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在甚低频(3—30 kHz)及以下频段, 与波长相比, 传统天线属于电小辐射体, 因此辐射效率很低. 如果将永磁体进行机械旋转, 可以获得时变电磁场. 与传统天线相比, 旋转永磁体将机械能向电能转换, 不需要阻抗匹配网络, 提高了辐射效率. 为了计算旋转永磁体的电磁特性, 本文基于安培环路电流理论, 研究了空间正交磁偶极子与机械旋转永磁体的场等效关系. 在此基础上, 采用无限大空间并矢格林函数, 并引入旋转永磁体的初始旋转角参数, 推导了空间正交磁偶极子场分布的通用解析表达式, 作为分析旋转永磁体及其阵列的近场和远场分布的理论模型. 仿真表明, 当钕铁硼(NdFeB)永磁体剩余磁感应强度 B r= 0.8 T、体积 V= 270 cm 3、转速为9600 r/min时, 对应频率为160 Hz, 在自由空间1 km的位置, 将产生15 fT的磁场; 在海水介质中, 当传输距离为250 m时, 磁场快速衰减到1 fT. 本文提出采用旋转永磁体阵列对近场分布特性进行调控, 仿真结果表明: 将两个相同的旋转永磁体组成二元阵, 将使近区磁感应强度提高3 dB; 改变阵元距离和初始旋转角, 可改变近区磁场分布方向图. 机械旋转永磁体为实现小型超低频发射天线提供了新的解决思路.Long wavelength results in the low radiation efficiency of a portable conventional antenna operating at very low frequency (VLF) and below. This has motivated one to develop an innovative approach to design an electrically small antenna in a frequency band lower than VLF. The time-varying electromagnetic fields can be generated by spinning a permanent magnet. In this way, the mechanical energy is converted to the electromagnetic energy, and the impedance matching networks with nonnegligible insertion loss are not required. Therefore, this mechanical antenna with spinning magnet can improve radiation efficiency in a low frequency band. In this paper, we give the detailed analysis procedure for the spinning magnet, which is seldom discussed in other published reports. In order to analyze the electromagnetic characteristics of the spinning magnet, in this paper we use the ampere return circuit theorem to investigate the equivalent relation between a spinning magnet and the orthogonal magnetic dipole. We introduce an initial spinning angle of the magnet into the dyadic green’s function. With this modification, we provide the rigorous analytic formula for field computation of the orthogonal magnetic dipole. Thus the electromagnetic characteristics of the spinning magnet and spinning magnet array can also be analyzed. For a spinning NdFeB magnet with a magnetization of B r= 0.8 T and a volume of V r= 270 cm 3as well as 9600 revolutions per minute, the simulation results reveal that the magnetic field of 15 fT at 1 km in air space can be obtained. But the magnetic field of the spinning magnet decreases quickly to 1 fT at 250 m in sea water. Considering the potential demand for increasing the field strength in the near field region, we recommend to use a magnet array with small-sized elements. The magnet array can be used to control the near field pattern. We take two magnets as an example for studying the performance. It can be found from the simulation results that the magnetic near field is increased by 3 dB with the linear magnet array consisting of two elements. With the initial spinning angle of the magnet element adjusted, the near field pattern of the magnet array can be controlled. This is analogous to beam steering of traditional phased array for high band operation. It can be concluded from our study that the spinning magnet is a possible alternative solution for low frequency small transmitter antenna.
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