Compact Microstrip Dual-Band Bandpass Filter Using a Novel Stub-Loaded Quad-Mode Resonator

This letter presents a novel stub-loaded quad-mode resonator. Due to the structure geometry, even-odd-mode analysis is applied twice to explain the modal characteristics. Each of the four modes equivalent circuits is a quarter-wavelength resonator, so the quad-mode resonator has a compact size. The stub-to-stub coupling is introduced to split two identical odd-modes. Meanwhile, these two coupling stubs are folded inward, and a new transmission zero is produced, which separates four modes into two parts corresponding to two passbands: the first one is formed by even-modes, and the second one is composed by odd-modes. The center frequency and bandwidth of each passband can be controlled by the corresponding physical dimensions. For demonstration, a dual-band bandpass filter (BPF) using the proposed quad-mode resonator is designed. The simulated and measured results with good agreement are presented.