Experimental and electromagnetic analysis of the minimal influence of top-surface roughness on S-band bandpass filters

Abstract The relationship between metallic surface morphology and high-frequency electromagnetic dissipation remains a fundamental question in conductor-loss physics. This work investigates how top-surface roughness affects the electrical behavior of S-band (2–4 GHz) bandpass filters using full-wave simulations and controlled measurements on ten samples. The top-surface roughness was varied from Ra = 1.13–1.89 μ m (Rq = 1.41–2.50 μ m). Across all cases, the maximum deviation in S 21 was < 0.2 dB, confirming that top-surface roughness has a negligible impact on insertion loss, passband characteristics, and resonant frequency. Field mapping further shows that power dissipation is instead dominated by current-crowding regions along conductor sidewalls and the metal-substrate interface. These results clarify the minor role of the top surface in conductor loss and provide materials-level insight for optimizing metallization and surface-processing strategies in high-frequency and optoelectronic systems.