Dual-Mode Circular Cavity Filters via Azimuthal Wave Propagation

Dual-mode circular cavity filters are conventionally analyzed by coupling between degenerate resonances. We develop an alternative formulation based on electromagnetic waves propagating in the azimuthal direction. The dispersion relation, derived from the radial boundary condition on Bessel functions, determines a frequency-dependent propagation constant that enters the transmission matrix of each angular section. Transmission zeros arise when counter-propagating waves traversing two parallel paths cancel at the output port. This formulation encodes dispersion, characteristic impedance, and electrical length in a unified transmission-matrix framework, enabling direct synthesis from the cavity geometry. Measured results for a 6.74~GHz dual-mode filter with two transmission zeros confirm the predicted response.