<p>This paper presents a narrowband bandpass filter based on a split-ring resonator (SRR) operating in the L band. An equivalent circuit model of the interdigital-arm SRR (IA-SRR) is proposed to provide deeper insight into the resonator’s electromagnetic behavior and to facilitate the design process. The proposed structure consists of a resonator, two capacitively coupled excitation lines, and three open stubs precisely integrated into the resonator. These stubs introduce transmission zeros near the passband, thereby significantly enhancing the filter’s selectivity. The design exhibits a compact size, ease of frequency response control, low insertion loss, and high out-of-band rejection, making it suitable for modern communication systems.</p>