Design of compact balanced filters with wide DM upper stopband and CM rejection band based on semi‐circular patch

A new design approach to achieve compact size and high common‐mode (CM) suppression of balanced filters is proposed with two back‐to‐back microstrip semi‐circular patch resonators. In feeding structures, metalized via holes are adopted to vertically feed patches by differential pairs. Then, TM 11 mode is excited and TM 31 mode can be rejected, resulting in wideband differential‐mode (DM) harmonic rejection. Besides, with a slotline etched in the common ground plane, desired coupling strength can be achieved for a target bandwidth. Meanwhile, by virtue of the stacked structure, source‐load coupling can be easily introduced to generate transmission zeros close to the passband. With theory guidance, two balanced filter prototypes centered at 2.4 GHz are designed. Simulation and measurement results both indicate that proposed balanced filters exhibit high DM and CM performances as well as a compact size about 0.26 λ g × 0.53 λ g .