3-D Printed W -Band Waveguide Twist With Integrated Filtering

—This work demonstrates the integration of a low- pass filter into a 90 ◦ waveguide twist at W -band (75–110 GHz), manufactured using polymer-based 3-D printing. For the first time, a 1-D periodic electromagnetic bandgap (EBG) structure is incorporated within a waveguide twist. Unlike conventional filters, implemented using irises and septa, EBG structures employing hollow cavities are structurally robust and mechani- cally insensitive to 3-D printing. The measured average passband insertion loss is only 0.48 dB at the W -band while using our unconventional split-block solution ( H -plane a -edge split with raised lips). A nontwist thru filter has also been demonstrated, as a reference. Our approach demonstrates the potential for the low-cost manufacture of compact and high-performance multifunctional integrated waveguide components at millimeter-wave frequencies.


3-D Printed
Abstract-This work demonstrates the integration of a lowpass filter into a 90 • waveguide twist at W -band (75-110 GHz), manufactured using polymer-based 3-D printing. For the first time, a 1-D periodic electromagnetic bandgap (EBG) structure is incorporated within a waveguide twist. Unlike conventional filters, implemented using irises and septa, EBG structures employing hollow cavities are structurally robust and mechanically insensitive to 3-D printing. The measured average passband insertion loss is only 0.48 dB at the W -band while using our unconventional split-block solution (H-plane a-edge split with raised lips). A nontwist thru filter has also been demonstrated, as a reference. Our approach demonstrates the potential for the low-cost manufacture of compact and high-performance multifunctional integrated waveguide components at millimeterwave frequencies.
In modern communications, radar, and sensor systems, waveguide twists and filters are both important passive components, providing respective polarization rotation and frequency selection functionalities, which are normally separate. Compared to the conventional approach of cascading these individual components, integration is more attractive because of its miniaturization, lightweight, and low-loss properties.
To date, a number of integrated twist-filter waveguide components have been reported in the open literature. For example, some use gradual rotation [6], [7], [8], while another employs a step-twist transition [9]. Diaphragm irises [6], [9], transverse offset irises [7], and E-plane septum insert [8] are used to realize the corresponding filter responses. Rotational irises and septa represent the most common approach to realize a twist filter. However, irises and septa become more structurally delicate and sensitive as the working frequency is increased, requiring more accurate manufacturing techniques-not suitable for 3-D printing.
An alternative method to provide frequency selection is the use of 1-D electromagnetic bandgap (EBG) structures. An EBG structure is a class of periodic metamaterial with unique properties that prevent wave propagation (stop bands) at certain frequencies. EBG structures are commonly used in gap waveguides to stop leakage from the air gap, without the need for electrical contact between the top and bottom waveguide parts. These periodic structures can employ different unit cell implementations, including mushroom [10], metal pin [11], [12], [13], and hollow cavity [14], [15] solutions. Mushroom-EBG structures are normally integrated into printed circuit boards, which are inherently more lossy than the fully metal-based solutions, due to dielectric losses. For the two metal-based pin and cavity solutions, the latter is structurally more robust and can be more cost-effective, when compared to the former. As a result, this approach is adopted here for low-cost and high-performance applications at millimeterwave (mm-wave) frequencies.
After an exhaustive literature search, to the best of our knowledge, we demonstrate the first proof of principle for a 90 • waveguide twist with integrated EBG filter. Note that the previously reported waveguide twist-filter examples have either bandpass filter (BPF) or low-pass filter (LPF) passbands within the frequency range between 10 and 33 GHz. In this letter, we also provide a low-cost manufacturing solution, using 3-D printing, which demonstrates high performance with a passband between 75 and 92.6 GHz.  EBG units and, therefore, g should be small enough so that the effects of EBG units are observable. However, a small gap between the top and bottom parts can cause strong reflections and, therefore, poor return loss responses. Therefore, g = 0.30 mm is chosen as a compromise when considering both effects. The unit cell periodicity p corresponds to half the guide wavelength at the mid-stopband frequency. Increasing the periodicity can narrow the stopband region. Increasing the unit cell height h provides better rejection within the stopband. Above h = 0.60 mm, the filter responses effectively remain unchanged. Here, h = 0.70 mm is chosen to compensate for fabrication errors. The increase of the radius r provides a denser unit cell distribution, and the rejection ability within the stopband is, therefore, improved. Fig. 1(a) and (b) shows the unit cell of a mirror-symmetric hollow structure and the 1-D periodic structure with N unit cells along the propagating x-axis, respectively. The physical dimensions for the unit cell are: a = 2.54 mm, g = 0.30 mm, p = 1.70 mm, h = 0.70 mm, and r = 0.55 mm.
All electromagnetic (EM) full-wave simulations are undertaken using Ansys High-Frequency Structure Simulator (HFSS), with bulk copper. Fig. 2(a) and (b) shows the dispersion diagram for the designed unit cell and eight identical unit cells cascaded together, respectively. These are obtained using the HFSS eigenmode solver and a standard Bloch analysis [16]. It can be seen that the lower waveguide EBG cutoff frequency for the first propagation mode is 64.1 GHz, which is higher than the 59.0-GHz cutoff frequency for the waveguide's TE 10 mode of propagation. The stopband is from 93.3 to 121.1 GHz, with a peak attenuation of 5.6 dB/unit cell at 106.8 GHz; both filters are designed to have a 3-dB cutoff frequency of 93 GHz.
Both EBG filters are designed to be compatible with standard WR-10 MPRWGs, having internal height and width cross-sectional aperture dimensions a = 2540 µm and b = 1270 µm, respectively. A split-block design is chosen, with "trough-and-lid" assembly, as this ensures the removal of all resin residues after 3-D printing and allows the waveguide's internal walls to be sufficiently metallized [2], [3]. Fig. 3(a) and (b) shows the plan views for the bottom parts of the designed filters. Clamp holes are designed for screw tightening and dowel holes are used for alignment. A close-in view of the thru filter's step transition is shown in Fig. 3(a), where the physical dimensions are: L s1 = 0.90 mm, L s2 = 0.30 mm, h s1 = 0.55 mm, h s2 = 0.15 mm, and h s3 = 0.27 mm. The step transition effectively reduces the waveguide's height from b down to g. The raised lips are designed to provide good ohmic contact between the bottom and the top parts, used to mitigate against EM energy leakage. Lip widths on both edges of the flanges are broadened to provide enhanced structural tolerances when the top and bottom parts are assembled [2], [3]. Fig. 3(c) shows the side view for the mirror-symmetric hollow cavities of the thru filter. Simulations show that the heights for the first and last two cavities have significant effects on the impedance matching and, therefore, they are reduced to form a discrete taper that minimizes reflections. The physical dimensions for the designed mirror-symmetric structure are: g = 0.30 mm, h e1 = h e8 = 0.15 mm, h e2 = h e7 = 0.27 mm, and h e3 = h e4 = h e5 = h e6 = 0.70 mm.
Our W -band 90 • twist has a flange-to-flange component length of 24.60 mm, which includes 4.65-mm thru lines within each flange. This gives a total twist length of 2.354λ gL , where λ gL = 6.50 mm is the guided wavelength at the lower bandedge frequency of 75 GHz (giving the worst case attenuation), having a rotational smoothness of 38 • /λ gL .
To transform from a waveguide thru to a twist, a smooth rotation is formed to realize the final 90 • polarization shift, as shown in Fig. 3(b). The mirror-symmetric hollow structure design is exactly the same as the one shown in Fig. 3(c), except for the physical rotation of 10 • between adjacent cavities. The transition design, shown in Fig. 3(b), is adjusted to accommodate for the rotation. The physical dimensions for the designed step transition are: L t1 = 0.85 mm, L t2 = 0.30 mm, h t1 = 0.50 mm, h t2 = 0.17 mm, and h t3 = 0.30 mm.

III. FABRICATION
The 3-D printing technology used in this work is lowcost masked stereolithography apparatus (MSLA). The Elegoo Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply. water washable photopolymer resin (Ceramic Gray) was used; mainly for its low shrinkage, ease of postprocessing, and low odor. Our Phrozen Sonic Mini 8K MSLA 3-D printer has a quoted 22-µm pixel resolution in the x-y build plane and the layer thickness in the z-direction is set to be 20 µm. The printing orientations for our filters are important. The thru filter is placed horizontally on the build plate, while the twist filter is rotated by 45 • to ensure the symmetric printing for both ports. The 3-D printed parts are initially electroless plated with a thin layer of nickel and then electroplated with a 25-µm layer of copper, which is compensated for in our simulations. This level of thickness is required to ensure that all edges and the bottom of the cylinders are sufficiently plated. We previously determined [2] that the average radius of hemispherical protrusions is 3.7 µm, having an average separation distance of 17 µm, giving an rms profile roughness of R q = 1.41 µm.

IV. MEASURED RESULTS
Measurements were made at Imperial College London, London, U.K., using the Agilent Technologies E8361A PNA vector network analyzer and 67-110-GHz frequency extension heads. Fig. 4(a) and (b) shows the disassembled 3-D printed filters and the assembled twist filter and its W -band test setup, respectively.
The simulation and measurement results for the thru filter and twist filter are shown in Fig. 5(a) and (b), respectively. Good agreement between the simulation and measurement results is achieved. The measured ripples are relatively very small and due to Fabry-Perot resonances caused by impedance mismatches at both measurement ports. For the thru filter, the measured 3-dB cutoff frequency is 90.5 GHz and the average passband insertion loss is 0.87 dB at the W -band. In the rejection band, the attenuation is greater than 20 dB above 93.1 GHz, where the peak attenuation of 46.1 dB is achieved at 107.7 GHz.
For the twist filter, the measured 3-dB cutoff frequency is 92.6 GHz and the average insertion loss is only 0.48 dB. In the rejection band, the attenuation is greater than 20 dB above 95.8 GHz, where the peak attenuation of 43.3 dB is  achieved at 107.9 GHz. According to the attenuation per unit cell given in Fig. 2(b), with both filters employing eight unit cells, the predicted peak attenuation is 44.8 dB. This is in good agreement with our measured values of 46.1 and 43.3 dB for the thru filter and twist filter, respectively.
A literature review has been undertaken of integrated twist-filter waveguide components and a comparison summary is given in Table I. As shown in Table I, only one previously reported example could be found at mm-wave frequencies [10].

V. CONCLUSION
In this letter, a new mm-wave multifunctional integrated waveguide component has been demonstrated. For the first time, a 1-D periodic EBG mirror-symmetric hollow structure is incorporated within a waveguide thru and twist to implement target LPF responses at the W -band. The measured twist filter meets its 93-GHz target cutoff frequency and its average passband insertion loss is only 0.48 dB. The measured thru-filter response, while also being low loss, has a 2-GHz reduction in its cutoff frequency that is believed to be due to random quantization errors in the MSLA printing [3]. Our 1-D periodic metamaterial solution is structurally robust and mechanically insensitive and, as a result, suitable for low-cost manufacture (e.g., using polymer-based 3-D printing), of high mm-wave frequency applications. Multifunctionality reduces the overall component assembly length and eliminates intercomponent flanges, dramatically reducing attenuation; opening up new opportunities for compact, low-loss, and low-cost integrated systems.