Iris-coupled Cavity Filter

Model

The first figure illustrates a three-cavity direct-coupled H-plane iris filter. All resonators are coupled via irises. These type of filters are suitable for mass-fabrication. They achieve low insertion loss and high breakdown voltage. The corners of the cavities, in practice, can never be a rectangular due to the finite radius of the milling tool. The accurate design of such filters with rounded corners is of practical interest for low-cost fabrication. A waveguide source with TE₁₀ mode excitation is used to feed the filter.

Simulation

A template broadband simulation is prepared and then the S-parameter Simulation Setup tool is used to build the necessary simulations to extract the two-port S-parameters of the filter. The maximum grid step was limited to 1.30 mm in the frequency region of interest. The minimum grid step was 0.04 mm. The UPML absorbing boundary conditions are used to truncate the domain.

Results

The S-parameter Evaluation tool is employed to calculate the S-parameters of the filter. The bottom figure shows the performance of the filter with radius R = 1.5 mm. The corner radius mainly a fects the effective resonator lengths which results in a frequency shift towards higher frequencies and a degradation in insertion loss.

Ref. G. Conciauro, et. al., Wideband Modeling of Arbitrarily Shaped H-plane Waveguide Components by the Boundary Integral-resonant Mode Expansion Method, IEEE Tran. Microwave Theory and Tech., vol. 44, no. 6, July 1996.