good agree- 

 ment for k' and some scattering of values for Xm'- This can be explained 

 by noting that the resonance frequency of the empty cavity enters into 

 the computation of Xm', but cancels out for k (equation 19). Conse- 

 quently, a very small change in the length of the cavity, as might be ex- 

 pected from opening and reassembling the device, will produce a notice- 

 able error in the low-field region. A change in cavity length of 10~^ inch 

 will produce a freciuency shift of 1 mc at an operating frequencj^ of 

 10,000 mc and introduce an error of the order of 0.02 into the measure- 

 ment of Xm'- This error may be minimized by using a relatively large disc. 



Discs Avith a diameter of 0.4 inch yielded good measurements of the 

 imaginary quantities Xm" ± k" in the low-field region. A typical result 

 for a low-loss ferrite (Fig. 8) show\s the measured quantities A{l/Q)^ 

 and the corresponding Xm" ± k" as a function of the applied magnetic 

 field. (The internal field in the ferrite is obtained by subtracting the 

 magnetization from the applied field). It is noted that only A{l/Q)_ 

 can be obserA-ed in the resonance region, whereas A(l /Q)+ becomes too 



