# Research on Ultra-Low-Frequency Communication Based on the Rotating Shutter Antenna
## 4 Experimental Results
### Communication Based on the Rotating Slutter Antenna  
We changed the distance between the transmitter and the receiver antenna to 4.5 m, while the other experimental parameters were the same as those mentioned above. Using control software to rotate the motor between 4800 rpm and 4950 rpm, we theoretically obtained 320 Hz and 330 Hz signals, which can represent different binary information symbols such as '1' and '0'. The time required for the motor to switch between different speeds was set to 100 ms and the time for a constant rotation was 80 ms, which determined the code rate of communication.  
The modulated signal after bandpass filtering is shown in Figure 11(a). Its corresponding spectrum, shown in Figure 11(b), demonstrates that the frequency of the 2 FSK signal did not reach the theoretical value, which may have been due to insufficient control accuracy. In addition, there was a very obvious frequency component between the frequency spectra of the 303.7 Hz and 316.2 Hz signals, which was introduced during the motor speed switching process. This problem could be solved by increasing the constant speed time of the motor, but this would reduce the communication code rate.  
It can be clearly observed from the spectrogram shown in Figure 12(a) that the signals with frequencies of 303.7 Hz and 316.2 Hz appeared alternately in sequence, which means that the information symbol '1010101...' was received. The received signal is processed by filtering out the power-frequency signal, passing through bandpass filtering and extracting the envelope of the filtered signal; then, the information symbol shown in Figure 12(b) can  
Figure 11: Path loss versus propagation range.  
Figure 12: (**a**) Modulated signals at the point 5.4 m away from the rotating shutter antenna; (**b**) spectrum of signals.  
be obtained through amplitude judgment. It was found, from the demodulated information, that there were 12.5 symbols in one second, which means that the code rate of communication was 12.5 bps.  
Table 3 summarizes the performance comparison between the previously proposed systems and the system adopted in this paper. It can be found that the frequency of the signal, the communication rate and the communication distance could not be taken into account simultaneously in the existing research study. Our system mainly considers a higher communication code rate, but the communication range was not the longest, which is also a focus for our future research work.  
Since the antenna operates in its near-field region, the signal strength decays with the third power of the distance, which limits the maximum communication distance of the communication system. Similarly, due to the low carrier frequency of the system, the data rate cannot be too high, which limits the diversity of communication. For example, the communication rate is only suitable for message transmission. In addition, an array with more shutter antennas would be a good choice to increase the magnetic moment, but assuring the control accuracy of the motor would be necessary and difficult to achieve.