Three-resonant-mode adjustment of the waveguide circulator

In its matched condition the scattering matrix eigenvalues of the three-port circulator lie equally spaced on a unit circle. As the frequency is varied these eigenvalues rotate in the complex plane at different speeds and the phase relation between them no longer corresponds to an ideal circulator. This is primarily due to the fact that the reference eigenvalue is normally associated with a nonresonant field pattern and the other two are associated with resonant ones. The purpose of this paper is to describe one circulation adjustment in which all three eigenvalues are associated with resonant field patterns. Experimental results obtained on a waveguide circulator are given for these two different adjustment procedures.