1170 THE BELL SYSTEM TECHNICAL JOUENAL, SEPTEMBER, 1953 



3. The polarization circulator. This is a modification of the one way 

 transmission system in which there are two connections, with polariza- 

 tions at 90° to each other, on either side of the ferrite rotating element. 

 This is shown schematically in Fig. 9, along with a symbol which has 

 been suggested for this element.^^ Energy sent into the device with polari- 

 zation A emerges with polarization B, polarization B is rotated into C 

 polarization C is rotated into D, and polarization D emerges as minus A. 

 One practical application of this device is as a TR box in a radar system. 

 Another, recently suggested by A. G. Fox, is as a device for separating 

 the various channels in a multichannel communication system. Referring 

 to Fig. 10, the signal comes in at A. Branch B is terminated in a filter 

 which accepts one channel but reflects the remainder of the signal which 

 is passed on to C. Here another filter accepts the second channel but 

 passes the remainder on to D. D in turn feeds a second circulator. This 

 process can go on until all channels are taken care of. 



4. Measurement of magnetic field strength. The phenomenon of ferro- 

 magnetic resonance suggests a means of making measurement of mag- 

 netic field strength by observing the resonance frequency for a ferrite 

 when subjected to the unknown field. ^^ Allen^^ has recently described a 

 magnetometer in which an unknown field is measured by observing the 

 Faraday rotation which it produces in a standard sample. 



5. Other applications. There are several ways in which the interaction 

 of the steady field with the microwave field may be utilized in designing 

 switches, attenuators, and modulators. For example, one might set the 

 two rectangular guides in Fig. 8 with their transmission planes at 90°. 

 Then by varying the current in the solenoid and thereby varying the 

 magnetic field applied to the ferrite, one may vary the amount of energy 

 accepted by the second waveguide. This is then an electrically controlled 

 attenuator. This same device offers the possibility of providing modula- 

 tion of the microwave signal by modulating the current in the solenoid. 



FERRITE 

 A 



"45° ROTATION 



c 



IB 

 POLARIZATION CIRCULATOR CIRCUIT SYMBOL 



Fig. 9 — Schematic representation of polarization circulator. The ferrite is 

 adjusted to 45° rotation by an external field, not shown. The circuit symbol for 

 the circulator is shown at the right. 



