Wide-Stopband Filtering Transformers for Antennas with Flexible Input Impedance

Wide-stopband filtering transformer with multiple useful functions suitable for RF front ends is proposed in this study. Based on the capacitor-attached half-wave (λ/2) short-circuited resonators, the bandpass filter using short-ended parallel-coupled lines (SE-PCLs) has been realized with wide stopband and miniature size. Specifically, the tapped-line feeds are adequately applied in the input and output ends to make the proposed filter with flexible impedance-transforming capability. As a result, the transformer is employed in the microstrip patch antenna possessing the input impedance unequal to the termination impedance to enhance its performance.