3-D Printed Circuits for RF and Microwave Applications

3-D printing is a promising technique to produce RF and microwave passive circuits. Compared with conventional machining techniques, 3-D printing enables the manufacture of complex structures with great geometrical complexity and/or printing the devices into a standalone part without the need of further assembly. Another great advantage of 3-D printing is the significant weight reduction, when the device is printed from polymer and metal coated. This paper presents a range of 3-D printed passive circuits investigated at the University of Birmingham. These circuits cover a wide frequency range, from 0.5GHz to 100GHz, and include a 0.5 GHz helical resonator filter [1], a 3 GHz combline filter [2], two 10 GHz waveguide filters based on spherical resonators [3]-[5], a 15 GHz filtering Butler matrix [6], a 30 GHz OMT [7], and two 90 GHz waveguide filters [8]. Among them, the 90 GHz filters represent the highest frequency 3-D printed filters reported in open literature to date. These devices will be discussed in detail in the sections below. Most of the devices are based on lightweight polymers, which are capable of handling high temperature when ceramic filled resin is used. 3-D metal printing is also utilised to produce several designs including a W-band filter. All of the circuits are optimised for 3-D printing, so that enhanced microwave performance and/or considerable weight reduction can be achieved. This cluster of work demonstrates the great potential of using 3-D printing for microwave applications. 0.5GHz Helical Resonator Filter [1] A filter working at a centre frequency of 0.5 GHz and a 4% fractional bandwidth (FBW) formed of four coupled helical resonators is designed and fabricated for potential applications in communication systems operating in the VHF and UHF ranges. The filter design as well as the fabricated device is shown in Figure 1. For each resonator the helix is fixed at both ends so that the resonator is less prone to vibration. The helix is designed to have variable width and this yields enhanced performance in terms of a higher unloaded quality factor and a spurious resonance at higher frequencies. Such a helix is ideally suited to 3-D printing which allows easy production of complex structures. Coupling into and out of the filter is realized through coaxial connectors. The filter is printed from a resin (Somos) using a stereolithography (SLA) printer, and is subsequently coated with around 25 μm thick copper. The filter is made in two pieces (the helical structures and a separate lid), in order to facilitate the metal plating process which requires access to inner surfaces. After metallization, these two pieces are assembled using bolts through holes 3-D printed through the whole side of the structure. The simulated and measured filter responses are shown in Figure 2. The measured return loss is better than 16.5 dB across the whole passband. The measured average passband loss is around 0.4 dB, which is around 0.1 dB higher than the expected value from simulations using conductivity of copper. Overall, there is a good agreement between measurement and prediction from CST simulation.