Design of Wideband Hybrid QMSIW and Coupled-Line Filter by Direct-Synthesis Approach in the Bandpass Domain

In this paper, a wideband hybrid bandpass filter using the quarter-mode substrate-integrated waveguide (QMSIW) and parallel-coupled lines is presented and discussed. For size- and radiation-reduction, the QMSIW cavity is folded and the striplines are shielded in an enclosed cavity. First, the working mechanism including the generation and coupling of the resonance is illustrated by the lumped-element equivalent circuits. Then, the filtering polynomials specified for a 5
^th
-order general Chebyshev response are derived from the synthesis methodology in the bandpass domain. And the initial dimensions of the proposed filter are obtained by applying the investigated direct mapping techniques between the polynomials and the values of the lumped elements. A transmission zero (TZ) introduced by the folded QMSIW is also taken into consideration in the synthesis process. Finally, a multilayer filter prototype is designed, simulated and implemented following the synthesis procedure. The proposed hybrid filter shows the advantages in terms of wide bandwidth (61.8 % 3-dB FBW), low insertion loss (0.76 dB), high rejection level (-60 dB @ 1.5 f
0
), low radiation and compact size (0.14 λ
g
× 0.14 λ
g
), etc.