Wide stopband QMSIW diplexer with large frequency ratio and flexible specification

In this letter, a wide stopband quarter-mode substrate-integrated waveguide (QMSIW) diplexer is proposed. By using coupling feeding lines and QMSIW cavities, the two passbands of the diplexer can be controlled independently by their respective QMSIW bandpass filters. In comparison to other counterparts, the proposed diplexer provides flexible passband specification, compact size, and high isolation. The design exhibits a wide 20-dB stopband up to 3.2f₀, a frequency ratio of 1.6, and a high isolation level larger than 42.2 dB across the passbands.