A 3-D-printing-oriented coaxial-line filter with wide out-of-band rejection

In this letter, a novel design for a 3-D-printed, self-supported coaxial-line X-band filter is presented. The filter is intended for Earth observation (EO) data downlink systems, where it must effectively reject signals in a wide frequency range. The filter design incorporates a 15th-order low-pass filter structure with a smooth profile, integrated with a short bandpass section with four $\lambda /4$ short-circuited stubs. The optimization of the low-pass section is attained by means of shape deformation, including the inner and outer coaxial conductors, and leads to a wide rejection band up to around 40 GHz, to suppress the third harmonic and other undesired out-of-band frequencies. A prototype was fabricated in one piece in an aluminum alloy using selective laser melting (SLM) and measured, exhibiting excellent agreement with simulations. In terms of out-of-band performance, the proposed coaxial-line filter is superior to other related state-of-the-art solutions.