Analysis and Design of a Millimeter-Wave Bandpass Filter with a Stopband in the Specified Higher Frequencies

The characteristics of a rectangular waveguide bandpass filter (BPF) are not well known in the high-frequency range where higher order modes can propagate. In this paper, the frequency response of a BPF, composed of symmetric inductive windows, is investigated. The range of frequency under investigation covers the region where not only the TE₁₀ mode but also higher modes can propagate in the waveguide. A window with incident waves of various modes is first characterized by a scattering matrix. The matrix elements for various modes are obtained in a closed form by means of the variational method. The overall characteristics of the BPF are then obtained by calculating the product of these matrices. A design of a BPF with a stopband in the specified higher frequencies is proposed. Prototype production proved the validity of the theoretical investigation.