3D-printed coaxial-line filters for earth observation applications

In this paper, we present the latest advancements in the design of 3D-printed, self-supported air-filled coaxial-line X-band filters. The filters are intended for Earth Observation (EO) data downlink systems, where they must effectively reject signals in a wide frequency range. Each filter design incorporates a low-pass filter (LPF) structure, integrated with a short bandpass filter (BPF) section. Two filter designs were developed, with the second design being an improvement over the first one. In the first design, the LPF follows a classic stepped-impedance (SI) filter structure. In the second design, the sharp SI low-pass section was replaced with a smooth-profile LPF. This improvement leads to a wider rejection band up to around 40 GHz, to suppress the third harmonic and other undesired out-of-band frequencies, as required by the target application. The smooth surface profile of the inner and outer coaxial conductors is attained by inverse scattering synthesis, and optimized by means of shape deformation. Several prototypes were fabricated in one piece in aluminum alloy using selective laser melting (SLM) and measured, exhibiting excellent agreement with simulations, thus validating both design concepts.