A coaxial-cavity tunable filter adopting spiral coupling loops

A compact continuous tunable coaxial cavity filter adopting spiral coupling loops was presented in this letter.The insertion of spiral loops can reduce the bandwidth variation across the tuning range of tunable filter notablely compare to the traditional coupling window.Compared to the filter with the traditional coupling structure, the new filter can generate a lower insertion loss and a tunable bandwidth without reprocessing the cavity.A 3-cavities tunable coaxial filter adopting this structure has been designed and fabricated.The experimental results show that both a tuning range of 56 percent and bandwidth variations of ±1.5 percent can be achieved.The tuning range of the tunable filter was operating from 230 MHz to 406 MHz,a 3 dB fractional bandwidth in 6.4MHz,and a pass-band insertion loss of less than 1.1 dB.