Abstract
This study proposes and designs a multiband branch antenna with a structure that imitates the Chinese classical pattern structure. The antenna radiator’s structure is a symmetrical rectangular stub fused with a Chinese classical pattern structure, and the rectangular stub is bent so that the outer and inner stubs are coupled to each other to generate multiple frequency bands. Microstrip line feeding is the feeding mode, and the grounding plate is a trapezoidal structure formed by subtracting two triangles from a rectangle. The overall size of the antenna is 60 × 60 × 1.6 mm3, and the dielectric board adopts FR4. The substrate dielectric constant εr = 4.4, the thickness h = 1.6 mm, and the dielectric loss tangent tanδ = 0.02. For antenna modeling and parameter optimization, HFSS electromagnetic simulation software is used. The antenna can cover 1.49 to 1.60 GHz, 1.87 to 2.51 GHz, and 4.63 to 5.34 GHz and generate three main frequencies: 1.57, 2.15, and 5.06 GHz, according to test result. The antenna has omnidirectional radiation characteristics and can be widely used in future mobile communication network coverage.
1. Introduction
Since the invention of the antenna, the antenna has played an increasingly important role in human social life. It is a vital radio device for transmitting and receiving electromagnetic waves, as well as an essential component of the radio communication system. The performance of the antenna has a direct impact on the communication system’s quality. Modern communications have paid increasing attention to the study of antenna multiband miniaturization performance to improve the efficiency and performance of antennas [1, 2].
Imitating the classical grille structure multiband antenna and combining the unique structure of the classic grille with the theory of antenna design, the changeable pattern inside the grille can make the antenna have multiband performance characteristics. By changing the structure of the window grille, the influence of its structure on the antenna performance is analyzed, and the structure that makes the antenna performance the best is found [3].
The door and window patterns that can be used for antenna design are broadly classified as geometric patterns, tree and flower patterns, animal patterns, and so on. Geometric patterns are those that use straight lines, curves, and shapes to create geometric patterns for decorations. It can be combined in many ways, including ice crack, step-by-step brocade, swastika pattern, set of square brocade, “回” pattern [4], and so on. In this antenna design, the structure of the “回” pattern is adopted, as shown in Figure 1.
Multifrequency band technology is to increase the path through which current flows on the surface of the radiator to generate resonance and realize multifrequency bands. Multipatch technology, slot-loading technology, multibranch structure, fractal structure, and other multiband implementation technologies are currently in use. By incorporating one or more patches, the radiator can operate in various resonant modes, generating multiple resonant frequency points to form a multiband antenna [5, 6]. Open one or more slots on the surface of the radiator to extend the path of the antenna surface current, change the path of the surface current, and make the multiple resonance frequencies which are generated by the radiator [7–9]. The antenna extends the radiator's branches and uses electromagnetic coupling to generate multiple frequency points [10–12]. Using the self-similarity of the fractal structure, after a certain number of iterations, each part of the radiator corresponds to a different resonance frequency to achieve the multiband performance of the antenna [13]. Taha A. Elwi et al. proposed a metamaterial (MTM) antenna series. It combines INP substrate and fractal technology. Part of the structure is similar to the “回” pattern structure, but the performance of the two differs due to differences in substrate material power supply mode [14].
In this paper, using the technique of multiple branches, combined with the classical pattern window grille structure, a design of a “回” pattern branch antenna with multiband characteristics is proposed and discussed. In the window grille structure, the “回” pattern is a relatively simple geometric pattern. The surface current path and length of the antenna are increased by bending the rectangular radiator into multiple branches, resulting in multiband performance. The antenna was designed to cover three frequency bands: 1.49–1.60 GHz, 1.87–2.51 GHz, and 4.63–5.34 GHz. It has a wide range of applications, including GPS, Beidou navigation systems, 3G, 4G, WLAN, Bluetooth, WIMAX, and other communication systems.
2. Design and Structure of Antenna
The electromagnetic simulation software HFSS is used to optimize the antenna parameters. Figure 2 depicts the final antenna structure model. Table 1 displays the size parameters. The dielectric plate is made of polytetrafluoroethylene glass cloth (FR4) with a dielectric constant εr = 4.4, a thickness of 1.6 mm, and a dielectric loss tangent tan of δ = 0.02. The physical size of the media board is 60 × 60 × 1.6 mm3.
3. Parameter Optimization
The main shape of the antenna is a rectangular stub, which incorporates a “回” pattern structure. The rectangular stub is bent into a “回” pattern structure, and a microstrip line structure is used for power feeding, bending the radiator and the electromagnetic coupling effect is primarily responsible for the multifrequency performance. Figure 3 depicts the evolution of the antenna.
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In Figure 3, model (a) is a basic antenna model. The antenna model mimics the structure of the “回” pattern, by bending the rectangular branches to form two inner and outer loops, with a fixed distance between each inner and outer branch. During the optimization of stimulation parameters, it was discovered that only two frequency points, 2.8 GHz and 5.8 GHz, have the antenna's return loss value less than −10 dB. On this basis, the grounding plate was changed, and the two right angles above the grounding plate were cut off, and the structure is shown in model (b) appeared. Model (b) has three more frequency points than model (a), 2.4 GHz, 3.8 GHz, and 5.1 GHz, and generates five usable frequency bands. Based on model (b), model (c) cancels the fixed distance between the inner and outer branches, resulting in each branch having its own parameter value. After a series of parameter optimization, the structure is shown in model (c) appears. The return loss diagram of the three evolution process is shown in Figure 3.
By comparing and analyzing the comparison chart of return loss curve in Figure 4, model (c) produced six resonance frequency points of 1.55 GHz, 2.4 GHz, 3.11 GHz, 4.1 GHz, 5.05 GHz, and 5.84 GHz, resulting in six frequency band: 1.49–1.58 GHz, 1.81–2.55 GHz, 2.74–3.42 GHz, 3.68–4.29 GHz, 4.72–5.38 GHz, and 5.59–6.12 GHz. At 2.4 GHz, the –10 dB bandwidth of model antenna (c) is 0.3 GHz narrower than that of model (b), and at 3.11 GHz, the –10 dB bandwidth of model antenna (c) is 0.4 GHz wider than model (b), and the return loss value is low. The return loss and bandwidth of the antenna do not change significantly in the high-frequency region. Comprehensive comparison, the model antenna (c) has superior performance in all aspects, so it is chosen as the final antenna model.
4. Antenna Simulation Results and Parameter Analysis
Figure 5 shows the return loss curve of the antenna. The antenna has six different resonant frequency bands. The center frequency points are 1.55 GHz, 2.4 GHz, 3.11 GHz, 4.1 GHz, 5.05 GHz, and 5.84 GHz, and the return loss values at the six resonance frequency points are −26.59 dBi, −15.89 dBi, −27.84 dBi, −19.56 dBi, −22.52 dBi, and −24.37 dBi. According to the return loss ≤−10 dB as the reference standard, from the return loss diagram, the −10 dB bandwidth corresponding to the center resonance frequency point on the return loss diagram is 1.49–1.58 GHz, 1.81–2.55 GHz, 2.74–3.42 GHz, 3.68–4.29 GHz, 4.72–5.38 GHz, and 5.59–6.12 GHz. Table 2 shows the the communication frequency bands covered by these frequency bands.
Figure 6 is the surface current and vector distribution diagram of the antenna at the resonance frequencies of 1.55 GHz, 2.4 GHz, 3.11 GHz, 4.1 GHz, 5.05 GHz, and 5.84 GHz.
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It can be seen from the surface current distribution of the antenna in Figure 6. When the center frequency is 1.55 GHz, the antenna surface current is primarily focused at the end of the radiator branch, the current is relatively smooth, and the impedance is matched. At 2.4 GHz, the current path becomes shorter as the frequency increases, and the current is primarily concentrated on the bottom of the radiator and the feeder. At the frequency point of 5.05 GHz, the surface current of the antenna is mainly concentrated on the radiator and the feeder, and the branches of the radiator are electromagnetically coupled with the ground plate so that the current spreads across the branches of the radiator. The antenna current is primarily concentrated on the feeder at the frequency point of 5.84 GHz. The antenna generates resonance frequency points primarily by changing the electromagnetic coupling between the current direction and the feeder stubs. A lower frequency corresponds to a longer current path, while a higher frequency corresponds to a shorter current path.
Figures 7(a)–7(d), respectively, list the 3D pattern and the E and H plane patterns of the antenna at different central frequencies. The red solid line represents the E plane pattern, while the blue dashed line represents the H surface direction map. Maximum gains are 1.74 dBi, 3.14 dBi, 2.90 dBi, and 4.18 dBi for antenna center frequencies of 1.55 GHz, 2.4 GHz, 5.05 GHz, and 5.84 GHz, respectively. Overall, the gain of the antenna at each frequency point is sufficient to meet the communication requirements, and the performance is satisfactory. The pattern of the E surface changes from a positive figure eight to a concave persimmon shape as frequency increases accompanied by the appearance of side lobes. The H surface gradually becomes concave and curved from a circle.
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5. Test Results of the Branch Antenna
To verify the correctness of the design, the antenna shown in Figure 8 has been manufactured. The size of the antenna is 60 × 60 × 1.6 mm3. The feeder is connected to a 50 Ω SMA adapter, and the prototype is tested using a vector analyzer and an antenna microwave anechoic chamber.
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The comparison between the actual measurement results and the simulation results of the return loss S11 is shown in Figure 9. According to the comparison diagram of the antenna return loss value between the simulation and the actual measurement, it can be seen that the antenna simulation and actual measurement trends are roughly the same.
At the 1.55 GHz frequency point, the frequency point did not move, and the return loss value changed from −18.9 dB to −21.9 dB. The −10 dB bandwidth at 2.4 GHz narrows. The bandwidth is reduced by 0.17 GHz from 1.76–2.56 GHz to 1.87–2.50 GHz. The antenna's resonant frequency is nearly constant at 5.05 GHz; the return loss value has changed from −23.82 dB to −30.78 dB, and the bandwidth is relatively consistent. In the actual measurement, the frequency point of 5.8 GHz is greater than −10 dB. Figure 10 depicts the antenna's actual measured 3D radiation pattern. It can be seen from the actual measured 3D pattern of the antenna that the gain values of the antenna at 1.57, 2.15, and 5.05 GHz are 3.1, 1.76, and 2.54 dBi, respectively. In the low-frequency band, the antenna has better omnidirectional radiation characteristics; as the frequency increases, more side lobe levels and side lobes appear in the 3D radiation pattern. The antenna maintains good radiation characteristics throughout its operating frequency range, and there is no zero point. Table 3 shows the specific commercial frequency bands that are available. There is a small difference between the antenna test results and the simulation results due to errors in the artificial manufacturing process of the antenna and electromagnetic interference during the antenna test. On the whole, the actual measurement results of the antenna match the simulation results and meet the design requirements.
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6. Conclusions
Based on the theory of branch antennas, combined with the trapezoidal ground structure, a microstrip branch antenna with multiband characteristics is designed by simulating the classical “回” structure. The comparison of simulation and actual measurement results validates the design's logic. The antenna has a lovely appearance and performs admirably. The antenna dimensions are 60 × 60 × 1.6 mm3. In Figure 10, the central resonance points at 1.57 GHz, 2.15 GHz, and 5.06 GHz reach −21.9 dB, −12.69 dB, and −30.79 dB, respectively, and the maximum gains are 3.1 dBi, 1.75 dBi, and 2.54 dBi, respectively. The pattern branch antenna has three frequency bands, 1.49–1.60 GHz, 1.87–2.51 GHz, and 4.63–5.34 GHz, with relative bandwidths of 7.0%, 29.77%, and 14.03%, respectively, and omnidirectional radiation characteristics in each bandwidth. The antenna can be applied to GPS, Beidou Satellite Navigation, Bluetooth (2.4–2.485 GHz), WLAN (2.4–2.48 GHz), LTE Band40 (2300−2400 MHz), ISM Band (2.420–2.4835 GHz), WiMAX (2.3 GHz), TD-SCDMA (1880−2025 MHz), WCDMA (1920−2170 MHz), CDMA2000 (1920–2125 MHz), and WLAN (802.11a/n:5.15–5.35 GHz). In future research, on the one hand, it is hoped that the antenna can be combined into an array to increase the gain; on the other hand, it is hoped that the antenna can be designed into a flexible material and applied to a flexible wearable terminal.
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 they have no conflicts of interest.
Acknowledgments
This work was supported in part by Natural Science Foundation of Hebei Province (no. F2021508009), National Key R&D Project (no. 2020YFC1511805), Fundamental Research Funds for the Central Universities (no. 3142018048), and Education and Teaching Reform Special Project of NCIST (nos. 01010403–55 and 0502010223–16).