Performance analysis of spinning magnet as mechanical antenna

Abstract

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 <i>B</i><sub>r</sub> = 0.8 T and a volume of <i>V</i><sub>r</sub> = 270 cm<sup>3</sup> as 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.

A miniaturized mechanical antenna based on FEP/THV unipolar electrets for extremely low frequency transmission

“…S5 also supports this conclusion. This is one of the advantages of an EBMA compared to a magnet-based mechanical antenna (MBMA), since the attenuation of the magnetic flux density generated by an MBMA with the distance is 42 – 44 . An EBMA has a small attenuation in the near field.…”

Analysis of Transmission Characteristics on an MSK-Based Rotating Magnet-Based Mechanical Antenna

“…e physical model of the spinning magnet is equated to a rotating magnetic dipole and can be regarded as two orthogonal vibrating magnetic dipoles with the same frequency in the same plane [27].…”

“…e rotational inertia J m � 0.0032 kg•m 2 (V � 500 cm 3 and λ � 2.4) is taken as the ideal value [28,29] in this paper.…”

“…e transmitter link model offers five modules: MSK signal generation, magnetic field distribution model [27], structural design and stress field analytical model [28], electromagnetic field analytical model of PMSM [32], and servo control model. According to the correspondence shown in Figure 3, the MSK signal generation module converts the input symbol data into the given speed.…”

Influence of the Spatial Distribution of Molecular Magnetic Moments on the Radiation Characteristics of Rotating Permanent Magnet Antennas

“…L ow frequency electromagnetic waves are widely used in both communication and navigation [1,2]. However, traditional transmitting antenna is limited by electric size and low radiation efficiency [3], which has reached the development limit at this stage. To realize the miniaturization and light weight of low-frequency transmitting antennas, since the Defense Advanced Research Projects Agency (DARPA) proposed the "A MEchanically Based Antenna" project in 2017 [4], permanent magnets have become the preferred object for the fabrication of mechanical antennas due to the rather high magnetic energy density [5].…”