Radiation from a Ferrite Cylinder

Abstract

A theoretical and experimental investigation has been made of a ferrite post in the aperture of a rectangular waveguide. This problem is representative of a general class of problems where an anisotropic obstacle is superimposed on an isotropic discontinuity. In the example treated by the diffraction approach, a simple solution has been obtained which predicts and explains the observed phenomena. A modified aperture is especially interesting because it offers good opportunity for directly measuring the effects of ferrite obstacles on rf fields. It also has the practical advantage of serving as a scanning antenna. The analysis of this radiating structure is also shown to apply to circulators that use cylindrical ferrites. In general, the theory shows that the energy in the far fields is directed at some angle φ with respect to pure forward scatter. A corresponding asymmetry is also present in the field plots inside the ferrite. The field displacement within the material is shown to explain this far field beam shift. Experimentally, continuous scanning from a waveguide was obtained over ±30° with a MgMn ferrite post 0.297 in. (0.754 cm) in diameter and with applied fields of less than 300 G. The frequencies were from 9.0–10.3 Gc/sec. The low reflection coefficients obtained make this a practical antenna. Beam patterns are presented, as are curves comparing the theoretical and experimental results.