Dual-Band Filters Based on Dual-Mode Ellipsoidal Cavities

A dual-band passband filter with ellipsoidal cavities is presented in this paper. The filter exploits the ellipsoidal cavity fundamental mode for obtaining one of the two bands and its degenerate for obtaining the other band. The basic building block of the filter is the ellipsoidal cavity with input-output irises rotated with respect to the cavity axes. This cavity produces two poles and one transmission zero (TZ). One pole is exploited in the lower band and the other in the upper band. The rotation angles of input-output irises are used for the fine positioning of the TZ, either in the lower stopband, in the middle stopband (between the two passbands) or in the upper stopbands. Filters with two N-order bands and N TZs are obtained in a modular design by cascading N cavities. Each of the N TZs can be independently positioned in each of the three stopbands. The filter presents an inline geometry and doesn't require the use of internal elements such as posts for controlling the frequency band separation. To validate the proposed approach, a 6-pole dual-band filter with TZ in the middle band has been designed, fabricated with a 3-D printer, and measured.