Abstract
A novel two-iteration binary tree fractal bionic structure antenna is proposed for the third generation (3G), fourth generation (4G), WLAN, and Bluetooth wireless applications in the paper, which is based on the principles of conventional microstrip monopole antenna and resonant coupling technique, combined with the advantages of fractal geometry. A new fractal structure was presented for antenna radiator, similar to the tree in nature. The proposed antenna adapted two iterations on a fractal structure radiator, which covers mobile applications in two broad frequency bands with a bandwidth of 44.2% (1.85–2.9 GHz) for TD-SCDMA, WCDMA, CDMA2000, LTE33-41, and Bluetooth frequency bands, and 11.5% (4.9–5.5 GHz) for WLAN frequency band. The proposed antenna was fabricated on a G10/FR4 substrate with a dielectric constant of 4.4 and a size of 50 × 40 mm2. The good agreement between the measurement results and the simulation results validate that the proposed design approach meet the requirements for various wireless applications.
1. Introduction
In recent years, due to the rapid development in wireless communication technology and applications, the demand for multiband wireless mobile terminals has been increasing. Therefore, it is necessary to design the multiband, compact, low profile, and low-cost antennas, and many types of antennas have been proposed to achieve multiband functions. In addition, wideband properties in resonance characteristics of monopole and dipole antennas have been broadly investigated over the years using various techniques [1, 2].
Various methods have been studied to obtain multiband enhancement, including slot-loaded technologies [3–5], coupling feed technologies [6, 7], loading the matching network [8], and fractal technologies [9]. The desired resonant frequency band is produced by controlling the current path and resonant mode of the antenna radiator. Alternatively, multiple monopole or dipole antennas may be employed as resonant branches having different operating frequencies to create multiple operating bands. In [10], the coupling branches or parasitic patches are isolated from the antenna, and the electromagnetic coupling between them is used to achieve multiband capabilities. In [11], slots are etched on the radiator to change the local current mode and produce different resonance frequencies.
At the same time, with the miniaturization of the terminal device, the radiator is usually bent and folded to increase the antenna electrical length to make the antenna operate at lower frequencies [12]. The bending and folding method of the antenna radiator structure is used to increase the electrical length of the antenna to achieve miniaturization of the antenna [13–15]. Among these design methods, fractal structure approaches are the most representative for their compact size, low profile, and multiband response, with fractal self-similarity and space filling used to increase the antenna electrical length and radiation efficiency [16–18].
From another perspective, the self-similarity of the fractal structure can be regarded as the genetic property of biology, that is, it has certain biomimetic structural characteristics. Most of the literatures use traditional structures such as Koch, Box, Sierpinski, Cantor, and Mandelbrot fractal, for the application of fractal structures in antenna engineering, but less research studies on fractal structures with biomimetic features. In [19–24], the appearance of octopus shape, ginkgo leaf shape, butterfly shape, tree structure, ancient coin-like structure, and curved branch tree are integrated into the antenna design. In [25], it presents a new planar antipodal Vivaldi antenna with nature fern-leaf inspired fractal structure which is around 19.7 GHz starting from 1.3 to 20 GHz.
In this work, the fractal structure of the binary genetic characteristics in nature is improved and applied to the antenna radiator structure design. A novel dual-band broadband antenna with a bionic fractal structure for dual-broadband mobile terminal is presented. The antenna covers mobile applications of TD-SCDMA (1880–2025 MHz), WCDMA (1920–2170 MHz), CDMA2000 (1920–2125 MHz), LTE33-41 (1900–2690 MHz), Bluetooth (2400–2483.5 MHz), and WLAN (802.11a/b/g/n: 2.4–2.48 GHz, 5.15–5.35 GHz) system.
2. Antenna Structure and Design Procedure
2.1. Characteristics of the Antenna Structure
In order to obtain more sunlight energy, plants in nature continue to grow to make full use of space and exhibit obvious genetic fractal characteristics in appearance. For antennas in a limited space, in order to improve the efficiency of radiating and receiving electromagnetic waves, the spatial filling of the fractal geometry can be used to increase the length of the radiator in a limited space. The binary geometry of nature is applied to the antenna engineering in the paper. Based on the binary tree, the binary branching is performed on each branch, and the structure is proposed as the antenna radiator after two iterations, as shown in Figure 1.
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The proposed planar antenna structure is shown in Figure 2 with dimensions in Table 1. The antenna has a two-iteration binary branches fractal structure. Miniaturization and multiband coverage are achieved by varying the length of the antenna surface current flow and through. The antenna is designed on FR4 substrate. A 50 Ω ladder structure coplanar waveguide structure feed line is used to extend the bandwidth, and the overall size is 50 × 40 × 1.6 mm3.
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2.2. Simulation Results of the Antenna
Simulations are conducted using the Ansoft HFSS software package. The basic shape of the antenna radiator is a V-shaped structure with an opening angle of 90 degrees, and the fractal is taken at 1/3 of the outermost side length, and then the fractal is performed once again at 1/3 of the length of each branch side. The evolution process is shown in Figure 3. The orange section is the copper radiator and the coplanar waveguide structure grounding plate, the green section is the dielectric material.
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V-shape radiator performance with different line widths are compared, and 1 mm is selected as the line width of the basic antenna radiator (no iteration), as shown in Figure 4.
The reflection loss of the no iteration antenna is lower than that of the higher iteration fractal antennas, but the working frequency band coverage is not as good as that of the higher iteration fractal antennas, as shown in Figure 5. The red solid line shows that the proposed antenna operates at two different wide frequency bands with three resonance frequencies centred at 2 GHz with a −18.1 dB return loss, 3.25 GHz with a −19 dB return loss, and 5.1 GHz with a −12.7 dB return loss. The simulated −10 dB return loss bandwidth is 71.2% for the first frequency band (1.7–3.58 GHz) and 22.2% for the second band (4.8–6 GHz). These bands cover several commercial application bands for the 3G, 4G, WLAN, and Bluetooth system technologies (see Table 2).
The simulated 3D gain and far-field normalized E/H-plane radiation patterns of the proposed antenna at the centre frequencies of 2, 3.25, and 5.1 GHz with peak gains of 2.3, 4.1, and 3 dBi are shown in Figures 6 and 7, respectively. The E- and H-plane patterns are omnidirectional at the low band and close to omnidirectional at all other bands, as the higher-order modes produce nulls and side lobes, and low level of cross polarization is obtained.
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The surface current amplitude and vector distribution on the conducting part of the proposed antenna at centre frequencies of 2, 3.25, and 5.1 GHz are shown in Figure 8. For the 2 GHz frequency band, the outer edges of the monopole have more current (see Figure 8(a)). As the frequency increases, the current becomes more concentrated at the inner iteration surface. For the 5.1 GHz frequency band, the current reaches a relative maximum at the edges of the radiator (see Figure 8(c)).
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3. Prototype and Measured Results
The prototype of the antenna is built on a 1.6 mm thick FR4 substrate with dielectric constant (εr) of 4.4 and loss tangent (δ) of 0.02 and copper on both sides, as shown in Figure 9. The antenna was tested by Satimo antenna measurement system SG24 in anechoic chamber and the test antenna was a dual-polarized horn antenna with a gain of 4–12 dBi. We used the same test process and analysis method as the two papers published [18, 24].
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The comparison of the measurement return loss and the simulation result is shown in Figure 10. The measured −10 dB bandwidths of the antenna are 1.85–2.9 GHz and 4.9–5.5 GHz, the return loss at the centre resonance frequency of 2.3 GHz is −23.8 dB, the return loss at 2.75 GHz is −23.8 dB, the return loss at 5.2 GHz is −16.3 dB, and the measured bandwidth of the antenna is matched with the simulated bandwidth. The antenna can cover wireless applications such as TD-SCDMA, WCDMA, CDMA2000, LTE33-41, Bluetooth, and WLAN, as shown in Table 3. Due to the fabrication accuracy of the antenna, the fabrication of the joint and the test error, there is a certain error between the test and the simulation data.
The measured 3D radiation pattern at the centres of the 2, 3.25, and 5.1 GHz resonance points is shown in Figure 11.
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Compared with the simulation graphs, the antenna has good omnidirectional radiation characteristics in all frequency bands. As the frequency increases, the side lobes gradually appear, but the zero point still does not appear; in addition, the measurement results are in good agreement with the simulation results.
4. Conclusions
This work developed a 2 iterations binary tree fractal bionic structure dual-broadband planar antenna for CDMA, LTE, Bluetooth, and WLAN applications. The multiband antenna covers two broadbands with a bandwidth of 44.2% (1.85–2.9 GHz) for TD-SCDMA, WCDMA, CDMA2000, LTE33-41, and Bluetooth frequency bands, and 11.5% (4.9–5.5 GHz) for WLAN frequency band. The measured results reveal omnidirectional radiation patterns. The good agreement between the measurement results and the simulation results validate that the proposed design approach meet the requirements for various wireless applications.
Data Availability
The simulation and test data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This work was supported in part by the National Natural Science Foundation of China (grant no. 61663014), Science and Technology Research Project for Colleges and Universities of Hebei Province (grant no. Z2017133), Hebei Higher Education Teaching Reform Project (grant no. 2018GJJG477), Fundamental Research Funds for the Central Universities (grant nos. 3142018047 and 3142017073), and Key Laboratory of Coal Mine Safety Monitoring and Control Technology Safety Production (grant no. 3142018048).