# Research on Ultra-Low-Frequency Communication Based on the Rotating Shutter Antenna
## 4 Experimental Results
### Measurement of the Rotating Shutter Antenna  
A prototype, as shown in Figure 7, with the same size as shown in Table 2 was manufactured and magnets were fixed on an aluminum plate through an aluminum alloy cover. The YASKAWA servo AC motors with model SGM7J-01 AFC6S were used as the driver, which could provide a rated torque of 637 mN\(\cdot\)m and a rated power of 200 W. The rotation axis of the shutter antenna and the ULF receiver shown in Figure 8 were placed in line to measure the B\({}_{2}\)-field. The distance between the antenna and the receiver was 3 m, as that in simulation. In the communication experiment, the information symbols were sent to the servo through software named MPE720 Ver. 7 to control the rotating shutter antenna in real time.  
The portable ULF receiver, shown in Figure 8, consisted of a magnetic sensor and an NI collector. The conversion coefficient of the magnetic sensor was 8 mV/nT, so we could directly convert the received voltage signal into field strength. The NI collector was utilized to digitize and store the received signal with a sampling rate of 3 Ksps.  
The measured result shown in Figure 9 shows that, when the motor speed was set to 4800 rpm, the frequency of the signal obtained was in good agreement with the simulated result, but the amplitude was larger than that obtained in the simulation, which may be ascribed to the fact that the material in the simulation was not consistent with our prototype and the metal substances in the experimental environment may also have had a certain influence on the signal strength. From the spectrogram of the field shown in Figure 9b, we can clearly see the 320 Hz signal.  
Figure 8: ULF receiver: (**a**) magnetic sensor; (**b**) NI sampler.  
Figure 7: The rotating shutter antenna.  
Figure 9: (**a**) Field at the point 3 m away from the shutter antenna. (**b**) Spectrogram of field.  
We measured the \(B_{z}\)-field by placing the receiving antenna along the axis direction of the rotating shutter antenna and changing the distance between them (such as 0.6 m, 1.2 m,..., 4.8 m and 5.4 m); the variation in the field strength with distance is shown in Figure 10. The propagation characteristic curve was fitted using MATLAB and the result is also shown in Figure 10. The fitting curve can be expressed as  
\[B_{z}(\mathrm{nT})=\frac{188}{r^{3}} \tag{3}\]  
The intensity of the \(B_{z}\)-field is inversely proportional to the third power of the distance, which is in line with the attenuation law of the near-field signal. However, different from the rotating permanent magnet antennas (\(B_{z}=\frac{\mu m}{2r^{3}}\)), the expression of the magnetic flux density of the rotating shutter antenna shown in Equation (3) cannot find the similar law related to the magnetic moment.  
In MI communication, the rotating shutter antenna is used as the transmitter. The path loss can be defined as  
\[PL=20\lg\frac{B_{r}}{B_{z}}\approx 137.4+60\lg(r) \tag{4}\]  
where \(B_{r}\) is the remanence of the permanent magnet and \(B_{z}\) is given in Equation (3).  
The path loss determined by the propagation range was simulated, as shown in Figure 11. The point with path loss of 303 dB (the received magnetic field was about 1 fT for \(B_{r}=1.4\)T ) is marked with red lines in Figure 11. If the receiver can handle magnetic field signals of 1 fT, the MI communication distance of up to 570 m could be achieved. The antenna prototype shown in Figure 7 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-unit antenna array would be required to make the maximum detection depth reach 450 m. The synchronization between the elements would also be difficult. Searching for a new method to enhance magnetic moments will be the focus of our research work.  
Figure 10: The change in \(B_{z}\)-field with distance.