A Compact Half-Mode Substrate Integrated Waveguide Bandpass Filter With Wide Out-of-Band Rejection

A compact bandpass filter exhibiting an ultra-wide out-of-band rejection is studied and developed. A combination of electromagnetic bandgap (EBG)-loaded half-mode substrate integrated waveguide (HMSIW) and composite right/left-handed (CRLH) HMSIW is used in this development. The proposed filter operates below the characteristic cutoff frequency of HMSIW. The CRLH HMSIW loaded with EBG structures is investigated first. The filter with a miniaturized size is then implemented with two slots etched on HMSIW to reduce coupling effects between EBG structures and interdigital capacitors. Defected microstrip structure (DMS) as feed lines are used to obtain an ultra-wide out-of-band rejection. Measured results show that this effective stopband can cover up to 10.3 times the design center frequency for a rejection level of 20 dB. The filter exhibits high selectivity, good stopband performance, and compact size.