Design of S-band combline coaxial cavity bandpass filter

This paper presents the design of high power, low loss combline coaxial cavity bandpass filter. The design is based on Chebyshev prototype element values for lowpass filters, which are used in calculating the normalized capacitances per unit length between adjacent resonators and resonator to ground. The resonator diameter depends on the capacitance between resonator and the ground, while its length is set equal to λ₀/8, where λ₀ is the free space wavelength at center frequency f₀. Similarly the spacing between resonators is calculated using the capacitances between adjacent resonators. In order to tune the filter fo and bandwidth, the tuning and coupling screws are added in the design. The diameter of screws is computed using coaxial capacitance formulations. Once the theoretical calculations are completed, the filter is optimized using 3-D EM simulator to finalize its dimensions. A 5^th-order prototype filter at 2.8 GHz with 9% FBW (fractional bandwidth) and 0.01 dB passband ripple is designed, optimized and fabricated for verification. A good agreement between the measured and simulated results validates the proposed synthesis technique.