Integrated large‐frequency‐ratio dual‐band tapered slot with monopole antenna for 4G/5G/B5G

With the advanced communications systems, the integration of 5G bands, including sub‐6‐GHz and millimeter‐wave (mm‐wave) bands with the existing lower bands of 3G/4G has become an essential need for the future mobile handsets. This article proposes an integration between a large‐frequency‐ratio dual‐band tapered slot antenna with a multisection meander monopole antenna on a handset board. The tapered slot Vivaldi antenna (TSVA) functions as a resonant open‐ended slot at low frequencies (sub‐6 GHz) while also acting as a high‐gain Vivaldi antenna at higher frequencies (mm‐wave 28 GHz). The modification of the slot geometry, that is, generating multisections within the slot, is shown to improve the impedance bandwidth. Furthermore, the meander‐line monopole antenna covers the bands of the existing mobile generation (3G/4G) at 0.8, 1.8, 2.1, and 2.3 GHz. Most importantly, the compact size of the proposed antenna is realized by utilizing the side wall of a mobile board.


| INTRODUCTION
2][3] IoT is described as an ecosystem of digital machines, interrelated computer devices, and objects that have the ability to communicate and transfer data to each other in real-time, with little to no human intervention. 1,4][7][8][9][10] It is noted that the upcoming handheld devices must support the millimeter-wave (mm-wave) standards besides the sub-6-GHz standards, industrial, scientific and medical (ISM) industband, and lower 2 G/3 G/4 G bands.
2][13][14][15][16][17] Separate antennas for the respective frequency bands were employed in most reported studies.The reported antennas in Sultan and colleagues 8,18 were introduced to serve only the mm-wave band of 5G, while the reported antennas in Zhang and colleagues [19][20][21] serve only the lower bands of 5G.On the other hand, the upper bands of 4G (around 2.3 GHz) and the mm-wave band of 5 G are covered by the designs in Fakih and colleagues 13,15,17,[22][23][24][25] and do not cover the lower sub-6-GHz band of 5G.On the other hand, the antennas in Ban and colleagues 11,12,14,26 cover the lower and the upper bands for 4 G besides the sub-6-GHz band for 5G and they do not cover the mm-wave band of 5G.
Footprint constraint in handsets for many singleband antennas is a very challenging task.As a result, the integration techniques of the sub-6-GHz and mm-wave bands have piqued the interest of researchers.Discovering a new method of exploiting the limited handset area with multiband antennas to integrate between lower mm-wave bands, sub-6-GHz band, and the mm-wave band is essential.In this article, a modified multisection tapered slot antenna (MS-TSA) is used, which offers a very large frequency ratio to be suitable for lower and upper bands of 5G/B5G applications.The MS-TSA is placed along the side of the device to cover 6 GHz-sub bands and 28 GHz band for 5 G applications.The contributions of this article include: • A folded and L-shaped monopole is proposed to cover the bands of 4G applications at 0.8, 1.8, 2.1, and 2.3 GHz.• The design of the folded/L-shaped monopole and the MS-TSA exploits the edge of the mobile board to reduce the footprint while realizing the most challenging lower band, that is, 0.8 GHz.• The proposed design is capable of most of the 2G, 3G, 4G, 5G, 5GB, ISM, WLAN, and IoT candidate bands. 26,30,[36][37][38]

| ANTENNA DESIGN AND INTEGRATION
Briefly, MS-TSA structure is basically an open-ended slot at sub-6-GHz band, with the resonance occurring when the slot length is roughly (see Figure 1).A 50-Ω microstrip line terminated by matched stubs is employed to excite this slot.At the same time, the structure serves as an end-fire Vivaldi antenna in the mmwave band (at 28 GHz), where L t is a multiple of wavelengths.In other words, the MS-TSA is a dual functionality slot operates as a resonance structure at lower frequency and a travelling wave structure at higher frequency.More details about the procedures of MS-TSA design have been reported by the same authors in Sultan et al. 44 It is noted that the MS-TSA can also be placed along different edges/sides of mobile phones to achieve MIMO configuration, which provides better space coverage and less sensitivity for user effects caused by hands.As for a mobile, it is still compulsory to provide different frequency bands to meet the requirements of other technologies.The MS-TSA is integrated with a monopole antenna and implemented together along the front side of the mobile handset due to the size constraint, as shown in Figure 1A.Such type of integration achieves multiband antenna to cover most of candidate applications with small footprint.The antenna is implemented on Rogers RT5880 with 0.51 mm of thickness, 2.2 dielectric constant, and 0.0009 tangential losses.
Two monopoles are used for the lower frequencies (3 G/4 G) to meet the multiband requirements.Specifically, a folded monopole is printed on the same substrate for the MS-TSA with the standard dimensions of a mobile phone board to generate a new band around 0.8 GHz with a large electrical length and small physical The radiation patterns of the antenna (A) at lower frequency bands 0.9, 2, and 3.1 GHz, (B) at upper-frequency bands at 27, 28, and 29 GHz.
area.The folded monopole is fed by a 50-Ω feed line with a width of 1.65 mm (see Figure 1B,C).The folded monopole is printed with a total length (L 4 ) that exhibits a quarter wavelength approximately at 0.8 GHz.][42][43][44] Here, a significant part of the antenna is folded and printed on the side of the device, which reduces the footprint's width of the antenna to be 10 mm (L k1 ) while providing an efficient radiation at the 0.8 GHz band.The width of the folded monopole is optimized to achieve wide bandwidth.The detailed geometry of the integrated antenna is shown in Figure 1.To cover the other bands of the 5 G and the long-term evaluation (LTE, 4 G), an inverted L-shaped monopole is printed on the backside of the substrate and connected to the ground plane, then an open stub with a length L p3 is used to adjust its matching.The L-shaped is printed inside the area of the folded monopole in the opposite direction and excited by the coupling from the front feed line.It exhibits a quarter wavelength at 2 GHz (L mm).All the dimensions are tabled in Table 1.
Figure 2A illustrates the simulated reflection coefficient of the multiband monopole antenna (performance of the MS-TSA will be shown in Section 3).The design starts with a simple folded monopole printed partially on the main and vertical wall PCBs.The reflection coefficient of folded monopole demonstrates that it generates resonant modes are f ( ) 0 and f ( ) 1 at 0.8 and 2.2 GHz, respectively.By printing the L-shape on the bottom side generates another mode f ( ) 2 at 1.9 GHz is integrated with f ( ) 1 to offer widens of the operational bandwidth at 2 GHz.An open-ended stub is added to the L-shape to improve the matching of the antenna.
To provide more understanding of the antenna performance, the current distributions of the antenna at different frequencies are shown in Figure 2B.It is confirmed that the current has a high concentration on the folded arm at f G H z ( ) 0.9 0  and on L-shaped at f G H z ( ) 1.9 2  .As expected, the current is only strong on the MS-TSA at 3.1 GHz when the MS-TSA is excited.

| ANTENNA PERFORMANCE
The simulated and measured S-parameters of the proposed antenna are illustrated in Figure 2C,D.It can be noticed that the antenna has the required impedance matching (<−6 dB) in the designed bandwidth.The multiband antenna operates at 0.9 GHz from 0.8 to 0.97 GHz, at 2 GHz from 1.79 GHz to 2.4 GHz while the MS-TSA operates at 3.15 GHz from 2.8 to 3.72 GHz and at 28 GHz from 26.5 to 31.5 (−10 dB impedance bandwidth).The isolation between the ports is more than 20 dB in the whole band.Figure 3A shows the simulated and measured radiation patterns at lower frequencies and upper frequencies.The results demonstrate good agreement between simulated and measured patterns.For the lower bands, the radiation patterns demonstrate that the antenna has a wide beamwidth like a monopole pattern which is required in such types of lower bands.The results for the lower band of the MS-TSA (around 3.1 GHz) are similar to the traditional radiation patterns of the slot antenna.The radiation patterns are presented at different frequencies for the mm-wave band, that is, 27, 28, and 29 GHz (see Figure 3B).The radiation patterns point to the endfire direction at different operating frequencies of mm-wave.There is a reasonable consistency between the simulated and measured radiation patterns.The realized gain and radiation efficiency at the lower band (monopole antenna and MS-TSA) and upper bands (MS-TSA) are shown in Figure 4A,B, respectively.It can be observed that the average gain is 4 and 10 dBi at the lower and upper bands, respectively.The measured and simulated radiation efficiency are mostly more than 90% in both lower and upper bands.Table 2 presents a comparison between the proposed antenna and the referenced antennas.In lower frequency bands, it is noted that the proposed antenna covers 0.8-0.97GHz, 1.79-2.4GHz, and 2.8-3.72 GHz which are suitable for 3 G/4 G applications in addition to lower bands (sub-6GHz) of 5 G application, while the antennas in Sun and colleagues, 23,30,39,45,46 supports three different bands for sub-6GHz 5 G application.Although the reported antenna in Kurvinen et al. 24 has similar performance at the lower band, it does not support the sub-6GHz band while having bulky and more complicated structure.The volume of antenna in Kurvinen et al. 24  3 for 4 G/5 G in the proposed antenna.
Furthermore, for mm-wave 5 G, the proposed antenna support 28 GHz with a simple structure, high gain, and high efficiency compared with the others reported antennas.In summary, the size and gain of the proposed antenna make it suitable for handheld devices and tablets/laptops, dongles, and so on, as similarly as presented in Kurvinen and colleagues.

1
Configuration of the proposed multiband antenna and photo of the fabricated samples.(A) The whole structure of the proposed antennas, designed geometry and fabricated prototype.The flat structure of the antenna before the folded (B) top view and (C) bottom view.

5 F
I G U R E 2 Simulated and measured performance of the antenna.(A) The reflection coefficient of the multiband antenna, (B) current distribution at different frequencies (the current distribution at 3.1 GHz is plotted when the multisection tapered slot antenna (MS-TSA) is excited, (C) reflection and isolation coefficients from Port 1 and Port 2 at the lower frequency band, and (D) reflection coefficients from Port 2 at the mm-wave band.

F
I G U R E 4 The simulated and measured (A) realized gain and (B) radiation efficiency at the lower band for monopole antenna and multisection tapered slot antenna (MS-TSA) and for upper band (MS-TSA).T A B L E 2 Comparison between the proposed antenna and antennas in the literature Ref.
24,[46][47][48][49]514 | CONCLUSIONA MS-TSA that combines with folded monopoles antenna to support 2 G/3 G/4 G/5 G/B5G/IoT applications has been presented in this work. Thetenna has been fabricated and tested to validate the proposed technique.The design offers multiband operation with a compact volume: it covers the frequency bands of 0.8-0.97GHz, 1.79-2.4GHz, 2.8-3.72 GHz (−6 dB impedance matching), and 26.5-31.5 (−10 dB impedance bandwidth) under a compact volume of 520  mm 3 .The simulated and measured results of the antenna in terms of S-parameters and radiation characteristics show a good agreement.With these characteristics, this article offers a versatile technique for designing 5 G/B5Gantennas.