Abstract
This paper incorporates the structural characteristics of chrysanthemums in nature and fractal geometry theory into the antenna structure design and a novel-structured fractal multiband fractal microstrip antenna with a chrysanthemum petal structure is proposed. The antenna can cover commercial frequency bands from the second to fourth generation (4G), satellite navigation, wireless local area networks, and Bluetooth. The antenna radiator imitates the structure of chrysanthemum’s petals in nature, and a basic arc shape is iterated many times according to a certain proportional coefficient. After simulation and comparison, the second iteration can achieve the best antenna performance, and the antenna adopted the coplanar waveguide feeding method to broaden the antenna bandwidth. The antenna covers three effective frequency bands of 1.58–2.68 GHz (64.7%), 2.76–4.01 GHz (33.9%), and 4.68–5.35 GHz (13.4%). The antenna dielectric board is made of FR-4 material, the dielectric constant is 4.4, and the actual size is 41 × 29 × 1.6 mm3. The antenna performs fractal iteration at a small size, and the approximate calculation of the frequency band is completed by comparing the ratio of each ring to the current vector diagram. The design of the antenna is to use HFSS for antenna modeling and parameter optimization, and the model under different conditions were compared and analyzed. By comparing the test results of the antenna prototype with the simulation structure in the electromagnetic anechoic chamber, the rationality of the antenna is verified.
1. Introduction
In the ancient Chinese book “Tao De Jing,” it is said: Dao is natural. It is the root of all things and a major source of human thoughts and applications. Observing the practices from ancient times to the present, human beings have gained many insights into the process of imitating natural creatures [1]. Bionics is an advanced technology that invents and creates by imitating the structure and function of creatures in nature to serve better human life and production [2, 3].
The bionic antenna is an important application of the antenna combined with bionics. In studying antennas, plant structures and animal organs in nature are analyzed and used to promote the generation of new antenna structures [4]. The working principle and performance of antennas are similar to those of the organisms in nature; hence, by imitating the structure of organisms to design the geometric structure of antennas, good performance can be obtained [5, 6]. Examples include the use of plane-opposed Vivaldi miniature antennas inspired by natural ferns [7], a broadband antenna that simulates the shape of a ginkgo leaf [8], multiband antennas imitating banana leaves [9], and slot-shaped antennas imitating the shadow of butterfly wings under the sun [10, 11]. The antenna can change the available frequency band from single frequency to dual frequency by adding semicircular arc branches to the circular monopole to form a “C + O” structure [12].
Recently, due to the miniaturization and multi-function of communication equipment, the requirements for multi-frequency, miniaturization, and integration of antennas have become much higher [13–15]. At present, many multiband antenna methods have been studied, such as coupling feed technology [16], slot-loading technology [17], and reconfigurable technology [18, 19]. Therefore, we should use bionics as a breakthrough point to explore new structures to simplify theoretical analysis. The performance comparison between the antenna proposed in this paper and the antenna in the reference is shown in Table 1.
In this paper, the structure of antenna radiator is innovated. The radiator integrates the plant structure, applies the chrysanthemum structure in bionics to the radiator of the antenna, and passes the basic semicircle structure through triple iteration fractal. Increase the current path on the radiator, thus increasing the electrical length of the antenna, so that the designed antenna can achieve multi band and miniaturization in a limited space.
This article proposes and discusses a chrysanthemum petal antenna with multiband characteristics designed using the principle of bionics. The chrysanthemum is a beautiful flower in nature with a regular structure. Inspired by the shape of chrysanthemum petals, the antenna designed in this paper realizes the design of a multiband bionic antenna. The designed antenna has three frequency bands that can be covered 1.51-2.31 GHz (64.7%), 3.32-3.8 GHz (33.9%) and 4.59-5.2 GHz (13.4%). It can be widely used in WLAN, WiMAX, Bluetooth, GPS and other communication systems.
2. Antenna Structure and Design Procedure
2.1. Characteristics of the Antenna Structure
The structure and parameters of the planar antenna are shown in Figure 1, and the size table is shown in Table 2. Inspired by the blooming chrysanthemums in nature, the radiator evolved into a curved dipole structure based on curved petals. The feed line of 50 Ω resistance is connected to the rectangular ground, and the feed mode of the coplanar waveguide is adopted. The antenna adopts the substrate FR-4 material; the thickness is 1.6 mm, the relative dielectric constant is 4.4, and the loss tangent is 0.02. Perform fractal iterations based on a single open ring, iterate sequentially according to the reduction ratio of ε =0.618 (golden ratio), and perform three iterations to achieve multiple frequency bands. After each iteration, the current path is increased to the greatest extent within a limited size. As shown in Figures 2(a)–2(d).
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2.2. Simulation Results
Modeling and analyzing antenna on high-frequency simulation software. As shown in Figures 2 and 3, figure 2(a) consists of an incomplete loop as a dipole to form the antenna’s radiator, resulting in two frequency bands of 1.34–2.7 GHz and 4.7–5.8 GHz. Fractal iterations based on a single ring were performed and iterated in sequence according to the reduction ratio ε =0.618. The blue dashed line in Figure 3 is the S11 curve of the 0th iteration of the antenna. Figure 2(b) is based on Figure 2(a) for the first iteration. Due to the increase in the current path, a new frequency band of 3.12–4.61 GHz is produced, and the bandwidth of the low-frequency band is reduced. The black rectangle dotted line in Figure 3 is the S11 curve of the 1st iteration of the antenna. Figure 2(c) The figure performs the second iteration. The low-frequency band is the same as before the iteration. The 3.5-GHz frequency band is slightly shrunk to form better isolation, and a new frequency band of 4.68–6 GHz is produced, finally reaching 1.34–2.76 GHz. In the three frequency bands of 3.26–4.1 GHz and 4.68–6 GHz, the isolation between each frequency band is ideal, and the S11 performance is good. The S11 curve is shown as the solid red line in Figure 3. Figure 2(d) The figure shows the third iteration. The frequency band generated at low frequencies is not as good as the performance of the second iteration, and the frequency band bandwidth generated at high frequencies is not as wide as the bandwidth of the second iteration. It is known from the S11 curves of the second order to the third order that since the size of the radiator added after the fractal iteration is small, the continuous iteration has little effect on the performance of the antenna. Therefore, the second iteration is used as the final model. The obtained antenna can cover multiple mobile commercial frequency bands, such as 2G, 3G, 4G, 5G, WLAN, and Bluetooth.
As shown in Figure 4, the length of L5 is swept from 0 mm to 13.5 mm. L5 determines the shape of the antenna ground. When L5 = 0 mm, the antenna ground is rectangular, and the return loss is shown by the long purple dotted line in Figure 4. The antenna performance requirements are not met at high frequencies. When L5 = 13.5 mm, the ground of the antenna is triangular. The return loss is shown by the yellow dotted line in Figure 4. The bandwidth is narrow in all frequency bands, and the frequency points do not meet the commercial frequency points. The result of frequency sweep analysis shows that when L5 = 9.5 mm, the antenna has the best performance and wider bandwidth, which can cover more commercial frequency bands.
The frequency sweep analysis of the antenna stub width W1 is shown in Figure 5. With the widening of W1, the two frequency bands with the center frequency of the antenna at 3.65 and 5.15 GHz gradually move to the high frequency, and gradually cannot cover the available frequency band. Therefore, the final width of the antenna is W1 = 1 mm.
As shown in Figure 6, the antenna works on three frequency bands; the center frequency points are 1.74 GHz, 3.68 GHz, and 5.15 GHz, and the S11 is −30 dB, −24.3 dB, and−20.1 dB, respectively. The simulated −10-dB bandwidth of the first frequency band (1.34–2.65 GHz) is 81%, which can cover GPS, DCS, TD-LTE, LTE33–37, TD-SCDMA, ISM2.4G, WLAN, and other wireless communication frequency bands. The bandwidth of the second frequency band (3.26–4.1 GHz) is 22%, which can cover LTE42/43 and WiMAX. The third frequency band (4.68–6 GHz) has a bandwidth of 25%, which can cover wireless applications such as WLAN (See Table 3).
The surface current and current vector diagrams of the antenna at 1.74, 3.68, and 5.15 GHz are shown in Figure 7 (see Figure 7(a)–7(c)). When the center frequency is 1.74 GHz, the current is mainly concentrated on the outer ring, and the current in the inner two rings is small. When the center frequency is 3.68 GHz, the current is concentrated in the middle ring. There is a small amount of current in the outer and inner rings; when the frequency increases to 5.15 GHz, the current is mainly concentrated in the inner loop. It is observed that the longer the length, the lower the resonance frequency point, and the shorter the length, the higher the resonance frequency point. The radiator is iterated to increase the electrical length of the antenna. Under the effect of mutual coupling between the radiators, the appropriate frequency point is found that increases the bandwidth.
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It can be seen from Figure 7(a) that the maximum current when the antenna appears in the main middle branch and part of the outer ring when the antenna is working at 1.74 GHz. Therefore, the length of the radiation patch that caused the first resonance can be calculated as:
According to the data in Table 2, La =41.25 is obtained; therefore,
Because the arc in the middle excites the second resonance, the maximum value of the current appears on the middle arc, as shown in Figure 7(b). According to the data in Table 2, the length of the maximum current path Lb is:
It can be seen from Figure 7(c) that the maximum current of the antenna at the resonance frequency of 5.15 GHz appears in most of the inner arcs and a small part of the middle arcs. The length of the maximum current path Lc is:
The 3D radiation patterns and cross-polarization patterns of the antenna at 1.74, 3.68, and 5.15 GHz are shown in Figures 8 and 9. At the center resonance frequencies of 1.74, 3.68, and 5.51 GHz, the 3D radiation pattern shows gains of −4, 0.59, and−0.04 dBi, respectively. In the 1.34–2.65 GHz and 3.26–4.1 GHz frequency bands, the antenna has omnidirectional radiation. There are some side lobes at high frequencies, but the antenna still maintains good radiation characteristics.
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The antenna is placed on the arm for the SAR value test, as shown in Figure 10. The peak value of SAR is 0.0352 W/kg. The maximum value can meet international standards.
3. Fabrication and Measured Results
The prototype and actual measurement of the antenna are shown in Figure 11. The antenna was eventually engraved with a 30 μm layer of copper on the FR-4 dielectric board. Antenna prototypes were fabricated and tested in an anechoic chamber to verify the broadband performance of multi-frequency planar antennas. The bandwidths of the three frequency bands of the antenna are 64.7% (1.51-2.31 GHz), 33.9% (3.32-3.8 GHz), and 13.4% (4.59-5.2 GHz). The bandwidth of −10-dB S11 is roughly identical to the simulation frequency band, as shown in Table 4 and Figure 12. Comparing the results of measurement and simulation, the S11 agrees well.
The measured S11 and simulated results are compared and good agreement is observed, as shown in Figure 12. However, some differences exist due to some reasons, such as the implementation of the antenna precision and interface deviation. The antenna is carved and polished by corrosion, and there will be some errors in the dimensions of the dielectric plate and the radiator. And the coaxial line will produce some loss. as shown in Table 4.
Figures 13(a)–13(c) show 3D radiation patterns measured and figure 14 shows the cross polarization of the E\H plane by the antenna at 1.74, 3.68, and 5.15 GHz. The outer curve is mainly polarized and the inner curve is cross polarized. The antenna has good omnidirectional radiation, side lobes appear when the frequency rises, and the measurement results agree well with the simulation manufacturing. The antenna has good radiation characteristics. In the available frequency band, the H surface has good omnidirectionality. However, the final performance will deviate slightly from the test performance due to manufacturing errors.
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The antenna gain curve is shown in Figure 15. It can be seen from the graph that the trend of the simulated and measured gain curves is roughly the same. The maximum gain in the three available frequency bands of the antenna is 2.41dBi, 3.86dBi and 3.91dBi, respectively. The gain of the antenna over the available frequency band meets the radiation requirements.
4. Conclusion
This article proposes a multi-band antenna that resembles a chrysanthemum. The final model is composed of half-arcs with the golden ratio gradually reduced. The frequency band and bandwidth tested are 1.51-2.31 GHz (64.7%), 3.32-3.8 GHz (33.9%) and 4.59-5.2 GHz (13.4%). The antenna covers multiple commercial frequency bands: LTE Band 40, WiMAX, SCDMA, LTE42/43, Bluetooth, WLAN, GPS, etc. The antenna radiator adopts the chrysanthemum like structure in bionics to achieve multi band and miniaturization in a limited space by fractal iteration. Due to the certain errors in the manufacturing accuracy and dielectric constant of the dielectric plate during the production process of the antenna, there will be a certain deviation between the test data and the simulation data. Still, the actual measurement and simulation of the antenna have good consistency, and the radiation on the H surface has full Tropism. The rationality of the antenna is verified, and it is suitable for most wireless applications.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The author(s) declare(s) that they have no conflicts of interest.
Acknowledgments
This work was supported in part by the Natural Science Foundation of Hebei Province (No. F2021508009) and the National Key Research and Development Program of China (No. 2020YFC1511805).