With the rapid development of the Internet of Things technology and increasingly mature, ultra-low power consumption wireless sensors have become an important constituent unit of the Internet of Things. The wireless sensor network uses radio communication to form a multi-hop network system with a multi-hop-enabled self-organizing network system with radio communication, and has been widely used. However, once the battery is used to replace the battery once the battery is exhausted, if the sensor node is distributed, the work required to manually replace the battery will not be ignored. As the ultra-low power chip technology is more mature, the radio frequency energy in the surrounding environment is collected to provide electrical energy into an effective new energy supply model. In recent years, with the rapid development of communication technology, the environment is filled with a large number of radio wave signals, mainly including mobile phones (GSM) bands and Industrial Communications (ISM) bands. For a long time in the future, a variety of communication networks coexist, and also provides a rich radio frequency resource for RF energy collection systems.
The most important part of the wireless energy collection technology is the analysis design of the receiving antenna, and is also a hot spot for experts from experts at home and abroad. The microstrip antenna has a low cost, light weight, ease of conformality, etc., is widely used in various communication systems. However, the microstrip antenna limits its practical application than the frequency band, increasing the parasitic unit or a rectangular patch element having different shape gaps, can overcome the narrow band characteristics of the microstrip antenna; currently in high frequency bands, gap the antenna at home and abroad A large number of research reports were conducted. The basic structure of the gap antenna is good, but there is also a narrow resistance bandwidth, which can only be fixed. Therefore, multi-frequency / broadband technology has become a hot spot in the study of gap antenna. The literature "Working in the design of the 2.4GHz / 5.2GHz dual-band microstrip gap the antenna" On the basis of the gap antenna, the characteristics of the two-frequency work of 2.4 / 5.2 GHz are achieved by loading the two-inverted U-shaped grooves; the new type of literature " Design of small dual-band slot microstrip antenna "Opened a F-type groove on the ground floor and feeds with microstrip wire, and the antenna works through the main dimension of the tank to operate at 2.4 / 5.8 GHz band. The design of a wide circular gap antenna is fed with a fork type microstrip line and opens a circular gap antenna in the ground plate, by adjusting the relative position of the microstrip terminal and the center of the gap and the radius of the circular gap. To get the best match, the antenna works at 2 GHz, and the band has reached 32.5%. However, since the 5 GHz band is low in the surrounding environment, these antenna design is not suitable for use in environmental wireless energy collection.
Through the analysis of the above literature, a small dual-band microproofing gap antenna suitable for use in wireless energy collected is proposed. The antenna is based on the fork type microstrip spring slit structure, which uses a reactance load method, which enables a dual-band operating characteristic by loading the microstrip section and the groove to improve the working bandwidth of the antenna, and overcome the microstrip gap the antenna while ensuring performance. Bandwidth defects. The general rules of the working frequency of the gap the antenna with the size of the slit size were obtained by simulation analysis.
1 Sliding antenna structure principle
Based on the microstrip antenna structure, the method of reactive load can be used to achieve dual-frequency operation. At this time, the dual frequency ratio can be adjacent. According to the cavity model theory, the microstrip antenna of the thin substrate is in the input impedance ZIN in the vicinity of the mode resonant frequency, can be equivalent to
In the formula, XR is the "resonant" reactance of the molded resonant equivalent circuit, XF is the synthesis effect of other modes. The characteristic equation of its resonant frequency is XR + XF = 0, and if the microstrip antenna is loaded with one reactance XL, the characteristic equation is turned into
Adjust the value of XL, you can get two zero points to achieve dual-frequency work.
Figure 1 is a modified antenna structure, the top of the antenna is a non-symmetric branch micropil line. Advantages of branching feeds are that the feed method can obtain a wide bandwidth and enable an antenna to achieve a good impedance match within a wide frequency range. In this design, two rectangular gaps are opened in the ground floor, and optimally matches the relative position of the microstrip line branch and the gap and the size of the rectangular gap.
Figure 1 Geometric model of antenna
In order to realize the impedance match of the interface, the characteristic impedance of the branch type microstrip wire main arm is 50 Ω, the characteristic impedance of the side arm is 100 Ω, according to the empirical formula (3), the formula (4) can calculate the width of the microstrip wire.
Where is the equivalent dielectric constant is
This calculates a width of 3.0 mm from a 50 Ω microstrip line, and a width of 100 Ω microstrip corresponds to 1.4 mm. Two rectangular gaps are etched on the bottom ground floor of the antenna, which is equivalent to introducing two reactive elements, resulting in two resonant points. The antenna uses FR - 4 as a dielectric substrate, the thickness of the substrate is 1.6 mm, the relative dielectric constant is 4.2, and the loss angle is tandem TAND = 0.0003. The size of the ground floor is 50 mm × 50 mm. Since the edge of the surface where the gap is located, the suitable medium substrate size can be selected, and a good far field direction map can be obtained. The feed point is in the center of the wide side, P1 and P2 are differential input ports.
2 parameter design and optimization analysis
In order to further explore the influence of the various geometric parameters of the antenna to the antenna echo loss, the working characteristics suitable for GSM 1900 MHz and ISM 2.4 GHz bands are obtained, and the ADS full-wave electromagnetic field simulation tool is used to parameter analysis and optimization. The physical dimension parameters of the antenna are shown in Figure 2.
By preliminary simulation, the echo loss of the antenna is sensitive to the length L1, L2, and width W3, W4 of the two rectangular gaps, thus selecting the above four parameters to perform parameter analysis. Each parameter selects an initial value when a parameter changes, and other parameters remain unchanged. The initial value of each parameter is shown in Table 1.
Figure 2 Design parameters of the gap antenna
Figure 3 shows the influence of the small slit length L1 on the antenna echo loss, the L1 size is selected from 22.9 mm, and the other main parameters remain unchanged. The simulation results can be found that the larger in the low frequency segment, the resonance point right Moving, when L1 = 23.9 mm, the echo loss is minimal; with the L1 increases, the resonant frequency point is left, the return loss is reduced, but the bandwidth is also reduced.
Figure 4 shows the influence of the large gap length L2 on the antenna echo loss, and L2 increases from 41.6 mm to each 1 mm, and other parameters remain unchanged. As can be seen from the figure, the smaller the L2 at the low frequency, the loose loss is large, and the bandwidth is also increased accordingly. The resonance point is basically unchanged; the larger the L2 at the high frequency section, the resonance point moves to the left, the smaller the echo loss, the antenna impedance is increasingly matched.
Figure 5 shows the influence of the small slit width W3 on the antenna echo loss, the size of W3 is increased from 10.6 mm to each 1 mm, and other parameters remain unchanged. The simulation results show that the impact of W3 on low frequency sections is almost very small; when the high frequency segment is in W3, the resonant frequency left shift, the echo loss and bandwidth remain unchanged.
Figure 6 is an influence of the large gap width W4 on the antenna echo loss, the W4 size is increased from 14.1 mm to each 1 mm, and the other parameters remain unchanged. As can be seen from the figure, the larger the low frequency band, the resonant frequency is slightly moved, and the return loss is getting larger and larger, the better the antenna, the bandwidth is also increased. Larctation in high frequency sections is the same as low frequency bands. Through the simulation results, it is found that the size of the adjustment gap can change the distance between the two resonant frequencies. According to the design of the design, the size of the selected gap size is L1 = 23.9 mm, L2 = 41.6 mm, W3 = 12. 6 mm, W4 = 18.1 mm, respectively. 6 mm, w4 = 18.1 mm. Finally, the best antenna size parameter is finally shown in Table 2.
Figure 3 Changes of resonance frequencies with L1
Figure 4 Changes of resonance frequencies with L2
Figure 5 Changes of resonance frequencies with W3
Figure 6 Resonance frequency changes with W4
The gain pattern of the antenna in the resonant frequency 1.9 GHz and 2.4 GHz is shown in Figures 7 and 8. As can be seen from the figure, the radiation of the gap antenna is bidirectional, the slit, the lower radiation field is stronger, and the radiation intensity is substantially the same. When the resonant frequency of the antenna is 1.9 GHz, the maximum gain on the Xoz surface is 1.4 DBI; the antenna resonant frequency is 2.4 GHz, the maximum gain on the Xoz face is 2.9 dBi. The direction of the antenna has a certain direction, but the gain of the antenna is not high, so this antenna can be used as a full-directional antenna, suitable for receiving the surrounding RF radio energy.
Figure 7 Gain (f = 1.9 GHz) on the antenna on the Xoz surface
Figure 8 gain of the antenna on the Xoz surface (f = 2.4 GHz)
3 test results
According to the parameter analysis and optimization results of the previous section, the antenna is made using the FR4 double-sided PCB board, and the antenna is tested through the Agilent Vector Network Analyzer, and the physical map of the antenna is shown in Figure 9.
Figure 9 Front view and opposite side of the active
Figure 10 shows the input echo loss simulation and measured curve of the antenna. From the simulation map, the center resonance point of the antenna is F1 = 1.9 GHz, F2 = 2.4 GHz. When the return loss S11 <- At 10 dB, the antenna in the low frequency band is from 1.82 to 1.96 GHz, with a bandwidth of 140 MHz, the antenna ranges from 2.34 to 2.45 GHz in the high frequency band, and the bandwidth is close to 110 MHz. The echo loss of the antenna at the resonance point is - 40 dB and - 20 dB, indicating that the antenna matches better. The result of the measured results is basically the same as the simulation results. The resonant frequency band at the low frequency section is approximately 1.92 GHz, and the resonance point at the high frequency section is slightly offset to the left, and the echo loss at the two harmonic points is reduced. The causes of errors include micro error in the process of size during processing antenna, and the SMA joint is welded, and there is energy loss, environmental interference, and other factors at the interface.
Figure 10 Echo loss test results
4 conclusion
A multi-band antenna method for gap loading binding two-wire feed is proposed, which is designed for a new small dual-frequency microstrip gap antenna for environmental wireless energy reception. The AGILENT's ADS is simulated and optimized, and the antenna is achieved in 1.9 GHz and 2.4 GHz dual-frequency work, respectively. At the low frequency end bandwidth of 140 MHz, the relative bandwidth is about 7.4%, at the high frequency end bandwidth of 110 MHz, the relative bandwidth is about 4.6%. The radio frequency energy receiving antenna can accommodate two frequency bands of GSM and ISM, small size, low production cost, strong practical and good application prospects.
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