This paper presents the design of an ultra-compact, low-loss, multi-band bandpass filter (BPF) tailored for sub-6 GHz 5G applications. The filter is based on a half-mode substrate-integrated waveguide (HMSIW) structure integrated with metamaterial-inspired unit cells, consisting of three circular and two symmetrical serrated complementary split-ring resonators (CSRRs). This configuration enables efficient and stable wave propagation even below the HMSIW cutoff frequency. The proposed structure significantly reduces the overall footprint while supporting multiple passbands. The filter occupies a compact area of 14.1 \(\times\) 14.1 \(\hbox {mm}^{2}\) , corresponding to less than 0.004 \(\lambda _{g}^{2}\) (with \(\lambda _{g}\) being the guided wavelength at 0.97 GHz), making it one of the smallest quad-band designs reported to date. It is highly suitable for seamless integration into wireless communication systems, including WiFi, WiMAX, WLAN, 5G, and other sub-6 GHz applications. The filter exhibits four distinct passbands centered at 0.97 GHz (0.87–1.11 GHz), 2.58 GHz (2.45–2.65 GHz), 4.5 GHz (4.05–4.6 GHz), and 5.6 GHz (5.45–5.65 GHz), with corresponding insertion (return) losses of − 0.38 dB (30 dB), − 1.1 dB (35 dB), − 0.84 dB (25 dB), and − 1.1 dB (22 dB), respectively. Additionally, the design offers reconfigurability, allowing easy adaptation to different frequency bands with minimal structural modifications. To validate the proposed concept, a prototype was fabricated and experimentally characterized. The measured results show strong agreement with simulations, confirming the efficiency and robustness of the design.
