A Compact Coupled-Line Butterworth Bandpass Filter for Sub-6 GHz Applications

This paper presents the design, fabrication, and experimental validation of a compact microstrip bandpass filter operating at 3.5 GHz for sub-6 GHz 5G wireless communication systems. The proposed filter is realized using a fourth-order Butterworth response implemented through a parallel coupled-line topology, aiming to achieve a maximally flat passband response, low insertion loss, and sufficient selectivity within the n77 frequency band (3.3-3.8 GHz). The design procedure starts with the synthesis of a normalized low-pass Butterworth prototype, followed by bandpass transformation and its implementation as an equivalent distributed microstrip structure. Electromagnetic analysis and parametric optimization are employed to accurately model coupling effects, transmission line behavior, and fabrication constraints. The filter is fabricated on a low-cost FR-4 substrate with a dielectric constant of 4.3 and a thickness of 0.8 mm using standard printed circuit board manufacturing techniques. Experimental characterization is performed using a vector network analyzer to evaluate the frequency response. Measurement results demonstrate a center frequency of 3.5 GHz with an impedance bandwidth of approximately 300 MHz, showing good agreement with the predicted performance. The proposed bandpass filter offers a compact footprint, simple geometry, and reliable frequency response, making it a suitable candidate for integration into 5G radio-frequency front-end modules and other modern wireless communication systems.