A quad‐band bandpass filter using sept‐mode double square‐ring loaded resonator

Abstract

Abstract A quad‐band bandpass filter (BPF) using a sept‐mode double square‐ring loaded resonator (DSRLR) is presented in this article. Based on the symmetrical geometric construction of the DSRLR, the even‐ and odd‐mode analysis with theoretical equations is employed to analyze the proposed BPF, and current density are obtained. Additionally, multimode splitting and resonant characteristics of the proposed DSRLR are explored. Accordingly, the design procedure is given after a pair of feedlines with dual‐finger structure is added to the I/O ports to introduce extra transmission zeros (TZs). Four TZs—including two created by the dual‐finger feedlines—are produced to separate four passbands that exhibit a sharp skirt and high stopband rejection. Finally, a microstrip quad‐band BPF is fabricated on a Teflon substrate with four measured passbands centered at 2.46, 3.41, 6.65, and 9.09 GHz with insertion losses of 0.68, 0.73, 0.81, and 1.23 dB, respectively. A satisfactory agreement is achieved between the measurement results and full‐wave simulation results.