Quasi-Elliptic Dual-Band Bandpass Filter Using Flexible Mixed Coupling of Inverted-L-Shape Coaxial Resonator

This letter introduces a compact inverted-L-shaped coaxial dual-band bandpass filter (BPF) with multiple transmission zeros (TZs). The structure uses mixed coupling between adjacent resonators to generate TZs. Notably, mixed coupling can produce $N-1$ TZs with the Nth-order filter, significantly enhancing the frequency selectivity of the dual-band filter. A key advantage of this design is the ability to achieve arbitrary placement of the TZs without requiring any additional structures, allowing for independent control of both electric and magnetic coupling. To validate this concept, the dual-band BPF was designed, fabricated, and tested, with the final test results showing good agreement with the simulated results.