Efficient Design of Large-Order Wide-Spurious-Free Dual-Band Evanescent-Mode Filters

The objective of this article is to discuss in detail the design procedure for large-order, wide-spurious-free dual-band filters. Two novel contributions are discussed in this context. A zero-volume diplexing junction and an approach to reduce the computational effort related to the tuning elements in the apertures. In this article, the design procedure is described in detail, going from an initial ideal inductive filter in rectangular waveguide to a final evanescent-mode filter structure that is suitable for manufacturing. In addition to theory, a filter prototype has been manufactured and measured showing excellent agreement with simulations, thereby fully validating both the filter structure that we propose and the complete design procedure. It is important to note that the final filter structure that we obtain is fully tunable so that the same structure, and the related design procedure, can effectively be scaled in terms of center frequency, filter order, and bandwidth. As a consequence, our contribution can indeed be used to design and manufacture a very large variety of dual-band filters, diplexers, and multiplexers.