A Cloud-Structured Fractal Multiband Antenna for 4G/5G/WLAN/ Bluetooth Applications

This study proposes a multiband printed planar antenna with cloud-like grooves. The outer contour of the antenna is shaped like a cloud, and the groove-like pattern is similar to the cloud-like pattern in ancient China. It can support 3G, 4G, 5G, WLAN, Bluetooth, WiMAX, and other applications. Based on the traditional monopole antenna, the antenna combines the advantages of a coplanar waveguide. The antenna uses an Archimedes helix to create grooves that resemble ancient Chinese cloud structures. Three effective frequency bands are obtained. The relative bandwidth of the first frequency band (1.8–2.6GHz) is 32.7%, covering 5G band n2 (1.85GHz–1.99GHz), WCDMA (1.9–2.17GHz), LTE33-41 (1.9–2.69GHz), Bluetooth (2.4–2.48GHz), WLAN (2.4–2.48 GHz), LTE Band40 (2.3–2.4GHz), ISM Band (2.42–2.4835 GHz), WiMAX (2.3GHz), and SCDMA (1.88–2.025GHz and 2.3–2.4GHz). The second frequency band (3.35–4.1GHz) has a relative bandwidth of 20.5%, covering LTE42/43 (3.4–3.8 GHz) and 5G band n78 (3.4GHz–3.8 GHz). The relative bandwidth of the third band (5.5–7.9GHz) is 40.3%, covering Emergency and Public Protection (5.85 GHz–5.925GHz) (WRC03). The antenna is printed on a G10/FR4 dielectric board with a size of 1 . 6 ∗ 45 ∗ 40mm 3 , the dielectric constant is 4.4, and the omnidirectional radiation pattern gain is 0.59–4.14dBi. The measurement results are in good agreement with the simulation results. The proposed design method is verified to meet the requirements of various wireless applications.


Introduction
With the wide application and development of radio, the demand for di erent antennas is also increasing, and the antenna performance requirements are getting higher and higher. In recent years, multiband and miniaturized antennas have become hot spots for mobile terminal equipment [1]. Due to the inherent characteristics of traditional monopole and dipole antennas, it is challenging to meet the practical requirements of miniaturization and multiband, so coupling feeding technology [2][3][4], fractal technology [5], matching network loading technology [6], and fractal techniques [5] to meet the relevant requirements for antenna performance. In addition, di erent feeding methods will also have a particular impact on the antenna's performance. For example, although the microstrip line feed mode has excellent antenna performance, its bandwidth is relatively narrow [7][8][9]. Coplanar waveguides have higher conductor losses than microstrip lines, although they have a wider frequency bandwidth and impedance range.
Antennas can also improve antenna performance by nesting a simple graph for multiple iterations and selfsimilarity [10,11]. is method enables the antenna to have similar surface currents at di erent positions of the radiator, space-lling [12,13], reducing the size of the antenna and making the antenna more compact and exible.
e antenna's performance can also be improved by making slots of di erent shapes in the antenna radiator.
In [14], the authors summarize the design idea of combining antenna fractal and slotting techniques. e antenna's performance can be considered while realizing multifrequency bands through the slotting and fractal design of the circular monopole patch. In [15], the authors used coplanar waveguide antennas to widen the frequency band. In [16] the authors discussed the interference of the external environment on each frequency band. Among them, the frequency band below 6 GHz has strong distortion immunity and penetrating ability to buildings because of the longer wavelength. And the frequency band below 6 GHz can meet the vast majority of communication needs. erefore, controlling the antenna coverage frequency band below 6 GHz helps to improve the robustness of the antenna system. In [17], the authors designed the reference ground plane of the coplanar waveguide to be a semielliptical shape. ey proposed the design of helical grooves on the ground plane to expand the bandwidth while meeting the performance requirements of multiple frequency bands. In addition, many other methods are used in antenna design, such as the Cartesian curve, the Sierpinski triangle type, the Koch curve type, and so on.
It has been shown that the frequency band of an antenna can be increased by adding branches and slits [18,19]. erefore, to avoid the poor antenna bandwidth and performance caused by the characteristics of traditional monopole or dipole antennas, an antenna with better performance and wider bandwidth can be designed, which can effectively combine the above technologies.
In this paper, a multiband antenna with a cloud-like structure is proposed, and the antenna radiator is slotted with an Archimedes spiral structure, and the groove structure is similar to the traditional Chinese cloud-like structure. Slotting has three fractal iterations. e commercial frequency bands covered by the antenna include 5G band n2 (1.  [14], the antenna proposed in this paper has a larger bandwidth and higher radiation efficiency.

Characteristics of the Antenna Structure.
e formula for calculating the relationship between antenna frequency and electrical length is where L is the electrical length of the antenna radiator, f is the frequency at the center frequency of the antenna, c is the speed of light in free space (3 * 108 m/s), and ε r is the dielectric constant of the dielectric substrate e structure of the coplanar waveguide antenna proposed in this study is shown in Figure 1, and its dimensions are shown in Table 1. An arc-shaped 50 Ω coplanar waveguide feeders is used to extend the bandwidth. e antenna has three iterations of cloud structure grooves similar to the fractal structure of ancient Chinese cloud-shaped ornaments, as shown in Figure 2. e cirrus pattern used for this coplanar waveguide antenna first appeared in the pre-Qin period in China. It is the source of all the cloud patterns in ancient China, representing auspiciousness, joy, and happiness and revealing the ancient Chinese people's yearning for a better life. As the representative of ancient Chinese traditional auspicious patterns, the cloud pattern is a unique representative of Chinese cultural symbols. It has profound cultural connotations, rich and complex symbolic meanings, and artistic sense. More importantly, this structure can increase the length of the radiator, thereby increasing the antenna's electrical length and radiation efficiency. e antenna was fabricated on an FR-4 substrate with a thickness of 1.6 mm, a dielectric constant of 4.4, and a loss tangent of 0.02. e basic geometry of ancient cloud structure fractal antennas is a cloud structure monopole with a helical slot. e formula for the Archimedes spiral structure is   where the initial value of _t is 0, end value of _t is 2.35 * pi, and width is 0.5 mm. e first-order Archimedes spiral model obtained by the formula is rotated by −45°to obtain the structure shown in Figure 3(a). e cloud-like structure shown in Figure 3(b) is obtained by mirror copying the structure of Figure 3

(a).
As shown in Figure 4, three iterations were performed to achieve a multiband response. In the evolution of the ground plane, the top of the trapezoidal ground plane is combined with the arc to obtain the ground plane model in this design. e three positioning points of the arc are (−15.2, 12.5, 0), (−20, 2, 0), and (−15.5, 6.8, 0). e evolution path is shown in Figure 5. e fractal formula of the cloud-like groove structure can be simply expressed as follows: Here, "D n " represents the nth order fractal e corresponding electrical length of the antenna at low frequencies can be calculated as follows:        International Journal of Antennas and Propagation calculated by the following formula: e corresponding electrical length of the antenna at high frequencies can be calculated as follows: calculated by the following formula:

Simulation Results
e ANSYS Electronics Desktop (HFSS) (Version 20) software package was used for the simulation. e basic model of the antenna can be regarded as a monopole antenna with a capacitive load similar to a cloud-shaped structure. e first-order radiator is formed by etching the cloud structure groove with the Archimedes spiral structure as the initial pattern evolution on the radiator. Model and generated five central resonance frequencies, which tended to stabilize as the number of iterations increased. Furthermore, it reaches the best at the third fractal iteration, which produces the center frequency we need. See Figure 6. e antenna proposed in this paper can work in three frequency bands: the center frequencies are 2.25 GHz, 2.5 GHz, 3.1 GHz, 3.55 GHz, 5.7 GHz, and 6 GHz, respectively, and the corresponding S11 are −19.9 dB and −13.54 dB, −16.6 dB, −19.97 dB, −20.90 dB, −21.04 dB. e simulated −10 dB S11 relative bandwidth is 29% of the first band (1.9-2.6 GHz), 8% of the second band (2.9-3.1 GHz), and 72% of the third band (3.35-7 GHz), See Figure 7. ese frequency bands cover commercial frequency bands such as 4G-LTE, WLAN, Bluetooth, WiMAX, public protection, emergency frequency bands, and 5G (see Table 2 for antenna radiators). e surface current amplitude and vector distribution of the antenna radiator part are shown in Figures 8(a)-8f, respectively. With the increase of the frequency, the changing trend of the surface current distribution of the antenna is from the edge of the radiator and the first-order groove of the antenna to gradually spread to the secondorder and third-order grooves. Among them, the surface current of the antenna is mainly distributed at the edge of the radiator and the first-order groove at the bottom at 2.25 GHz and 2.5 GHz, and the surface current of the antenna at 3.1 GHz and 3.55 GHz is mainly distributed between the first-order groove and the second-order groove. At 5.7 GHz and 6 GHz, the surface current of the antenna gradually spreads to the third-order groove. e overall trend is that the surface current of the antenna tends to concentrate towards the third-order groove as the frequency increases. e peak gains are 2.4 dBi, 2.4 dBi, 1.0 dBi, 2.4 dBi, 2.9 dBi and 2.4 dBi at center frequencies of 2.25 GHz, 2.5 GHz, 3.1 GHz and 3.55 GHz, 5.7 GHz and 6 GHz, respectively. e simulated 3D far-field radiation pattern and the far-field normalized E/H surface radiation pattern are shown in Figures 9 and 10. It can be seen from Figure 9 that the omnidirectional performance of the antenna at each frequency point is good, but side lobes appear at 3.1 GHz, 5.7 GHz, and 6 GHz. It can be seen from Figure 10 that the  overall degree of cross-polarization in each frequency band of the antenna is low. But as the frequency increases, the cross-polarization of the antenna gradually increases relative to the main polarization. However, the isolation degree of the overall cross-polarization from the main polarization is within an acceptable range.

Fabrication and Measured Results
e actual antenna was fabricated and tested to verify the correctness of the bandwidth, frequency, and performance in the design scheme. e antenna size is 45mm * 40mm printed on a 1.6 mm thick Polytetra (G10/FR4) dielectric board. e dielectric constant of the dielectric plate is ε r � 4.4 and the loss tangent is 0.02. e antenna has been placed into an electromagnetically shielded darkroom for testing (See Figure 11). e actual antenna S11 obtained from the test is compared with the simulated S11, as shown in Figure 12. e first frequency point is shifted from 2.25 GHz to 2.35 GHz and its S11 value is −18 dB. e second frequency point is shifted from 2.5 GHz to 2.55 GHz and its S11 value is −11.9 dB. e S11 value of the third frequency shifted from 3.1 GHz to 2.85 GHz is −21.2 dB. e fourth frequency point is shifted from 3.55 GHz to 3.65 GHz and its S11 value is −15.5 dB. e fifth frequency shifted from 5.7 GHz to 5.6 GHz and its S11 value was −14.6 dB and the sixth frequency shifted from 6 GHz to 6.3 GHz and its S11 value was −27 dB. ere are four frequency bands corresponding to the above frequency points, and the first frequency band (1.8 GHz-2.6 GHz) has a phase bandwidth of 32.7%. e relative bandwidth of the second frequency band (2.8 GHz-2.9 GHz) is 3.5%. e relative bandwidth of the third frequency band (3.35 GHz-4.1 GHz) is 20.5%. e relative bandwidth of the fourth frequency band (5.5 GHz-7.9 GHz) is 40.3%. ere is little difference between the specific coverage application and the simulation, as shown in Table 3.
e simulation results are in good agreement with the actual test results, but there are still some errors. e cause of the error may be related to the fabrication and welding process of the antenna. Figures 13(a)-13(f ) are the 3D far-field radiation pattern of the antenna at each frequency point and the comparison diagram of the main polarization and cross-polarization of the E/H plane of the antenna. It can be seen that as the frequency increases, the cross-polarization of the antenna gradually increases and the isolation gradually decreases, but the isolation of each frequency point of the antenna still meets the performance requirements of the antenna.
ere are three effective frequency bands for the antenna: the first frequency band (1.8-2.6 GHz), the second frequency band (3.35-4.1 GHz), and the third frequency band (5.5-7.9 GHz). e gain and efficiency of the actual antenna are shown in Figure 14. As can be seen from the figure, the highest gain of the antenna reaches 4.56 dBi, and the maximum efficiency reaches 81%. In the effective frequency band of the antenna, the first frequency band gain range is 1.79-3.26 dBi, and the efficiency range is 45%-58%.
e second band has a gain range of 0.59-2.92 dBi and an efficiency range of 71%-81%. e third-band gain is 1.27∼4.14 dBi, and the efficiency is 55%∼72%. is antenna can meet most application needs.
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Conclusions
is paper develops a multiband coplanar waveguide antenna with a cloud structure for LTE, Bluetooth, WLAN, WiMAX, SCDMA, LTE42/43, and 5G. With a relative bandwidth of 41% (1.85-2.8 GHz), the first frequency band covers WCDMA, LTE33-41, Bluetooth, WLAN, LTE band40, ISM band, WiMAX, and SCDMA. In the second frequency band, the relative bandwidth is 17.94% (3.6-4.3 GHz), covering LTE42/43 and WiMAX, while the third frequency band, with a relative bandwidth of 38.55% (5.72-8.15 GHz), covers the 5G frequency band. According to the actual measurement of the antenna, the antenna gain in the frequency band below -10 dB is 0.59-4.14 dBi, and the antenna efficiency range is 28%-81%. erefore, the antenna proposed in this experiment is suitable for most wireless applications due to its good radiation characteristics and novel structure.
Data Availability e data used in this study are included in the figure files.

Conflicts of Interest
e authors declare that there are no conflicts of interest in this article. International Journal of Antennas and Propagation 9