Wideband Band-Stop X-Band Filter Using Electrically Small Tightly Coupled Resonators

An electrically small, coupled resonator X-band filter capable of exhibiting a 25% fractional bandwidth band-stop response is presented. Different than the existing designs, the capacitively-loaded open loop resonators of the filter are realized over a 7 μm thin Benzocyclobutene (BCB) layer in order to bring them in close proximity of a microstrip feed line to achieve the desired wideband response. The resonator shape was specifically modified to minimize the width of the filter to 2.5 mm (i.e., λ₀/13) at the center frequency of 9.2 GHz in order to be employed within the gaps of a tightly packed tile antenna array. The filter exhibits a measured band-stop performance with > 25 dB insertion loss from 8.1 to 9.64 GHz. Within the band-pass regime (<; 7.3 GHz and > 11.2 GHz), the filter is shown to operate with <; 1.5 dB measured insertion loss.