Fully 3D-Printed Filter Based on Helical Resonators with Elliptical Line Cross-Section

In this paper the design and preliminary measurement results of a 3rd order VHF filter centered at 350 MHz with 25 MHz bandwidth are presented. The proposed filter is based on quarter-wavelength helical resonators which allow for improved results in terms of volume occupation with respect to traditional coaxial cavity resonators, while maintaining relatively high Q-factor values. The proposed filter is fully fabricated through a metal additive manufacturing process where aluminum alloy has been used. The shape of helical resonators has been carefully designed in order to achieve the best compromise between compactness, high unloaded Q-factor and feasibility with respect to the selected 3D-printing process. The use of additional support structures has been minimized during the printing process thanks to a proper elliptical cross-section of helical wire. Although surface finishing processes have not been performed before silver-plating, the experimental results are very promising.