Self-diplexing mixed-coupling bandpass filter based on single slotted SIW cavity

Abstract

A self‐diplexing mixed‐coupling bandpass filter based on a single slotted substrate‐integrated‐waveguide (SIW) cavity is proposed. By etching a wide slot, two unequal half‐mode SIW parts are constructed and then fed by four coaxial ports to realise two bandpass filters with different centre frequencies. Since this etched slot can serve as a divider and an isolator, a flexible frequency ratio of 0.60–1.66 and high isolation of better than 20.0 dB is achieved. By further cutting the other two narrow slots, four quarter‐mode sub‐cavities are obtained where the mixed coupling is induced in the pass bands and a transmission zero (TZ) is introduced at the stopbands. The coupling and TZ are controllable by simply adjusting the length of these cut slots, which makes the proposed design easily redesigned for practical applications. A prototype with a size of 0.59λ0 × 0.59λ0 × 0.03λ0 has been fabricated. The measured centre frequency, TZ, 3‐dB bandwidth, and insertion loss of the lower pass band is 2.51 GHz, 2.96 GHz, 340 MHz, and 1.67 dB, respectively, whereas those of the upper pass band is 3.06 GHz, 2.53 GHz, 370 MHz, and 2.44 dB, respectively.