An ultra-compact microstrip bandpass filtering coupler with suppressed harmonics and low group delay: A novel structure for 5G applications

This study introduces a novel microstrip design methodology for an ultra-compact bandpass filtering coupler, emphasizing a systematic approach to its development. The coupler is accurately designed, optimized, fabricated, and measured, achieving a small layout area of 0.003 lambda g2, excluding feeding ports. The design process begins with a comprehensive mathematical analysis of a fundamental microstrip layout, followed by simultaneous optimization techniques that enhance performance characteristics. Electromagnetic simulations and measurements are conducted to validate the design, revealing an excellent correlation between the two and confirming precise performance metrics. The coupler demonstrates a filtering response with a suppressed 3rd harmonic and exhibits S21 and S31 values of -3.135 dB and -3.103 dB, respectively, indicating low insertion losses and balanced output magnitudes. Its isolation factor exceeds -19.5 dB across the frequency range from DC up to 1.875 GHz, making it particularly suitable for 5G applications. Additionally, the coupler features flat channels with minimal group delays, with S21 and S31 showing the maximum group delays close to 0.8 ns. Our design methodology not only integrates these advantages but also sets a new benchmark for compact bandpass filtering couplers, delivering exceptional performance within a remarkably constrained footprint.