# Research on Ultra-Low-Frequency Communication Based on the Rotating Shutter Antenna
## 1 Introduction  
The electromagnetic waves at radio frequencies have short wavelengths and poor penetrating capabilities, which make them lose part of their energy in a complex electromagnetic environment [1]. For example, the permittivity and conductivity of soil have a great relationship with the water content, which causes significant attenuation of high-frequency electromagnetic waves. This leads to the fact that radio-frequency communication cannot be widely used in underground communication. To overcome this problem, magnetic induction communication technology with ultra-low-frequency magnetic fields is widely used in underground communication, such as Through-The-Earth communication [2]. The permeability of non-magnetic materials is almost the same, which provides a relatively stable channel for the propagation of magnetic field signals and the stable channel enables the magnetic induction communication to avoid the shortcomings of multipath propagation, large propagation delay and high bit error rate of acoustic transmission through the ground [3]. Normally, magnetic inductive communication systems use coils to generate and receive magnetic field signals in near-field zone [4]. However, in the ultra-low-frequency band, thesize of the coil is also very large, which brings great inconvenience to the placement and layout of the system [5; 6]. To overcome the shortcomings of traditional low-frequency antennas, such as large size, low efficiency and high-power consumption, in 2017, the US Department of Defense Advanced Research Project Agency (DARPA) proposed the AMEBA plan [7]. Different from traditional antennas, AMEBA mainly generates alternating electromagnetic waves directly through electrical charges or magnetic moments in mechanical motion, which is a process of converting mechanical energy into electromagnetic energy [8; 9]. As a kind of mechanical antenna, the rotating permanent magnet antenna (RPMA) drives the permanent magnet to rotate so as to obtain an alternating magnetic field. It is likely to be used as the transmitting antenna in the magnetic induction communication system. After AMEBA was proposed, many researchers have become interested in RPMA and began to study it, including field generation and measurement [10; 11] and radiation power analysis [12]. [PERSON] et al. analyzed the efficiency of RPMA and proposed a method to build an RPMA array to improve antenna efficiency [13]. [PERSON] et al. proposed a prototype consisting of an array of magnetic pendulums in oscillatory motion at ULF. The transmission efficiency of the magnetic pendulums array is higher than a bare coil through the measurement in the near field [14; 15]. The communication system which uses RPMA as the transmitter mostly adopts direct antenna modulation (DAM). For example, we can obtain an OOK signal by controlling the driver switch and an FSK signal by controlling the speed of motion [16; 17; 18; 19; 20]. Besides, [PERSON]. [21; 22; 23; 24] proposed that amplitude and phase modulation can be achieved by using an external modulator. However, these methods require additional energy to control the modulator in real time, which is difficult to implement. To effectively avoid the limitation of the motor to the antenna's operating frequency range, we used a rotating shutter antenna as our transmitting antenna, which could obtain a magnetic field signal that is four times the rotating frequency. ANSYS Maxwell was used to analyze the magnetic field distribution of the rotating shutter antenna and the results show that the interaction between the high-permeability shutter and the mutual cancellation of magnets decreased the transmitting magnetic moment. A prototype antenna was manufactured and the experimental results are in great agreement with the simulated results, while the magnitude of the measured signal was larger.