Design of a small dual-band RF energy receiving antenna
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With the rapid development and increasing maturity of the Internet of Things technology, ultra-low power wireless sensors have become an important component of the Internet of Things. Wireless sensor networks deploy a large number of sensor nodes in the monitoring area and use radio communication to form a multi-hop self-organizing network system with a dynamic topology. It has been widely used. However, once the battery of the sensor node using the traditional power supply mode is exhausted, it needs to be replaced. If the sensor nodes are distributed in large numbers, the work required for manual battery replacement cannot be ignored. With the increasing maturity of ultra-low power chip technology, collecting wireless RF energy in the surrounding environment to provide power has become an effective and feasible new energy supply mode. In recent years, with the rapid development of communication technology, the environment is full of a large number of radio wave signals, mainly including mobile phone (GSM) frequency band and industrial communication (ISM) frequency band. For a long time in the future, the coexistence of multiple communication networks also provides abundant RF resources for RF energy harvesting systems.
The most important part of wireless energy harvesting technology is the analysis and design of the receiving antenna, which is also a hot topic of concern for relevant experts and scholars at home and abroad. Microstrip antennas have many advantages such as low cost, light weight, and easy conformality, and are widely used in various communication systems. However, the narrow frequency band of microstrip antennas limits their practical application. Adding parasitic units or rectangular patch elements with different shapes of slots can overcome the narrow band characteristics of microstrip antennas; currently, a large number of research reports on slot antennas have been conducted at home and abroad in the high frequency band. The slot antenna with a basic structure has good performance, but it also has inherent defects such as narrow impedance bandwidth and can only work at a single frequency. Therefore, multi-frequency/broadband technology has become a hot spot in slot antenna research.
1 Slot Antenna Structure Principle
Based on the microstrip antenna structure, dual-frequency operation can be achieved by using the reactance loading method, and the dual-frequency ratio can be adjusted to be closer. According to the cavity model theory, the input impedance Zin of the microstrip antenna on a thin substrate near the mode resonance frequency can be equivalent to
In the formula, Xr is the "resonant" reactance of the parallel resonant equivalent circuit of this mode, and Xf is the synthetic effect of other modes. The characteristic equation of its resonant frequency is Xr + Xf = 0. If a reactance XL is used to load the microstrip antenna, the above characteristic equation becomes
By adjusting the value of XL, two zero points can be obtained to achieve dual-frequency operation.
Figure 1 shows the improved antenna structure, with a left-right asymmetrical branched microstrip line at the top. The advantage of branched feeding is that this feeding method can obtain a wider bandwidth and achieve good impedance matching of the antenna over a wide frequency range. In this design, two rectangular slots are opened in the ground plane, and the best match is obtained by adjusting the relative position of the microstrip line branch and the slot and the size of the rectangular slot.
Figure 1. Antenna geometry model
In order to achieve impedance matching of the interface, the characteristic impedance of the main arm of the branched microstrip line is 50 Ω, and the characteristic impedance of the side arm is 100 Ω. The width of the microstrip line can be calculated according to the empirical formula (3) and formula (4).
The equivalent dielectric constant is
It is calculated that the width of the 50 Ω microstrip feed line is 3.0 mm, and the width of the 100 Ω microstrip line is 1.4 mm. Two rectangular slots are etched on the bottom ground plane of the antenna, which is equivalent to introducing two reactance elements and generating two resonance points. The antenna uses FR-4 as the dielectric substrate, the thickness of the substrate is 1.6 mm, the relative dielectric constant is 4.2, and the loss tangent is TanD = 0.0003. The size of the ground plane is 50 mm × 50 mm. Since there is a strong diffraction field at the edge of the ground where the slot is located, a good far-field radiation pattern can be obtained by selecting a suitable dielectric substrate size. The feed point is at the center of the wide side, and p1 and p2 are differential input ports.
2 Parameter design and optimization analysis
In order to further explore the influence of various geometric parameters of the antenna on the antenna return loss and obtain the working characteristics suitable for the GSM 1900 MHz and ISM 2.4 GHz frequency bands, the ADS full-wave electromagnetic field simulation tool is used to perform parameter analysis and optimization of the antenna. The physical size parameters of the antenna are shown in Figure 2.
Through preliminary simulation, the return loss of the antenna is sensitive to the changes of the length L1, L2 and width W3, W4 of the two rectangular slots, so the above four parameters are selected for parameter analysis. Each parameter selects an initial value, and when one parameter changes, the other parameters remain unchanged. The initial values of each parameter are shown in Table 1.
Figure 2 Schematic diagram of slot antenna design parameters
Figure 3 shows the effect of the small gap length L1 on the antenna return loss. The size of L1 is selected to increase by 1 mm from 22.9 mm, and other main parameters remain unchanged. The simulation results show that in the low frequency band, the larger the L1, the more the resonance point moves to the right. When L1 = 23.9 mm, the return loss is the smallest. In the high frequency band, as L1 increases, the resonance frequency point moves to the left, the return loss decreases, but the bandwidth also decreases.
Figure 4 shows the effect of the large slot length L2 on the antenna return loss. L2 increases by 1 mm from 41.6 mm, while other parameters remain unchanged. It can be seen from the figure that the smaller L2 is in the low frequency band, the greater the return loss is, and the bandwidth increases accordingly. The resonance point remains basically unchanged; in the high frequency band, the larger L2 is, the resonance point moves to the left, the return loss is smaller, and the antenna impedance becomes increasingly mismatched.
Figure 5 shows the effect of small slot width W3 on antenna return loss. The size of W3 increases by 1 mm from 10.6 mm, while other parameters remain unchanged. The simulation results show that W3 has little effect on the low frequency band; in the high frequency band, when W3 increases, the resonant frequency shifts to the left, and the return loss and bandwidth remain unchanged.
Figure 6 shows the effect of the large gap width W4 on the antenna return loss. The size of W4 increases by 1 mm from 14.1 mm, and other parameters remain unchanged. It can be seen from the figure that the larger the W4 is in the low frequency band, the resonant frequency shifts slightly to the right, the return loss becomes larger and larger, the better the antenna matching is, and the bandwidth increases accordingly. The rule in the high frequency band is the same as that in the low frequency band. Through the simulation results, it is found that adjusting the size of the gap can change the distance between the two resonant frequencies. According to the design requirements of the frequency band, the sizes of the gaps finally selected are L1 = 23.9 mm, L2 = 41.6 mm, W3 = 12.6 mm, and W4 = 18.1 mm. Finally, the optimal antenna size parameters are obtained, as shown in Table 2.
Figure 3 Resonant frequency changes with L1
Figure 4 Resonant frequency changes with L2
Figure 5 Resonance frequency changes with W3
Figure 6 Resonant frequency changes with W4
The gain patterns of the antenna at the resonant frequencies of 1.9 GHz and 2.4 GHz are shown in Figures 7 and 8. It can be seen from the figure that the radiation of the slot antenna is bidirectional, and the radiation field above and below the slot is the strongest, and the radiation intensity is basically the same. When the resonant frequency of the antenna is 1.9 GHz, the maximum gain on the XOZ plane is 1.4 dBi; when the resonant frequency of the antenna is 2.4 GHz, the maximum gain on the XOZ plane is 2.9 dBi. The antenna pattern has a certain degree of directivity, but the gain of the antenna is not high, so this antenna can be used as an omnidirectional antenna, suitable for receiving surrounding RF wireless energy.
Figure 7. Gain of the antenna on the XOZ plane (f = 1.9 GHz)
Figure 8. Gain of the antenna on the XOZ plane (f = 2.4 GHz)
3 Test Results
According to the parameter analysis and optimization results in the previous section, the antenna was manufactured using a double-sided FR4 PCB board, and the antenna was tested using an Agilent vector network analyzer. The actual image of the antenna is shown in Figure 9.
Figure 9: Front and back views of the actual antenna
Figure 10 shows the input return loss simulation and measured curves of the antenna. It can be seen from the simulation that the central resonance points of the antenna are f1 = 1.9 GHz and f2 = 2.4 GHz. When the return loss S11 <- 10 dB, the antenna operates in the low-frequency band from 1.82 to 1.96 GHz, with a bandwidth of 140 MHz. The antenna operates in the high-frequency band from 2.34 to 2.45 GHz, with a bandwidth of nearly 110 MHz. The return losses of the antenna at the resonance point are -40 dB and -20 dB, respectively, indicating that the antenna is well matched. The measured results are basically the same as the simulation results. The resonance frequency in the low-frequency band is shifted to the right by about 1.92 GHz, and the resonance point in the high-frequency band is slightly shifted to the left. The return losses at the two resonance points are reduced. The causes of the error include slight errors in the size during the antenna processing, poor welding at the SMA connector, energy loss at the interface, and environmental interference.
Figure 10 Return loss test results
4 Conclusion
This paper proposes a multi-band antenna method combining slot loading with dual-line feeding, and designs a new small-size dual-band microstrip slot antenna suitable for ambient wireless energy reception. Through ADS simulation and optimization analysis, the antenna can work at 1.9 GHz and 2.4 GHz. The bandwidth at the low frequency end is 140 MHz, the relative bandwidth is about 7.4%, and the bandwidth at the high frequency end is 110 MHz, the relative bandwidth is about 4.6%. The RF energy receiving antenna can adapt to both GSM and ISM frequency bands, has small size, low production cost, strong practicality and good application prospects.
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提升卡
变色卡
千斤顶