Half-wavelength resonator bandpass filters (BPFs) with improved skirt chara
cteristics are proposed in a heuristic manner. 'Tapping' for I/O and/or int
erstage coupling is the key concept here because the resulting half-wavelen
gth open-ended resonators have open stubs which lead to anti-resonances nea
r the resonant frequencies. Hence, transmission zeros appear near the reson
ant frequencies, i.e. the attenuation poles are created close to the passba
nds. Using this concept, excellent bandpass characteristics with multiple a
ttenuation poles are obtained by using both I/O and interstage tap-coupling
. The results of an experiment involving a coplanar structure are presented
which confirm the concept.