The Research And Application Of Near-field Antenna At Low Frequency

As the development of the communication, well logging systems, electromagnetic compatibility, the array antenna design and the imaging systems, the near-field analysis is more and more important. At present, the use of antenna near field also becomes very wide, such as radio frequency identification(RFID), near field communication(NFC), magnetic coupling resonant wireless energy transmit, near field measurement, biomedical engineering, and etc. So the accurate and efficient method of near field analysis plays an important role in control of near field distribution and strategy of near field guidance. Analysis of near field so far is not deep enough, but engineering request of near field application is urgent, so near field theory should be deeply studied to reveal the character of its distribution and its energy flow. Moreover, a low frequency antenna is an electrically small antenna, then the problems concerning small antennas including the efficiency, the impedance matching, and other issues also need to be studied. And in terms of computational electromagnetics, low frequency collapse problem is also a difficulty, how to solve the problem is key to the study in low frequency.Based on theoretical research of magnetic dipole(electrically small loop) and electric dipole in the near field, this paper analyzes the average energy flow density(i.e., the poynting vector), the average electric field energy density, and the average magnetic energy density. Following by studying the near-field energy density distribution and the poynting vector, and then energy flow in the near field image. This paper also calculates the radial component of the poynting vector, and then analyzes the antenna near-field radiation energy and the non-radiation energy.In addition, this paper studies the deformation small electric loop(including saddle loop, cambered loop). In order to improve the near field uniformity in the interested region, this paper compares their near field with that of a rectangular electric loop. These loop can be used in radio frequency identification(RFID) system to improve the reading efficiency.Finally, this paper further uses the above conclusions and the electromagnetic simulation software FEKO to analyze narrowband antennas(including microstrip antenna, half wave antenna) and broadband antennas(including Vivaldi antenna, double cone antenna). Average energy flow density and energy density distribution of these antennas are studied, and the energy flow characteristics are analyzed through the poynting vector. This paper also analyzes the antennas’ near-field polarization, amplitude and phase, and compares the near field with their own far field respectively. Meanwhile, the different near fields of several kinds of antennas are also compared with each other.