Compact branch-line coupler filter with transmission zeros

A compact filtering branch-line coupler is developed with short-ended half-wave resonators. For input/ output matching, tapped-line feeds incorporate a skew-symmetric configuration with magnetic inter-resonator coupling, so that transmission zeros can be created on high side of the passbands in the coupled and through outputs, as well as the isolated port. Circuit analysis is conducted based on the transmission line theory for prediction of these zeros. Measured responses of an experimental circuit show good agreement with simulated results.