A Wide‐Stopband Bandpass Filter Using < λ/16 Capacitive‐Coupled Short‐Circuited Resonators in Wide‐Gap Coplanar Waveguide Structure

A compact bandpass filter (BPF) using a very short capacitively coupled resonator exhibits wide-stopband characteristics. It is based on two <λ/16 short-circuited resonators built on a coplanar waveguide (CPW) having unusually wide gap. The filter's input/output ports are connected to the resonator through microstrip-to-CPW broadside-coupled transition structure. The proposed filter is simple and compact without employing any additional structural elements that are often used in conventional approaches. It is found that the stopband characteristic can be significantly extended only by properly setting the coupling capacitance of the broadside coupling structure, the length of the resonator, and the CPW gap. A second-order BPF with a center frequency (f0) of 2.45 GHz and a 3-dB bandwidth of 10.6% is fabricated on a liquid crystal polymer substrate, demonstrating dramatically extended stopband suppression of 30 dB up to 9.8f0. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1752–1755, 2013