Dual-Band Gysel Filtering Power Divider with a Frequency Transform Resonator and Microstrip/Slotline Phase Inverter

This paper presents a novel dual-band Gysel filtering power divider (FPD) with an excellent isolation performance and a significantly wide isolation bandwidth. Although Gysel power dividers have been extensively studied in the field of radio frequency (RF), the integration of filtering functionality and the expansion of isolation bandwidth remain challenging. The proposed design addresses these challenges by incorporating frequency transform resonators (FTRs) and a microstrip/slotline (M/S) phase inverter into the classic Gysel topology. The FTR is directly connected to the output port to provide a dual-band response, enabling the Gysel FPD to operate without external coupling between the resonator and the port. The M/S phase inverter is a dual-layer 180 degrees phase shifter, designed to replace the conventional 180 degrees transmission lines loaded between the two isolation resistors of the Gysel FPD, achieving a wide isolation bandwidth. To validate the proposed design method, a dual-band Gysel FPD with center frequencies of 1.4 GHz and 1.7 GHz is designed, fabricated, and measured. The measured results show that the in-band return loss is greater than 20 dB, and the in-band insertion loss is about 0.6 dB, and the amplitude and phase imbalance characteristics are good. In addition, the 20 dB-isolation fractional bandwidth achieves 97% (0.78-2.25 GHz). The measured results show excellent agreement with the simulation results, validating the effectiveness of the proposed design methodology.