Dual-Band Coaxial Cavity Bandpass Filter With Helical Feeding Structure and Mixed Coupling

In this letter, stepped-impedance resonators are used in coaxial cavity filter to generate dual-band response. Two key merits of the proposed filter are the helical feeding structure and the introduction of mixed coupling for coupling control and generation of transmission zero. The helical feeding structure makes it possible to conveniently control the external quality factors of two bands. To illustrate the concept, a third-order dual-band coaxial cavity filter is designed, fabricated and measured. The measured results are in good agreement with the full-wave simulation results.