1358 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



number of half -wavelengths long at the low end of the frequency band 

 in order to increase the rotation at that point. The ferrite chamber may 

 be designed as a resonant cavity very tightly coupled so that over the 

 band the input impedance satisfies the matching requirements even 

 though there are multiple reflections inside the cavity. 



Some ellipticity of the resultant wave will be introduced by this 

 method as the wave will be linearly polarized only at those frequencies 

 at which the ferrite element is an integral number of quarter-wavelengths 

 long. However, if only a small correction is applied in this way the ellip- 

 ticity will be tolerable. 



The first and second broadbanding techniques may be combined in a 

 manner which was discovered in the course of our measurements to 

 determine bandwidth. Consider a circular waveguide such that at the 

 low end of the band the wave is just slightly above cut-off. Then the 

 wave impedance in the air-filled pipe will be high. If now the pencil of 

 ferrite is supported in a polystyrene holder, the wave impedance in the 

 ferrite region is much lower than that in the air filled region and multiple 

 reflections will be set up. In order to reduce the magnitude of the re- 

 flections to achieve just the right degree of correction each end of the 

 polystyrene can be tapered at the proper angle to give optimum cor- 

 rection. The polystyrene alone gives some improvement in bandwidth 

 according to the arguments first submitted, and the internal reflections 

 provide the rest of the compensation. The data shown in Fig. 14 show 

 the variation of rotation with frequency obtained in this manner. The 

 dotted extension of the low end of the curve indicates the expected ro- 

 tation in the absence of the correction. By means of this compensation 

 we are able to restrict the variation in rotation to ±4 per cent over a 

 band wider than 15 per cent. Over this band the ellipticity defined as the 

 ratio in db of the maximum to the minimum field strength was over 50 

 db which corresponds to perfectly linear polarization within our ability 

 to measure it. As a further advantage of this system the rotation obtained 

 at all frequencies from the ferrite pencil is increased through the use of 

 the polystyrene. This amplification of rotation follows directly from an 

 extension of the plane theory. ^^ 



The rotation per unit length is given by the relation 



\ = '^^Ve{vr--vjr^) (19) 



where /li± are the effective permeabilities seen by the circularly polarized 



" Such an amplification of rotation was first observed and studied by J. P. 

 Schafer of Bell Telephone Laboratories at Deal, N. J. 



