# Research on Ultra-Low-Frequency Communication Based on the Rotating Shutter Antenna
## 5 Conclusions  
This paper used ANSYS Maxwell 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 was manufactured; the measured results are in great agreement with the simulated results, while the magnitude of the measured signal was larger. We used FEKO to analyze the propagation characteristics of RPMA in air-soil-rock and the simulated results show that the signal strength was greater than 1 fT at a depth of 450 m from the antenna, whose moment was 1 Am\({}^{2}\). Since the phase difference between \(B_{r}\) and \(B_{\varphi}\) remained constant at \(\pm\)90\({}^{\circ}\), we could use a multi-directional receiver to receive the multi-directional field components; then, the signal strength could be enhanced by shifting  
Figure 13: (**a**) Spectrogram of 2 FSK signal; (**b**) demodulated code.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Frequency** & **Data Rate** & **Detection Distance** \\ \hline Ref. [10] & 500 Hz & / & \textless{}130 m \\ Ref. [11] & 22 Hz & 8 bps & 0.7 m \\ Ref. [14] & 1.03 kHz & 2 bps & 30 m \\ Ref. [17] & 100 Hz & 0.25 bps & 5 m \\ Ref. [20] & 30 Hz & 5 bps & 100 m \\ Ref. [22] & 40 Hz & \textless{}2 bps & 5 m \\ Ref. [23] & 50 Hz & 40 bps & 0.25 m \\ Our work & 320 Hz & 12.5 bps & 5 m \\ \hline \hline \end{tabular}
\end{table}
Table 3: Performance comparison of rotating permanent magnet antennas.  
the phases of the \(B_{r}\)-field component and adding it to another component, such as a \(B_{\theta}\)-field component. The formula shows that the path loss of the rotating shutter antenna at 570 m was 303 dB, which means that the signal strength was 1 fT. The rotating shutter antenna was equivalent to an RPMA with a transmitting magnetic moment of 0.94 Am\({}^{2}\). If the antenna was used as the transmitting antenna in the magnetic induction communication, a two-element antenna array would be required to make the maximum detection depth reach 450 m. The synchronization between the elements would make the control of the system very difficult. Finding a new method to increase the transmitting magnetic moment and the synchronization of the elements will be the focus of our next research studies. FSK modulation could be realized by controlling the motion state of the motor and the constant time of the motor determined the symbol rate. The prototype was used to achieve indoor communication with a code rate of 12.5 bps in the ULF.  
Conceptualization, F.S., X.M. and F.Z.; methodology, F.S. and X.M.; software, F.S. and Z.G.; validation, F.S. and F.Z.; formal analysis, F.S.; investigation, F.S. and X.M.; resources, Y.J.; data curation, F.Z. and Z.G.; writing--original draft preparation, F.S.; writing--review and editing, F.Z. and Y.J.; visualization, F.S.; supervision, G.F.; project administration, Y.J.; funding acquisition, G.F. All authors have read and agreed to the published version of the manuscript.  
This research study was funded by the National Natural Science Foundation of China, grant number 61827803.  
Not applicable.  
The authors declare no conflict of interest.