Bandpass Filters Using Stepped Coupled Lines With Super Wide Stopbands

A class of wide-stopband bandpass filters (BPFs) using 5-stage stepped coupled lines (SCLs) and two quarter-wavelength shorted stubs (SSs) is proposed in this paper. The 5-stage SCL with total electrical length of
$\boldsymbol {\lambda }$
/4 is able to suppress the harmonics up to
$7f_{0}$
(
$f_{0}$
: center frequency) or even up to 9f
$_{0}{}_{~}$
through well-designed coupling pattern, resulting in super wide stopband. Two additional quarter-wavelength SSs will introduce transmission zeros (TZs) at even times of the center frequency, which are utilized to improve the passband selectivity. For demonstration, two compact microstrip prototypes have been experimentally characterized, in good agreement with the simulated ones. The proposed BPFs, featuring wide stopband and compact size, will have significant potentials in miniaturized wireless systems.