Researchers have developed a single-layer spatially resonant frequency (FSR) filter capable of operating simultaneously in four frequency bands (quad-band) within the S and C electromagnetic spectrum bands. This advance is significant because traditional FSR filters are typically single-band or dual-band, and integrating multiple operating bands into a single layer simplifies device design and manufacturing. The new design exhibits enhanced angular stability, meaning its performance remains consistent even when the incident wave arrives from different angles, a crucial feature for many practical applications.