Reduced to silence

Abstract

A collaboration of researchers from the University of Birmingham and Shenzhen University have presented a Letter detailing a manufactured slotted hemispherical resonator which suppresses spurious modes by interrupting the corresponding surface current. The filter is monolithically prototyped by employing metallic additive manufacturing technology and demonstrates passband insertion and return losses of 0.9dB and greater than 20dB respectively. With the rapid advancements in additive manufacturing (AM) technologies, 3D printed filters have become a focused research topic for scientific and engineering communities. Waveguide bandpass filters are good examples on which to test the quality of the utilised AM processes as RF performance of the fabricated filters is generally sensitive to the process-induced dimensional tolerance. The tolerance can be caused by limited printing resolution, surface and geometrical imperfections, and shrinkage of the utilised printing materials. Therefore, to achieve high-performance filters, optimised filter structures and precise control of the AM process are required. Team members Dr. Jin Li (left) and Dr. Guan-Long Huang (right) holding the metallic 3D printed waveguide filters and (far right) the direct metal laser melting printer used during the fabrication process. Photograph of the metallic 3D printed X-band fourth-order slotted hemispherical resonator waveguide filter. Practical waveguide bandpass filters require low-loss passbands and wide, highly rejective stopbands. The stopband performance is usually limited by the passbands of unwanted spurious modes. Specifically, non-slotted hemispherical resonators have an intrinsically high unloaded quality factor, and spurious modes are located spectrally close to the fundamental TM101 mode. Therefore, it is necessary to eliminate these spurious modes without significantly interfering with the TM101 mode. In this work, a slotted hemispherical resonator is designed with the spurious modes effectively suppressed by introducing a simple rectangular slot on the hemispherical shell. The suppression contributes to a significant extension in the spurious-free stopband of the bandpass filter with metallic 3D printing enabling relatively fast and cost-effective prototyping. The filter was monolithically printed, without need of either assembly nor any fastening elements. Redundant structures of the filter are removed, and the filter weight is minimised. Co-author Jin Li describes the AM methods employed: “Direct metal laser sintering (DMLS)/selective laser melting (SLM) is an AM technique for building structures with powder-based metallic materials layer by layer. The process started with loading the 3D electronic model of the waveguide filter into a file preparation software package of the 3D printer. The model was then automatically sliced into multiple layers in a specified layer thickness, controllable according to the vertical printing resolution of the printer, creating 2D patterns of each layer were created. During the printing, thin layers of aluminum alloy powder were evenly coated onto the substrate plate. Once each layer had been distributed, each 2D slice of the filter geometry was fused by selectively melting the powder with a laser beam. The process was repeated layer after layer until the model was complete. After printing the external surface of the printed filter was manually polished, followed by other polishing processes such as sandblasting and vibration grinding to further minimise the surface roughness.” It should be noted that 3D printed slotted waveguide/resonator filters have been reported prior to this work, where slots and holes were put at current nulls on the cavity shells allowing lighter weight structures as well as allowing the plating liquids to penetrate inside the cavities. The main emphasis of slotting in previous works was to facilitate the metal plating process of the filters made by polymer-based 3D printing, instead of improving the spurious-free stopband. The focus of this work lies in maximising the suppression of spurious resonances by interrupting the surface current with slots. Another suppression approach is tuning the spurious-mode resonances spectrally far away from the fundamental mode, making the spurious-free region intrinsically wide. This can be achieved by capacitively loading the resonators with rods, but at the expense of degrading the unloaded quality factor. Alternatively, this can be realised by geometrically shaping the resonators. However, the design and modelling of such shaped resonators is complicated. This work offers a straightforward and universal approach to effectively suppress spurious modes in hemispherical resonators and can also be applied to other types of resonators and higher order filters. The stopband performance can be further enhanced with 3D printing which allows complex slotting structures structures to be readily fabricated with reduced cost and improved efficiency. The design focus was to maximise the spurious suppression without significantly altering the passband. To resolve this, the slot dimensions, were carefully designed to trade off the suppression level and the unloaded quality factor of the resonator. High-precision metallic 3D printing techniques were employed to prototype the complex filter into a simple part and make fabrication practical. The team's future studies will include both cavity geometries and conformal slot patterns on the cavity shells to further enhance the filter performance. Generally, functional slotted patterns can be adopted to other cavity-based components such as multiplexers and antennas. Again, 3D printing allows such slotted cavity structures to be easily fabricated. Doctor Li notes that several imperfections in this work need to be further improved. “First, the frequency shift in the passband due to volume shrinkage of the printed alloy can be corrected by performing a structural compensation to the filter model before the printing. The shrinkage can be induced in the laser melting process by the contraction of solids down to ambient temperature. Second, the passband insertion loss can be reduced by electroplating a thin layer of copper onto the printed alloy.” The next decade is likely to see an influx of 3D printed components operating from ultrahigh frequency to terahertz bands, with significant technical upgrades expected in the design and fabrication processes. The commercialisation of more high-precision 3D printing techniques, 3D printers, advanced printing materials, and associated services will be part of this future growth.