Compact single-layer bandwidth-enhanced balanced bandpass filter using half-mode substrate-integrated waveguide

A compact single-layer balanced bandpass filter using just one half-mode substrate-integrated waveguide (HMSIW) multimode resonator with slots is proposed. The pair of microstrip direct-feed lines with space about 0.5 lambda(g) (lambda(g) is the guide wavelength of HMSIW at the centre frequency f(0)) makes TEHM 103, TEHM 104 and TEHM 105 modes of the HMSIW multimode resonator operate in differential mode and TEHM 102 and TEHM 106 modes operate in common mode (CM). As a result, the features of wide bandwidth and low insertion loss (IL) can be achieved when compared with the state-of-art substrate-integrated waveguide balanced filters. Furthermore, the etched slots can be slightly tuned to make up for the performance deviations caused by the manufacturing error. A prototype is designed with a total size of 1.2 lambda(0) x 0.14 lambda(0) (lambda(0) is the wavelength in free space at f(0)), the 3 dB fractional bandwidth (FBW) of 28% at the centre frequency of 6 GHz, the minimum IL of 0.86 dB, and the bandwidth of 20 dB CM suppression of 0.25f(0).