558 



ANTENNAS AND RF COMPONENTS 



tional to frequency, gyromagnetic resonance occurs and the permeability for 

 the positive sense of circular polarization reverses abruptly between 

 extreme values. In this same region the positive circularly polarized wave 

 experiences a high resonance absorption. Except in the region of resonance, 

 the ferrite is practically lossless for both senses of circularly polarized waves. 

 Various regions of the unusual and controllable permeability charac- 

 teristics shown in Fig. 10-29 and the resonance loss characteristic are 

 exploited in many ways to achieve the remarkable reciprocal as well as 

 nonreciprocal properties of the microwave ferrite devices. ^^'^^ Most of these 

 new devices depend upon the differential permeability for their operation, 

 while the resonance absorption isolator^^ exploits the high absorption 

 differential between positive and negative circularly polarized waves in 

 the region of gyromagnetic resonance. 



Faraday Rotation. A linearly polarized wave incident on a longitudi- 

 nally-magnetized ferrite element, as shown in Fig. 10-30, will undergo a 



Output Polarization 



Solenoid 



To Current Supply 

 Fig. 10-30 Principle of the Ferrite Microwave Faraday Rotator. 



rotation of its plane of polarization. This is because of the different 

 permeabilities seen by the positive and negative circularly polarized 

 components of the linearly polarized wave. (A linearly polarized wave can 

 be resolved into two oppositely rotating circularly polarized waves of equal 

 amplitudes.) Each circularly polarized component manifests a different 

 phase delay in traversing the ferrite section; and if losses are negligible, 

 they combine in a linearly polarized wave rotated by some angle 6 from the 

 input wave. The microwave Faraday effect, which permits electronic 



■i^C. E. Hogan, "The Elements of Nonreciprocal Microwave Ferrite Devices," Proc. IRE44:j 

 1345-1368 (1956). 



''^Henjamin Fax, "Frequency and Eoss Characteristics of Microwave Ferrite Devices," 

 op. a/., pp. 1368-1386. 



s^Max T. Weiss, "Improved Rectangular Waveguide Resonance Isolators," IRE Trans. 

 MTT-4, No. 4, 240-243 (October 1956). 



