An Additive 3D-Printed Hemispherical Lens With Flower-Shaped Stub Slot Ultra-Wideband Antenna for High-Gain Radiation

This paper presents a 3D-printed hemispherical lens integrated with a planar ultra-wideband (UWB) antenna. The flower-shaped stub slot UWB antenna is made of 0.8-mm FR-4. The operating frequency of the UWB covers 3.10 GHz – 11.6 GHz with a nominal gain at zero degrees of 1.74 dBi. To enhance the UWB antenna’s high-gain radiation, a 3D-printed additive hemispherical lens is designed and fabricated from acrylonitrile butadiene styrene (ABS). The electrical properties, i.e., relative permittivity and loss tangent, of ABS are 2.66, and 0.003, respectively. Four different lens radii (8 mm, 10 mm, 12 mm, and 14 mm) are chosen to investigate the gain of the antenna. In all four cases, the 3D-printed lens is fixed in place in front of the UWB antenna with an optimum gap of 3 mm chosen to reduce the wave reflection between the lens and source antenna. Based on the measurement results, the reflection coefficient, $S_{11}$ , of four conditions still covers the UWB frequency range. The nominal gain at zero-degree values for lens radii of 8 mm, 10 mm, 12 mm, and 14 mm are 3.43 dBi, 4.22 dBi, 4.73 dBi, and 5.18 dBi, respectively. The proposed additive 3D-printed dielectric lens antenna also offers many advantages, i.e., ease of design and assembly, low-cost fabrication, and size reduction for high-gain antennas. Furthermore, the high-gain antenna provides a narrow half power beamwidth, which can be implemented to increase the resolution of the imaging system.

A dielectric lens mounted on a planar antenna can be fabricated using the micro-machining process; however, the micro-machining process involves many fabrication steps and hence is expensive. As an alternative, 3D-printing technology is receiving considerable attention in the fabrication of lenses for planar antennas.
The 3D-printed lens is an alternative technique to enhance antenna gain and improve antenna performance by reducing the complexity of the fabrication process. Various shapes of dielectric lenses have been investigated [16], [17], [23], [24], [25], [26], [27], [28], [29]. In [23] and [25], the grin lens was designed to be placed on top of the antenna source, and the number of layers of the grin lens was used to control the antenna gain. Hemispherical lenses [29], and conical lenses [16] were designed based on the elliptical shape to conform the radiation pattern in the UWB range. The advantages of 3D-printing techniques include ease of fabrication, rapid prototyping capability, and low cost [18], [22], [30], [31]. Additionally, 3D printing techniques allow the use of a variety of conductor and dielectric materials.
This paper presents an additive 3D-printed lens embedded with a UWB antenna. The structure of the UWB antenna is designed from a flower-shaped stub slot, which is made of a conventional epoxy substrate (FR-4) with a thickness of 0.8 mm. The measured nominal gain of the UWB antenna is 1.74 dBi at zero degrees. The structure of the 3D-printed dielectric lens antenna is hemispherical; the ratio of the lens radius and extended lens configuration is discussed in [20]. The electrical properties of the acrylonitrile butadiene styrene (ABS) material, which is used for fabrication, were investigated using the Keysight 85072A 10-GHz split cylinder resonator method [32]. The relative permittivity and loss tangent values of ABS are 2.66 and 0.003, respectively. Four lens radius values (8 mm, 10 mm, 12 mm, and 14 mm) are chosen to investigate the antenna characteristics, i.e., the reflection coefficient, gain, radiation pattern, and half power beamwidth (HPBW). Based on the measurement results, the reflection coefficient, S 11 , of the four studied conditions still covers the UWB frequency range, with values of 3.10 GHz to 11.60 GHz. The nominal gain at zero-degree for the four lens radii of 8 mm, 10 mm, 12 mm, and 14 mm increased to 3.43 dBi, 4.22 dBi, 4.73 dBi, and 5.18 dBi, respectively. At 7 GHz, which represents the center of the UWB range, the HPBW values for the lens radii of 8 mm, 10 mm, 12 mm, and 14 mm are 121.1 degrees, 89.2 degrees, 73.4 degrees, and 59.9 degrees, respectively. , respectively. The antenna is designed in a flower-shaped stub in front view, and its back view is designed with an inverted T-shaped slot and an inverted U-shaped slot, which are etched on the ground plane. This flower-shaped antenna was previously studied in [33], in which it was made using an FR-4 substrate and operated at a frequency range of 3.1 GHz -12.5 GHz corresponding to a relative permittivity, ε r , of 4.3 and a loss tangent, tanδ, of 0.025. The dimensions of the UWB antenna in [23] were a width, W s , of 16 mm, a length, L S , of 24 mm, and a substrate thickness, S t , of 0.8 mm. The average antenna gain at zero-degree was 1.74 dBi. The radiation pattern in the xz-plane is nearly omnidirectional at low frequency and bi-directional at high frequency. Based on the above, the flower-shaped antenna has low gain. At high frequencies, the beam is not situated in the center of the antenna and its direction cannot be controlled. Accordingly, a hemispherical lens was selected in this study and incorporated with the flower-shaped stub slot antenna to achieve a configurable beam and increase the antenna gain by increasing the lens radius. However, the previous size of the flower-shaped stub slot antenna cannot connect with the 3D-printed dielectric lens antenna due to its smaller active area. Therefore, the authors of the present study modified the UWB antenna by adding an active locking area (t 1 and t 2 ), as shown by the green region in Figures 1(a) and 1(b). The length values of t 1 and t 2 are 6 mm and 2 mm, respectively. The final modified UWB antenna has a width of 28 mm and a length of 26 mm.

B. 3D-PRINTED HEMISPHERICAL LENS DESIGN
A 3D-printed hemispherical lens design [20] was selected to enhance antenna performance, i.e., the antenna gain, HPBW, and radiation pattern. Figures 1(c) and 1(d) depict the 3D geometry and a cross-section view of the hemispherical lens, respectively. The hemispherical lens comprises three key sections. The first section is the lens radius, L r , which is chosen to enhance the antenna's gain. The second section is the lens extension length, L e , which is used to decrease the overall incidence of the side lobe of the antenna radiation pattern. To determine L r and L e , the ratio of these quantities is fixed at 1 to achieve the maximum realized gain at zero degrees [20]. The third section is the housing section, which is used for the locking lens UWB antenna with a flower-shaped stub slot.
The key parameters of the designed 3D-printed hemispherical lens are listed in Table 1.
The hemispherical lens is made of ABS, an engineering plastic widely used in many industries due to its excellent mechanical properties. It is a versatile material capable of withstanding high-impact forces and high temperatures, thus making it suitable for printing both functional prototypes and end-use parts. To characterize the electrical properties, i.e., ε r and tanδ, of the 3D-printed ABS material, the Keysight 85072A 10-GHz split cylinder resonator method [32] was chosen. The size of the analyzed sample was 60 mm × 60 mm with a thickness of 2 mm; this sample was printed with an infill density of 95%. The 3D printer used in this study is a Zortrax M200 [34]. The measured values of ε r and tanδ at 10 GHz for the ABS used in this study were 2.66 and 0.003, respectively.
The hemispherical lens was designed with four L r values from 8 to 14 mm in steps of 2 mm, and the efficiency of the antenna gain and the radiation pattern for each configuration were simulated. Table 2 shows the comparison of the total antenna efficiency without and with four lens radii, L r . The optimized gap, g, between the flower-shaped stub slot UWB antenna and the 3D-printed hemispherical lens is set to 3 mm, which was simulated using the 3D EM simulation tool CST Studio Suite [35]. The simulated reflection coefficient was determined both with and without a hemispherical lens, as illustrated in Figure 2(a). The simulated results show that the antenna without a lens has a frequency range of 3.06 GHz -11.48 GHz when calculated at a reflection coefficient below -10 dB. The antennas with dielectric lenses of 8 mm, 10 mm,    91228 VOLUME 11, 2023 Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply. gain at zero degrees of the antenna without a 3D-printed lens antenna is 1.77 dBi, whereas the average gain values for the 3D-printed lens antennas of 8 mm, 10 mm, 12 mm, and 14 mm are 4.65 dBi, 5.26 dBi, 5.77 dBi, and 6.56 dBi, respectively. Based on the simulated results for lens radii of 8 mm to 14 mm, the antenna gain at zero degrees will increase with increasing lens radius. Figure 3 shows a comparison between the simulated results of the 2D-normalized radiation pattern of the flower-shaped stub slot UWB antenna both with and without a hemispherical lens by fixing the lens radius at 12 mm. Three frequencies (3.5 GHz, 7 GHz, and 10 GHz) were selected to compare the low-, middle-, and high-frequency performance of the UWB antenna. Based on the simulated results, for the flower-shaped stub slot UWB antenna only, the antenna's radiation pattern is bi-directional. In contrast, when the 3D-printed hemispherical lens antenna is integrated, the radiation patterns are changed to directional. Thus, these results indicate that the 3D-printed lens antenna can reconfigure the radiating beam from a bi-directional to a directional beam antenna.

C. FABRICATION AND ASSEMBLY
The completed models of the proposed flower-shaped antenna were exported from the simulation tool, CST Microwave Studio Suite, using the DXF file format. The top view and bottom view of the antenna were routed and fabricated using an LPKF ProtoMat S100 circuit board plotter [36], which has a high travel speed of up to 50 mm/sec and a cutting resolution of 0.25 µm. When finishing with circuit board plotter, the fabricated prototype of a flower-shaped antenna connecting with a 50-ohm SMA Panel Jack 2-Hole Flange Straight Flat Tab WR-SMA [37].
The Zortrax M200, a fused deposition modeling machine, was used to fabricate the hemispherical lens used in this study. The proposed lens antenna was constructed from ABS with a filament diameter of 1.75 mm. The 3D EM simulation program CST Studio's STL file format was used to export the final 3D models of the proposed hemispherical lens.  For printing, the layer printing thickness was set to 90 µm using a nozzle diameter of 0.4 mm and a 95% infill density. Figure 4 shows the fabricated prototype of the flower-shaped stub slot UBW antenna with the four different 3D-printed hemispherical lens radii.

A. MEASUREMENT SETUP
A Rohde & Schwarz ZVB-20 vector network analyzer (VNA) was used to measure the reflection coefficient, S 11 , and radiation pattern of the four fabricated hemispherical lens antennas. To measure S 11 , the Through-Open-Short-Match (TOSM) calibration method was selected. The start and stop frequency values were set to 3.0 GHz and 12 GHz, respectively. The number of points was set to 901. To measure the radiation pattern of the proposed lens antenna, a linearly polarized R&S®HF907 double-ridged wave horn antenna [38] was used as a reference antenna. The frequency range of the standard horn is 800 MHz to 18 GHz. A DAMs Heavy-Duty Antenna Model-5100 turntable was used to rotate the device under test (DUT); this was connected with the VNA and controlled using Diamond Studio Antenna Measurement Studio Software [39]. The distance between the reference antenna and the DUT was set from a far-field condition. The turntable was automatically rotated from 0 • to 360 • in steps of 1 • . Figure 5 shows the measurement setup of the 3D-printed lens antenna and a standard antenna in an anechoic chamber.

B. MEASUREMENT RESULTS
The reflection coefficients, S 11 , for the flower-shaped antenna with all four hemispherical lens sizes are shown in Figure 6. Based on the measured results, the lens radius, L r , of the hemispherical lens at 8 mm, 10 mm, 12 mm, and 14 mm can cover the UWB frequency range, when calculating the reflection coefficient, S 11 , lower than -10 dB. The reflection coefficient values, S 11 , corresponding to dielectric lens radii of 8 mm, 10 mm, 12 mm, and 14 mm are 3.10 GHz -11.58 GHz, 3.08 GHz -11.76 GHz, 3.08 GHz -11.68 GHz, and 3.05 GHz -11.74 GHz, respectively.
The measured radiation patterns are shown in Figure 7. The three sampling frequencies of 3.5 GHz, 7 GHz, and 10 GHz were selected to represent the low, middle, and high frequencies of the UWB range. To clearly visualize the enhancement antenna gain, the normalized condition was calculated for only the flower-shaped stub slot UWB antenna. At a zerodegree angle and a low frequency of 3.5 GHz, increasing lens radii of 8 mm, 10 mm, 12 mm, and 14 mm correspond to gain increases to +0.14 dBi, +0.47 dBi, +0.50 dBi, and +1.06 dBi, respectively, as shown in Figure 7(a). At an intermediate frequency of 7 GHz, Figure 7(b) shows that the antenna gain increased to +1.87 dBi, +2.96 dBi, +4.57 dBi, and +4.77 dBi for lens radii of 8, 10, 12, and 14 mm, respectively. Finally, for the high-frequency 10 GHz band, the antenna gain values for increasing lens radius increased to +1.89 dBi, +2.73 dBi, +5.12 dBi, and +5.33 dBi, as shown in Figure 7(c). Based on the measured radiation pattern results  for a zero-degree angle, combining the hemispherical lens and flower-shaped antenna achieves high gain and reconfigures the radiation beam from bidirectional to marginally unidirectional, depending on the radius of the hemispherical lens. Table 3 summarizes the results of the measured HPBW for the four fabricated lens antennas, compared with the bare flower-shaped stub slot UWB antenna. At a low frequency of 3.5 GHz, the HPBW slightly decreases with increasing lens radius. However, at the middle frequency of 7 GHz and high frequency of 10 GHz, the HPBW rapidly drops from 155 degrees to 53 degrees and from 109 degrees to 47 degrees, respectively. Thus, this narrow HPBW could be potentially applied in many applications, including increasing antenna resolution or detecting small objects.

IV. CONCLUSION AND FUTURE WORK
This study proposed an ultra-wideband antenna with a flower-shaped stub combined with a 3D-printed hemispherical lens. The UWB antenna has a width of 28 mm, a length of 26 mm, and a thickness of 0.8 mm and was assembled with a dielectric hemispherical lens. Four lens radii of 8 mm, 10 mm, 12 mm, and 14 mm, were chosen to investigate the antenna's characteristics. Based on the measured results, the reflection coefficient, S 11 , of the antenna with and without a dielectric lens can cover the operating frequency of UWB (3.1 GHz -10.6 GHz). The antenna gain at zero degrees will increase with increasing lens radius. Table 4 extensively shows the figure-of-merit comparisons of the lens processes with other published works, [16], [17] and [23], [24], [25], [26], [27], [28], [29]. This technique provides many advantages, including ease of design, low complexity of fabrication and plug & play function, enhancing the antenna gain, and providing reconfigurable beams; thus, the proposed technique can be applied to improve the resolution of various imaging applications.   Then, he returned to TU Dresden to lead a research team in microsensors and MEMS ICs for the Chair for Circuit Design and Network Theory. In 2013, he was appointed as a Lecturer (an Assistant Professor) with the School of Electronic and Electrical Engineering, University of Leeds, Leeds, U.K., where he is currently an Associate Professor. Since 2022, he has been appointed to lead a research team as an Adjunct Faculty Member of the Micro and Nanosystems Department, KTH Royal Institute of Technology. His research interests include integrated smart highfrequency components, sustainable micro and nanosystems, and low-cost microfabrication processes. Dr