A Design Method for Highly Selective Dual-Band Filters Based on Frequency-Variant Triplets

This letter presents a method for the design of highly selective dual-band filters. The frequency-variant triplets (FVTs) are applied to dual-band filter design. In addition to different passbands with different orders, bandwidths, or return losses, this method can generate
${N}$
-1 transmission zeros (TZs) for the specified
${N}$
th-order filter, significantly improving the frequency selectivity of dual-band filters. Moreover, the limitation of TZs achieved by the frequency-variant couplings (FVCs) structure widely used in coaxial cavity filter design is discussed for the first time. Furthermore, to facilitate physical implementation, a strategy is proposed to guide the selection of the production sequence of TZs in dual-band filters formed by inline FVCs and FVTs. Finally, a dual-band filter with four TZs is fabricated, whose synthesis, simulation, and test results perform the desired responses and are well-matched with each other.