Innovative Antenna Solutions for 5G and Wearable Technologies YAHYA AHMADI

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Wideband MIMO Antennas for 5G Mobile Terminals

Over the years, antenna design for mobile terminals has become increasingly difficult. This is mainly attributed to a limited space to house multiple antennas and wide frequency band requirements.

The fifth generation (5G) wireless communications makes it even more challenging for mobile terminal antenna designers since it requires a large number of multiple input/multiple output (MIMO) antennas having very wide frequency bands. In these antenna designs, though antennas are placed very closely to one another, high isolation between the antennas has to be satisfied [1. I. R. Barani and K.-L. Wong,

“Integrated inverted-F and open-slot antennas in the metal-framed smartphone for 2×2 LTE LB and 4×4 LTE M/HB MIMO operations,” IEEE Trans. Antennas Propag., vol. 66, no. 10, pp. 5004–5012, Oct.

2018], [2. H. Xu, H. Zhou, S. Gao, H. Y. Wang, and Y. J. Cheng, “Multimode decoupling technique with independent tuning characteristic for mobile terminals,” IEEE Trans. Antennas Propag., vol. 65, no.

12, pp. 6739–6751, Dec. 2017], [3. A. Ren, Y. Liu, and C.-Y.-D. Sim, “A compact building block with two shared-aperture antennas for eight-antenna MIMO array in metalrimmed smartphone,” IEEE Trans.

Antennas Propag., vol. 67, no. 10, pp. 6430–6438, Oct. 2019], [4. Y. X. Li, C.-Y.-D. Sim, Y. Luo, and G. L. Yang, “High-isolation 3.5 GHz eight-antenna MIMO array using balanced open-slot antenna element for 5G smartphones,” IEEE Trans. Antennas Propag., vol. 67, no. 6, pp. 3820–3830, Jun.

2019]. To save antenna volume and to place more antennas in a limited space, the concepts of designing antennas in pair with high isolation have been proposed [5. K.-L. Wong, C.-Y. Tsai, and J.-Y. Lu, “Two asymmetrically mirrored gap-coupled loop antennas as a compact building block for eightantenna MIMO array in the future smartphone,” IEEE Trans. Antennas Propag., vol. 65, no. 4, pp. 1765–1778, Apr. 2017], [6. J. Sui and K.-L. Wu, “Self-curing decoupling technique for two inverted-F antennas with capacitive loads,” IEEE Trans. Antennas Propag., vol. 66, no. 3, pp. 1093–1101, Mar. 2018], [7.

L. B. Sun, Z. H. Feng, Y. Li, and Z. J. Zhang, “Tightly arranged orthogonal mode antenna for 5G MIMO mobile terminal,” Microw. Opt. Technol. Lett., vol. 60, no. 7, pp. 1751–1756, Jul. 2018], [8. L. B. Sun, H. Feng, Y. Li, and Z. Zhang, “Compact 5G MIMO mobile phone antennas with tightly arranged orthogonal-mode pairs,” IEEE Trans. Antennas Propag., vol. 66, no. 11, pp. 6364–6369, Nov. 2018], [9. ] Z. Y. Ren, A. P. Zhao, and S. J. Wu, “MIMO antenna with compact decoupled antenna pairs for 5G mobile terminals,” IEEE Antennas Wireless Propag. Lett., vol. 18, pp. 1367–1371, 2019], [10. C. J.

Deng, D. Liu, and X. Lv, “Tightly arranged four-element MIMO antennas for 5G mobile terminals,”

IEEE Trans. Antennas Propag., vol. 67, no. 10, pp. 6353–6361, Oct. 2019], [11. J. W. Sui, Y. H. Dou, X. D. Mei, and K.-L. Wu, “Self-curing decoupling technique for MIMO antenna arrays in mobile terminals,” IEEE Trans. Antennas Propag., vol. 68, no. 2, pp. 838–849, Feb. 2020]. The concept of common mode (CM) and differential mode (DM) has been recently introduced to further reduce the antenna volume and to improve the isolation [12. H. Xu, S. S. Gao, H. Zhou, H. Y. Wang, and Y. J.

Cheng, “A highly integrated MIMO antenna unit: Differential/common mode design,” IEEE Trans.

Antennas Propag., vol. 67, no. 11, pp. 6724–6734, Nov. 2019], [13. ] L. Chang, Y. F. Yu, K. P. Wei, and H. Y. Wang, “Polarizationorthogonal co-frequency dual antenna pair suitable for 5G MIMO smartphone with metallic bezels,” IEEE Trans. Antennas Propag., vol. 67, no. 8, pp. 5212–5220, Aug.

2019], [14. L. Chang, Y. F. Yu, K. P. Wei, and H. Y. Wang, “Orthogonally polarized dual antenna pair with high isolation and balanced high performance for 5G MIMO smartphone,” IEEE Trans. Antennas Propag., vol. 68, no. 5, pp. 3487–3495, May 2020], [15. L. B. Sun, Y. Li, Z. J. Zhang, and H. Y. Wang,

“Self-decoupled MIMO antenna pair with shared radiator for 5G smartphones,” IEEE Trans. Antennas Propag., vol. 68, no. 5, pp. 3423–3432, May 2020], [16. Y. Ye, X. Zhao, and J. Y. Wang, “Compact high-isolated MIMO antenna module with chip capacitive decoupler for 5G mobile terminals,” IEEE

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Antennas Wireless Propag. Lett., vol. 21, pp. 928–932, 2022]. However, the main problem of the above existing work is that antennas with compact size, such as a half wavelength in electric length, usually have narrow bandwidth, since only one antenna mode or resonance is employed to cover the required very wide frequency bands. In [17. L. Chang, G. L. Zhang, and H. Y. Wang, “Dual-band antenna pair with lumped filters for 5G MIMO terminals,” IEEE Trans. Antennas Propag., vol. 69, no. 9, pp. 5413–

5423, Sep. 2021], [18. K.-L. Wong, B.-W. Lin, and W.-Y. Li, “Dual-band dual inverted- F/loop antennas as a compact decoupled building block for forming eight 3.5/5.8-GHz MIMO antennas in the future smartphone,” Microw. Opt. Technol. Lett., vol. 59, no. 11, pp. 2715–2721, Nov. 2017], [19. K.- L. Wong, Y.-H. Chen, and W.-Y. Li, “Decoupled compact ultra-wideband MIMO antennas covering 3300 6000 MHz for the fifth-generation mobile and 5GHz-WLAN operations in the future smartphone,”

Microw. Opt. Technol. Lett., vol. 60, no. 10, pp. 2345–2351, Oct. 2018], [20. L. B. Sun, Y. Li, Z. J.

Zhang, and Z. H. Feng, “Wideband 5G MIMO antenna with integrated orthogonal-mode dual-antenna pairs for metalrimmed smartphones,” IEEE Trans. Antennas Propag., vol. 68, no. 4, pp. 2494–2503, Apr. 2020], a variety of wideband antenna pairs have been proposed to circumvent the narrow bandwidth problem, however the penalty is that the overall electric size of the antennas is much larger or the antenna configurations are very complex. Furthermore, in most of the work outlined above, the focused and examined frequency bands of 5G New Radio (NR) are N77, N78 and N79 with frequency ranging from 3.3-5.0 GHz, whereas attentions have not been given to the N1, N2, N3, N7, N38 and N41 with frequency ranging from 1.71-2.69 GHz, in which 4 x 4 MIMO antennas are also compulsory for 5G NR. The antenna with large physical size, being accommodated in a very limited space due to the lower operating frequency bands, makes antenna design even more difficult, complex and challenging.

In this paper, the CM and DM theory proposed in [12. H. Xu, S. S. Gao, H. Zhou, H. Y. Wang, and Y. J. Cheng, “A highly integrated MIMO antenna unit: Differential/common mode design,” IEEE Trans. Antennas Propag., vol. 67, no. 11, pp. 6724–6734, Nov. 2019] has been significantly further developed for more compact antenna size and wider bandwidth. In comparison with the antenna in [12.

H. Xu, S. S. Gao, H. Zhou, H. Y. Wang, and Y. J. Cheng, “A highly integrated MIMO antenna unit:

Differential/common mode design,” IEEE Trans. Antennas Propag., vol. 67, no. 11, pp. 6724–6734, Nov. 2019], the CM antenna and DM antenna in the proposed antenna pair share the same radiator, rather than using two separated antenna radiators, for the purpose of antenna size reduction, and each of the CM and DM antennas has dual resonances for impedance bandwidth improvement, instead of a single resonance. The principle on how to achieve these features using an antenna structure that supports both CM and DM has been rigorously analyzed. A realistic antenna design example has been presented and examined to demonstrate the effectiveness of the proposed CM and DM approach for 5G MIMO antenna design of mobile terminals. Overall, the new contributions of this paper can be classified as:

(a) the 5G NR MIMO antennas have more compact size thanks to a co-radiator being utilized to combine the CM antenna and the DM antenna; (b) The 5G NR MIMO antennas have wider bandwidth because of the innovative dual-resonance approach, and they have a half wavelength, rather than one wavelength or longer; (c) The examined frequency bands for the 5G NR MIMO antenna pair are N1, N2, N3, N7, N38 and N41 with frequency ranging from 1.71-2.69 GHz, which have seldom been studied by existing work published in the literature.

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A Dual Wide-Band Mushroom-Shaped Dielectric Antenna for 5G Sub-6-GHz and mm-Wave Bands

Growing wireless data traffic requires high-rate data transmission, which necessitates a substantial increase in communication frequencies. This demand has led to the inception of fifth- generation (5G) wireless standards operating at sub-6-GHz and mm-wave band, which provides a 10 to 100 times increase in the data transmission rate. As a result, attempts have been made to develop wideband/multi-band antennas to 5G and beyond for telecommunication systems. Multi-band antennas have various applications in point-topoint and satellite communication systems. Moving toward the mm-wave band necessitates the design of high-gain antennas to compensate for signal losses. DRAs are known for their high efficiency, wide bandwidth, low fabrication cost, and simplicity of excitation.

Previous works employed a single DRA element covering several bands close to one another by exploiting special feeding mechanisms and by employing the fundamental and highorder DRA modes [1. P. Kumar et al., “Electronically controlled beam steerable dual-band star-shaped DRA for UAS and Wi-Fi data link applications,” IEEE Trans. Antennas Propag., vol. 68, no. 10, pp. 7214–7218, Oct.

2020], [2. S. Mishra et al., “Three-dimensional dual-band dielectric resonator antenna for wireless communication,” IEEE Access, vol. 8, pp. 71593–71604, 2020], [3. H. Tang, J.-X. Chen, W.-W. Yang, L.-H. Zhou, and W. Li, “Differential dual-band dual-polarized dielectric resonator antenna,” IEEE Trans. Antennas Propag., vol. 65, no. 2, pp. 855–860, Feb. 2017], [4. X.-C. Wang, L. Sun, X.-L. Lu, S.

Liang, and W.-Z. Lu, “Singlefeed dual-band circularly polarized dielectric resonator antenna for CNSS applications,” IEEE Trans. Antennas Propag., vol. 65, no. 8, pp. 4283–4287, Aug. 2017], [5. Z.-X. Xia, K. W. Leung, P. Gu, and R. Chen, “3-D-printed wideband high-efficiency dual-frequency antenna for vehicular communications,” IEEE Trans. Veh. Technol., vol. 71, no. 4, pp. 3457–3469, Apr. 2022].

Recently, a high-gain dual-band hemispherical DRA with resonant frequencies of 7.65 and 10.04 GHz has been presented [6. S. Varghese, P. Abdulla, A. M. Baby, and P. M. Jasmine, “High-gain dual-band waveguide-fed dielectric resonator antenna,” IEEE Antennas Wireless Propag. Lett., vol. 21, no. 2, pp.

232–236, Feb. 2022]. The antenna is fed by a waveguide. The impedance bandwidth of the antenna is 4.98% and 12.68% at the lower and higher bands, respectively. However, this approach does not apply to 5G in which the frequency bands are far apart.

Dual-band operation with a high-frequency ratio is essential in the new generation of wireless communication systems and various designs have been proposed for that [7. X.-H. Ding, W.-W. Yang, W. Qin, and J.-X. Chen, “A broadside shared aperture antenna for (3.5, 26) GHz mobile terminals with steerable beam in millimeter-waveband,” IEEE Trans. Antennas Propag., vol. 70, no. 3, pp. 1806–1815, Mar. 2022], [8. T. Li and Z. N. Chen, “Shared-surface dual-band antenna for 5G applications,” IEEE Trans. Antennas Propag., vol. 68, no. 2, pp. 1128–1133, Feb. 2020], [9. Y. Su, X. Q. Lin, and Y. Fan,

“Dual-band coaperture antenna based on a single-layer mode composite transmission line,” IEEE Trans.

Antennas Propag., vol. 67, no. 7, pp. 4825–4829, Jul. 2019], [10. J. F. Zhang, Y. J. Cheng, Y. R. Ding, and C. X. Bai, “A dual-band shared-aperture antenna with large frequency ratio, high aperture reuse efficiency, and high channel isolation,” IEEE Trans. Antennas Propag., vol. 67, no. 2, pp. 853–860, Feb. 2019]. The approaches mainly rely on employing two distinct elements; one for the lower frequency band and the other for the upper-frequency band [11. T. Zhihong, Y. P. Zhang, C. Luxey, A.

Bisognin, D. Titz, and F. Ferrero, “A ceramic antenna for tri-band radio devices,” IEEE Trans. Antennas Propag., vol. 61, no. 11, pp. 5776–5780, Nov. 2013], [12. L. Zhang, K. Y. See, B. Zhang, and Y. P.

Zhang, “Integration of dual-band monopole and microstrip grid array for single-chip triband

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application,” IEEE Trans. Antennas Propag., vol. 61, no. 1, pp. 439–443, Jan. 2013], [13. Z.-X. Xia, K.W. Leung, N. Yang, and K. Lu, “Compact dual-frequency antenna array with large frequency ratio,”

IEEE Trans. Antennas Propag., vol. 69, no. 4, pp. 2031–2040, Apr. 2021], [14. T. Smith, U. Gothelf, O. S. Kim, and O. Breinbjerg, “An FSSbacked 20/30 GHz circularly polarized reflectarray for a shared aperture L- and Ka-band satellite communication antenna,” IEEE Trans. Antennas Propag., vol. 62, no.

2, pp. 661–668, Feb. 2014], [15. T. Li, and Z. N. Chen, “Metasurface-based shared-aperture 5G ${S}$

-/ ${K}$ -band antenna using characteristic mode analysis,” IEEE Trans. Antennas Propag., vol. 66, no. 12, pp. 6742–6750, Dec. 2018], [16. Y. R. Ding and Y. J. Cheng, “Ku/Ka dual-band dual-polarized sharedaperture beam-scanning antenna array with high isolation,” IEEE Trans. Antennas Propag., vol.

67, no. 4, pp. 2413–2422, Apr. 2019], [17. Y.-X. Sun and K. W. Leung, “Substrate-integrated two-port dualfrequency antenna,” IEEE Trans. Antennas Propag., vol. 64, no. 8, pp. 3692–3697, Aug. 2016], [18. W.-W. Yang, X.-H. Ding, T.-W. Chen, L. Guo, W. Qin, and J.-X. Chen, “A shared-aperture antenna for (3.5, 28) GHz terminals with end-fire and broadside steerable beams in millimeter wave band,”

IEEE Trans. Antennas Propag., vol. 70, no. 10, pp. 9101–9111, Oct. 2022]. For wide-band operation, it is necessary to increase the overall size of the antennas. In order to decrease the overall size while maintaining the wide-band operation, a single radiator with two backto- back folded plates was proposed to design a compact dual-band antenna [19. L. Y. Feng and K. W. Leung, “Dual-frequency folded-parallel-plate antenna with large frequency ratio,” IEEE Trans. Antennas Propag., vol. 64, no.

1, pp. 340–345, Jan. 2016], [20. K. Lu, Y. Ding, and K. W. Leung, “A new Fabry–Perot resonator antenna fed by an L-probe,” IEEE Trans. Antennas Propag., vol. 60, no. 3, pp. 1237–1244, Mar. 2012].

This antenna provides bandwidths of 9.7% and 2.1% at center frequencies of 2.4 and 24 GHz (ISM bands), respectively. A dual-band antenna that integrates a microwave hollow DRA with a high-gain mmwave dielectric Fabry–Perot resonator antenna (FPRA) was proposed in [21. L. Y. Feng and K. W.

Leung, “Dual-fed hollow dielectric antenna for dual-frequency operation with large frequency ratio,”

IEEE Trans. Antennas Propag., vol. 65, no. 6, pp. 3308–3313, Jun. 2017]. This design provides a bandwidth of 4.67% at the mm-wave band which is again not sufficient for 5G mm-wave communications. The feed at the mm-wave band is a rectangular metallic waveguide, which is not easy to integrate antenna on RF printed circuit boards. In another study, a microwave DRA and an mm- wave metallic FPRA were integrated to form a dual-band antenna [22. L. Y. Feng and K. W. Leung,

“Wideband dual-frequency antenna with large frequency ratio,” IEEE Trans. Antennas Propag., vol.

67, no. 3, pp. 1981–1986, Mar. 2019]. At the lower band, the antenna exhibits good performance.

However, the antenna has a low radiation efficiency at the upper band due to the conductor loss. A large ground plane and conventional feeding for the lower frequency band is used. Most recently, the idea of encapsulated dielectric resonator antennas (E-DRAs) is introduced in [23. R. S. Malfajani, H.

Niknam, S. Bodkhe, D. Therriault, J.-J. Laurin, and M. S. Sharawi, “A 3D-printed encapsulated dual wide-band dielectric resonator antenna with beam switching capability,” IEEE Open J. Antennas Propag., vol. 4, pp. 492–505, 2023], which offer efficient performance for dual wideband applications and deliver bandwidths of 33% and 27% centered around frequencies of 3.6 GHz and 30.5 GHz, respectively.

This work presents a novel wideband, high gain, single dielectric antenna element covering two widely separated bands. The design methodology enables the design of a dualband antenna at the desired frequency ranges. The dielectric antenna used in this work is mushroom-shaped. It consists of a large cDRA on top of a smaller dielectric cylinder acts as a dielectric rod (cDR). In addition, a dielectric lens (DL) is placed on top of the larger DRA. The large dielectric cylinder and the DL serve as a dielectric resonator antenna at sub-6-GHz frequencies. At the mm-wave band, the small cDR acts as a dielectric rod waveguide while the large DR and the DL form a radiating aperture to enhance the gain of the mm-wave signals. Different feeding schemes are utilized at the two bands. A microstrip-

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fed aperture is used to feed the antenna in the millimeter wave band. At sub-6-GHz, the DRA is fed by a substrate-integrated cavity (SIC) through an annular slot. The same technique has been used in [24. R. S. Malfajani, J.-J. Laurin, and M. S. Sharawi, “Wideband substrate integrated cavity-backed dielectric resonator antenna at sub-6-GHz band,” IEEE Open J. Antennas Propag., vol. 4, pp. 60–68, 2023] to improve the bandwidth at sub-6-GHz and stabilize the gain patten. However, in contrast to [24. R. S. Malfajani, J.-J. Laurin, and M. S. Sharawi, “Wideband substrate integrated cavity-backed dielectric resonator antenna at sub-6-GHz band,” IEEE Open J. Antennas Propag., vol. 4, pp. 60–68, 2023], the SIC feed is single-ended in this work (compared to a differential one used before). The antenna presented here is a dual band antenna operating at sub-6-GHz and mm-wave bands but the one in [24. R. S. Malfajani, J.-J. Laurin, and M. S. Sharawi, “Wideband substrate integrated cavity-backed dielectric resonator antenna at sub-6-GHz band,” IEEE Open J. Antennas Propag., vol. 4, pp. 60–68, 2023] is operating at only sub-6-GHz band. Thus, the contribution in this work is different than that in [24. R. S. Malfajani, J.-J. Laurin, and M. S. Sharawi, “Wideband substrate integrated cavity-backed dielectric resonator antenna at sub-6-GHz band,” IEEE Open J. Antennas Propag., vol. 4, pp. 60–68, 2023] from an antenna perspective (microwave and mm-wave bands in this work) and the use of the DL.

In this work, fused filament fabrication (FFF) as an additive manufacturing (AM) technology is used to fabricate the proposed design. The major benefits of employing AM are the possibility of fabrication of more complex geometries and reducing the fabrication time [23. R. S. Malfajani, H.

Niknam, S. Bodkhe, D. Therriault, J.-J. Laurin, and M. S. Sharawi, “A 3D-printed encapsulated dual wide-band dielectric resonator antenna with beam switching capability,” IEEE Open J. Antennas Propag., vol. 4, pp. 492–505, 2023], [25. F. Calignano et al., “Overview on additive manufacturing technologies,” Proc. IEEE, vol. 105, no. 4, pp. 593–612, Apr. 2017], [26. A. I. Dimitriadis et al.,

“Polymer-based additive manufacturing of high-performance waveguide and antenna components,”

Proc. IEEE, vol. 105, no. 4, pp. 668–676, Apr. 2017], [27. B. Zhang, Y.-X. Guo, H. Zirath, and Y. P.

Zhang, “Investigation on 3-D-printing technologies for millimeter-wave and terahertz applications,”

Proc. IEEE, vol. 105, no. 4, pp. 723–736, Apr. 2017], [28. S. Alkaraki et al., “Compact and low-cost 3- D printed antennas metalized using spray-coating technology for 5G mm-Wave communication systems,” IEEE Antennas Wireless Propag. Lett., vol. 17, no. 11, pp. 2051–2055, Nov. 2018], [29. G.

Addamo et al., “3-D printing of high-performance feed horns from Ku- to V-bands,” IEEE Antennas Wireless Propag. Lett., vol. 17, no. 11, pp. 2036–2040, Nov. 2018], [30. M. Liang, C. Shemelya, E.

MacDonald, R. Wicker, and H. Xin, “3-D printed microwave patch antenna via fused deposition method and ultrasonic wire mesh embedding technique,” IEEE Antennas Wireless Propag. Lett., vol. 14, pp.

1346–1349, 2015], [31. P. Nayeri et al., “3D printed dielectric reflect arrays: Low-cost high-gain antennas at sub-millimeter waves,” IEEE Trans. Antennas Propag., vol. 62, no. 4, pp. 2000–2008, Apr.

2014], [32. J. Huang, S. J. Chen, Z. Xue, W. Withayachumnankul, and C. Fumeaux, “Impact of infill pattern on 3D printed dielectric resonator antennas,” in Proc. IEEE APCAP, 2018, pp. 233–235], [33.

Z.-X. Xia, K. W. Leung, and K. Lu, “3-D-printed wideband multi-ring dielectric resonator antenna,”

IEEE Antennas Wireless Propag. Lett., vol. 18, no. 10, pp. 2110–2114, Oct. 2019], [34. F. P. Chietera, R. Colella, and L. Catarinucci, “Dielectric resonators antennas potential unleashed by 3D printing technology: A practical application in the IoT framework,” Electronics, vol. 11, no. 1, p. 64, 2022].

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FIGURE 1. Geometry of proposed single feed SIC cDRA (a) Disassembled view (b) Top view (c) Bottom view (d) Perspective view (e) Cross-section from side view

FFF technology has been used to fabricate slot [28. S. Alkaraki et al., “Compact and low-cost 3-D printed antennas metalized using spray-coating technology for 5G mm-Wave communication systems,”

IEEE Antennas Wireless Propag. Lett., vol. 17, no. 11, pp. 2051–2055, Nov. 2018], horn [29. G.

Addamo et al., “3-D printing of high-performance feed horns from Ku- to V-bands,” IEEE Antennas Wireless Propag. Lett., vol. 17, no. 11, pp. 2036–2040, Nov. 2018], patch [30. M. Liang, C. Shemelya, E. MacDonald, R. Wicker, and H. Xin, “3-D printed microwave patch antenna via fused deposition method and ultrasonic wire mesh embedding technique,” IEEE Antennas Wireless Propag. Lett., vol.

14, pp. 1346–1349, 2015], reflectarray [31. P. Nayeri et al., “3D printed dielectric reflect arrays: Low- cost high-gain antennas at sub-millimeter waves,” IEEE Trans. Antennas Propag., vol. 62, no. 4, pp.

2000–2008, Apr. 2014], and DRA [23. R. S. Malfajani, H. Niknam, S. Bodkhe, D. Therriault, J.-J.

Laurin, and M. S. Sharawi, “A 3D-printed encapsulated dual wide-band dielectric resonator antenna with beam switching capability,” IEEE Open J. Antennas Propag., vol. 4, pp. 492–505, 2023], antennas at microwave and mm-wave bands. In these studies, different type of antennas has been considered with customized filaments, and the effect of 3D printing parameters on the antenna response has been investigated. The remaining sections of this paper are structured as follows. The design of a SIC- backed DRA at sub-6-GHz is described in Section II. In Section III, the designed DRA at sub-6-GHz is modified to act like a lens at mmwave. A dual-band wide-band mushroom-shaped dielectric antenna is described in Section IV. Characterizations of 3D printing materials and the fabrication process are

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described in Section V. In Section VI, a detailed analysis of the measurement and simulation results is presented. Finally, a conclusion is drawn in Section VII.

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Screen-Printed, Flexible, Parasitic Beam-Switching Millimeter-Wave Antenna Array for Wearable

Applications

Millimeter (mm)-wave antennas are required for a wide range of applications, such as 5G communication, radio frequency (RF) identification, automotive radars, and more. Object detection or sensing through electromagnetic waves is also becoming popular for wearable applications. One possible application for such wearable sensing systems is to provide assistance to visually impaired people when walking [1. T. Kiuru et al., “Assistive device for orientation and mobility of the visually impaired based on millimeter wave radar technology—Clinical investigation results,” Cogent Eng., vol.

5, no. 1, 2018, Art. no. 1450322]. Such antennas require either multidirectional beams or certain types of beam switching or steering. In addition, they require a low profile to ensure they can be easily embedded in modern miniature designs. Flexibility is also desirable for wearable applications. Finally, lower cost is a consideration for almost all wearable applications. However, designing a beam-steerable antenna system that is low-cost, low-profile, and flexible with a simple beam-steering mechanism and decent performance (i.e., gain, efficiency, and beamwidth) is quite challenging. Continuous beam steering can be achieved using techniques such as mechanical scanning, electrical scanning (e.g., phase shifters), and frequency scanning. Mechanical scanning requires bulky components, which makes it infeasible for modern miniature designs. Furthermore, utilizing discrete phase shifters at each element for mm-wave frequencies would make the antenna array expensive, narrow-banded, and lossy.

By contrast, beam switching (i.e., beam steering at discrete points) offers a comparatively simpler approach. Beam switching may be achieved using Butler and Blass matrices, a Rotman lens, or parasitic antenna arrays. However, couplers, crossovers, and delay lines in the Butler matrix add extra power losses to the system. Similarly, Rotman lenses are unsuitable for wearable applications because of their significantly large dimensions. Moreover, parasitic antenna arrays exhibit potential for low-loss and flexible beam-switching systems.

A parasitic antenna array directs the antenna beam using parasitic elements that are close to a driven (i.e., directly fed) antenna. Current is induced into the parasitic elements through mutual coupling from the main driven antenna.

The radiation from these parasitic elements changes the total radiation pattern of the array. A parasitic antenna array has only one driven element, which helps to avoid distribution network losses. Some designs of various parasitic antenna arrays have been presented in [2. S. L. Preston, D. V. Thiel, J. W. Lu, S. G. O’keefe, and T. S. Bird, “Electronic beam steering using switched parasitic patch elements,” Electron. Lett., vol. 33, no. 1, pp. 7–8, Jan. 1997]–[6. S.-J. Lee, W.-S. Yoon, and S.-M. Han, “Planar beam steerable parasitic array antenna system design based on the Yagi-Uda design method,” Int. J. Antennas Propag., vol.

2019, Mar. 2019, Art. no. 8023712]. In [2. S. L. Preston, D. V. Thiel, J. W. Lu, S. G. O’keefe, and T. S.

Bird, “Electronic beam steering using switched parasitic patch elements,” Electron. Lett., vol. 33, no.

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1, pp. 7–8, Jan. 1997], a patch antenna array with parasitic patches was employed to track polar-orbiting satellites. Beam switching through a parasitic monopole antenna array was utilized in [3. S. L. Preston, D. V.

Thiel, T. A. Smith, S. G. O’Keefe, and J. W. Lu, “Base-station tracking in mobile communications using a switched parasitic antenna array,” IEEE Trans. Antennas Propag., vol. 46, no. 6, pp. 841–844, Jun. 1998] to determine the direction of the nearest base station. An array of four parasitic circular patches was used in [4. M. Jusoh, T. Sabapathy, M. F. Jamlos, and M. R. Kamarudin, “Reconfigurable four-parasitic- elements patch antenna for high-gain beam switching application,” IEEE Antennas Wireless Propag.

Lett., vol. 13, pp. 79–82, 2014] to switch the beam in four different directions for a WiMAX application.

Finally, a wide switching range was demonstrated in [5. T. Sabapathy, M. Jusoh, R. B. Ahmad, M. R.

Kamarudin, and P. J. Soh, “A ground-plane-truncated, broadly steerable Yagi-Uda patch array antenna,” IEEE Antennas Wireless Propag. Lett., vol. 15, pp. 1069–1072, 2016], which utilized parasitic antenna elements. Paper [6. S.-J. Lee, W.-S. Yoon, and S.-M. Han, “Planar beam steerable parasitic array antenna system design based on the Yagi-Uda design method,” Int. J. Antennas Propag., vol. 2019, Mar. 2019, Art. no. 8023712] describes a design of a parasitic monopole antenna array. Although the parasitic antenna array-based beam-switching mechanism is simpler and has lower loss, it has the disadvantages of a wide beamwidth and lower directivity. Furthermore, unlike conventional antenna arrays with a feed network, the beamwidth does not become narrower when the number of array elements is increased. Because narrow beams are desirable for increasing detectability in most beam steering and switching applications, design techniques must be adapted to narrow the beam for parasitic arrays.

Typically, narrow beams (enhanced directivity) can be achieved through lenses, reflectors, or transmit arrays. However, these methods are generally unsuitable for lowprofile, flexible, and planar designs. An alternate approach is a partially reflective surface (PRS), which can be realized on top of the antenna through a thin flexible superstrate layer. Studies have demonstrated multiple antennas with a PRS layer [7. G. V. Trentini, “Partially reflecting sheet arrays,” IRE Trans. Antennas Propag., vol. 4, no. 4, pp. 666–671, Oct. 1956]–[15. L.- Y. Ji, P.-Y. Qin, Y. J. Guo, S. Genovesi, H.-L. Zhu, and Y. Zong, “A reconfigurable partially reflective surface antenna with enhanced beam steering capability,” in Proc. 13th Eur. Conf. Antennas Propag.

(EUCAP), Krakow, Poland, Apr. 2019]. However, the concept of PRS has never been applied to enhance the directivity of a parasitic antenna array or flexible antennas.

This paper presents a beam-switching parasitic antenna array that is loaded with a PRS. Because of these design

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choices (i.e., a parasitic array and PRS), the antenna system is low profile. It was realized on a flexible substrate to enhance its suitability for wearable applications. The antenna was developed through screen printing to make it cost-effective. Table 1 compares our work with the published parasitic antenna arrays. It indicates that our antenna array is the only one that is flexible and was realized through additive manufacturing technique.

The antenna has a ±32◦ beam-switching range, which is said to be the limit of the parasitic antenna array concept. This limit can be further extended up to 50◦ using the ground-plane truncation method described in [5. T. Sabapathy, M. Jusoh, R. B. Ahmad, M. R. Kamarudin, and P. J. Soh, “A ground- plane-truncated, broadly steerable Yagi-Uda patch array antenna,” IEEE Antennas Wireless Propag.

Lett., vol. 15, pp. 1069–1072, 2016]. This method was not employed in the present study because of the fabrication limitations of a W-band antenna dues to its small dimensions. To the authors’ knowledge, the presented antenna array has the highest gain among the published parasitic patch array designs (11.2 dBi) as a result of PRS integration. Furthermore, it is apparent that we have one of the lowest profiles and a comparable gain with other published PRS antennas. The low profile, flexibility, narrow beamwidth (i.e., high directivity), simple beam-switching mechanism, and low cost are features of the proposed design that are attractive for various wearable sensing applications.

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