A Coplanar Stripline Filter With Out‐of‐Band Absorption for Wideband Operation

ABSTRACT This article presents a coplanar stripline (CPS) reflectionless filter that integrates out‐of‐band absorption, exploring a novel configuration that promotes applications of reflectionless filters. By taking advantage of the CPS structure, the CPS absorptive resonator is initially proposed and studied to keep lossless transmission at its resonance and produce sufficient absorption at complementary frequencies. Next, a wide passband is achieved when introducing broadside coupling between two back‐to‐back CPS absorptive resonators. To guide the design, a transmission‐line model representing the wideband CPS reflectionless filter is developed so that the wideband synthesis can be used to obtain all the circuit parameters for the specified transfer function. Subsequently, a 4th‐order prototype is implemented. Simulated results reveal the filter achieves good impedance matching across the entire band with significantly reduced out‐of‐band reflection. Finally, the prototype is fabricated with transitions at its external ports. The de‐embedded results in measurement are found in good agreement with the simulated ones, validating its compact size, wide reflectionless range, and efficient design process.