Abstract
In this article, we present the first demonstration of distributed and
symmetrical all-band quasi-absorptive filters that can be designed to
arbitrarily high orders. The proposed quasi-absorptive filter consists
of a bandpass section (reflective-type coupled-line filter) and
absorptive sections (a matched resistor in series with a shorted
quarter-wavelength transmission line). Through a detailed analysis, we
show that the absorptive sections not only eliminate out-of-band
reflections but also determine the passband bandwidth (BW). As such, the
bandpass section mainly determines the out-of-band roll-off and the
order of the filter can be arbitrarily increased without affecting the
filter BW by cascading more bandpass sections. A set of 2.45-GHz one-,
two-, and three-pole quasi-absorptive microstrip bandpass filters are
designed and measured. The filters show simultaneous input and output
absorption across both the passband and the stopband. Measurement
results agree very well with the simulation and validate the proposed
design concept.