With the rapid development and increasing maturity of Internet of Things technology, ultra-low power wireless sensors have become an important component of the Internet of Things. Wireless sensor networks have been widely used by deploying a large number of sensor nodes in monitoring areas and forming a multi hop self-organizing network system with a dynamic topology through wireless communication. However, once the battery of sensor nodes using traditional power supply mode runs out, they need to replace the battery again. If sensor nodes are widely distributed, the work required for manual battery replacement cannot be ignored. With the increasing maturity of ultra-low power chip technology, collecting wireless radio frequency energy from the surrounding environment to provide electricity has become an effective and feasible new energy supply mode. In recent years, with the rapid development of communication technology, the environment has been filled with a large number of radio wave signals, mainly including the mobile phone (GSM) frequency band and industrial communication (ISM) frequency band. For a long period of time in the future, multiple communication networks will coexist, providing abundant RF resources for RF energy collection systems.
The most important part of wireless energy harvesting technology is the analysis and design of the receiving antenna, and DYTP123A 61430001-TW is also a hot topic of concern for domestic and foreign experts and scholars. Microstrip antennas have many advantages such as low cost, light weight, and easy conformability, and are widely used in various communication systems. However, the narrow frequency band of microstrip antennas limits their practical applications. Adding parasitic elements or rectangular patch elements with different shaped gaps can overcome the narrowband characteristics of microstrip antennas; At present, a large number of research reports have been conducted on slot antennas in the high-frequency range both domestically and internationally. The basic structure of the slot antenna has good performance, but it also has inherent defects such as narrow impedance bandwidth and can only operate on a single frequency. Therefore, multi frequency/broadband technology has become a hot topic in the research of slot antennas. The literature “Design of a 2.4GHz/5.2GHz Dual Band Microstrip Slot Antenna” achieves dual band operation at 2.4/5.2GHz by loading two inverted U-shaped slots on the basis of the slot antenna; The literature “Design of a novel miniaturized dual band slot microstrip antenna” has an F-shaped slot on the ground plane and is fed with microstrip lines. By adjusting the main size of the slot, the antenna operates in the 2.4/5.8 GHz frequency band. The literature “Design of a Bandwidth Circular Slot Antenna” uses a forked microstrip line feed and a circular slot antenna is opened on the ground plane. The optimal match is achieved by adjusting the relative position of the microstrip line terminal and the center of the slot, as well as the radius of the circular slot. The antenna operates at 2 GHz and the frequency band reaches 32.5%. However, due to the low signal power spectral density in the surrounding environment in the 5 GHz frequency band, these antenna designs are not suitable for wireless energy harvesting in the environment.
Through the analysis and research of the above literature, a small dual band microstrip fed slot antenna suitable for wireless energy harvesting is proposed in the paper. This antenna is based on a forked microstrip feeding slot structure and adopts the reactance loading method, which achieves dual band operating characteristics by loading microstrip branches and slots to improve the antenna’s operating bandwidth, while ensuring performance and overcoming the narrow bandwidth defect of microstrip slot antennas. And the general pattern of the operating frequency of the slot antenna changing with the size of the slot was obtained through simulation analysis.
1. Principle of slot antenna structure
Based on the microstrip antenna structure, dual frequency operation can be achieved using the method of reactance loading. At this time, the dual frequency ratio of DYTP123A 61430001-TW can be adjusted to be closer. According to the cavity model theory, the input impedance Zin of a microstrip antenna with 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 composite effect of other modes. The characteristic equation of its resonant frequency is Xr+Xf=0. If a reactance XL is used to load a microstrip antenna, the above characteristic equation becomes
By adjusting the value of XL, two zeros can be obtained to achieve dual frequency operation.
Figure 1 shows the improved antenna structure, with a left-right asymmetric branching microstrip line at the top. The advantage of branch feeding is that this feeding method can obtain a wide bandwidth and achieve good impedance matching of the antenna over a wide frequency range. In this design, two rectangular gaps are opened on the grounding plate, and the optimal match is achieved by adjusting the relative position of microstrip line branches and gaps, as well as the size of the rectangular gaps.