1356 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



1200 1600 2000 



APPLIED FIELD IN OERSTEDS 



2400 



2800 



Fig. 13 — -Dependence of rotation upon ferrite cylinder diameter. 



To illustrate the effect of the diameter of the ferrite specimen upon 

 the Faraday Rotation the following experiment was performed. A one- 

 quarter inch cylinder suitably tapered on both ends was measured in 

 the rotation measuring equipment and then ground down in steps of 

 about 0.020" and remeasured after each cut. The particular ferrite chosen 

 was one having very low dielectric loss and a dependence upon sample 

 diameter which is typical of many ferrites. The data are shown in Fig. 13. 

 At the largest diameter we observe a rotation which agrees with neither 

 the plane wave theory nor the small perturbation approach. This be- 

 havior is evidently due to the presence of a higher order mode of propaga- 

 tion in the section of waveguide containing the ferrite cylinder. 



Higher order mode effects are undesirable from the standpoint of 

 bandwidth as the rotation produced thereby is much more frequency 

 sensitive than that obtained from the TEn mode. This is evident from 

 consideration of the fact that the partially filled waveguide is much 

 closer to the cutoff for the higher order modes than for the dominant 

 mode, and we will show that frequency dependence is increased as 

 waveguide approaches cut-off for a given mode. 



