THE FIELD DISPLACEMENT ISOLATOR 879 



2. Reverse loss ^ 30 db 



3. Return loss ^^ 30 db 



The extremely low forward loss strongly suggested the existence of an 

 electric field null in the plane of the resistance material. Consequently, 

 a theoretical investigation of the null condition was made and a set of 

 criteria estal)lished for the existence and utilization of the null. (E. H. 

 Turner^ independently developed the same null conditions.) An exten- 

 sion of the analysis leads directly to a set of scaling laws which permits 

 the ready design of isolators of comparable performance at other fre- 

 quency bands. 



i II. DESCRIPTION OF OPERATION 



I 



In Section IIA we will show how the "point-field" approach^ is used 

 to predict the ciualitative behavior of the structure and in Section IIB 

 we will apply a more rigorous analysis to the determination of the op- 

 timum design parameters. 



.1. Qualitative 



Prior to introducing the actual isolator configuration, we shall re- 

 ^'iew some elementary properties both of the ferrite medium and of an 

 unloaded rectangular waveguide. It is in terms of these properties that 

 we can understand, in a qualitative sense, the interaction of an rf wave 

 with a ferrite in such a waveguide. Since the behavior of a ferrite medium 

 in the presence of a static magnetic field and a small rf field has been 

 discussed in the literature^ the following resume is not intended to be 

 detailed. It is presented, however, to maintain continuity. 



If a static magnetic field is applied to a ferrite medium the unpaired 

 electron spins, on the average, will line up with the field. If now an rf 

 magnetic field, transverse to the dc field, excites the spin system these 



1 electrons will precess, in a preferential sense, about the static field. The 

 precession gives rise to components of transverse permeability at right 



{ angles to the rf magnetic field, leading to a tensor characterization of 

 the medium. This tensor has been given by Polder^ and may be diag- 

 onalized in terms of circularly polarized wave components. Correspond- 



, ing to the appropriate sense of polarization we use the designation -|- 



' and — . When the polarization is in the same sense as the natural pre- 

 cessional motion of the spin system, gyromagnetic resonance occurs for 

 an appropriate value of the static magnetic field. The scalar permea- 

 bilities /i_ and M+ are shown in Fig. 2 as functions of the internal static 

 magnetic field as would be observed at an arbitrary frequency. 



