Tunable Dual-Band Filters and Diplexers Using a Single Tuning Element

This article proposes a novel design for a tunable coaxial dual-band filter capable of simultaneously changing the center frequency of both bands. The resonator consists of a metallic rod with two fins of different dimensions inserted inside a cavity housing. Each fin represents a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(\lambda /4)$ </tex-math></inline-formula> resonator that is responsible for covering one band. The housing is shaped to change the capacitive loading on the two <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(\lambda /4)$ </tex-math></inline-formula> resonators and change their resonance frequencies as the rod rotates inside the housing. Specific iris shapes are designed to provide proper coupling between adjacent dual-band resonator and realize the tunable dual-band filter. A feature of the proposed design is that different shapes of housings can result in different directions of movement of the passbands within each range. Additionally, this design utilizes a single tuning element in the middle of the housing to rotate the resonators, independently of the filter’s order. To verify this concept, two reconfigurable dual-band six-pole filters were designed and tested. The only difference between these two designs is the shape of their housings, which demonstrates different directions of movement of the two passbands. The proposed concept demonstrates the feasibility of realizing tunable dual-band filters using a single tuning element for the first time. This article also shows how the concept can extended to realize a tunable diplexer with a single tuning element.