As a new type of low-frequency magnetic antenna, the rotating permanent magnet antenna (RPMA) has great application potential in underwater and underground communication. An RPMA array based on near-field polarization modulation is presented in this letter. The near-field polarization modulation is proposed because the RPMA can radiate circularly polarized magnetic fields in the direction of the rotation axis. Since the RPMA is intrinsically an electrically small antenna, it can only work in the near-field area where the field decays with the third power of the distance. The 1 × 2 array, which can enhance the field strength, is manufactured and tested. The measured results of the axial ratio and phase difference show that the proposed method is effective when the offset angle is not greater than 5.7°. A communication experiment is also carried out, and the results show that the RPMA array can achieve a transmission rate of 2 bps.

This paper proposes a rotating shutter antenna that can directly generate 2FSK signals in the ULF band and it is expected to be used as the transmitter for magnetic induction (MI) underground communication systems. The antenna was modeled using ANSYS Maxwell and the magnetic field distribution was simulated. The results show that the interaction between the high-permeability shutter and the mutual cancellation of magnets decreased the transmitting magnetic moment of the antenna. A prototype antenna was manufactured and the time and frequency properties of the measured B${}_z$ field were the same as the simulated results, while the magnitude of the measured signal was larger. The propagation characteristics of the antenna in air–soil–rock were simulated using FEKO and the results show that the signal strength was greater than 1 fT at a depth of 450 m from the antenna whose magnetic moment as 1 Am${}^2$. The relationship between different magnetic components and azimuth could be used to enhance the signal strength. The formula of the B${}_z$ field was derived using the measured magnitude versus distance and the path loss was also analyzed. Finally, the 2FSK modulation property of the antenna was verified by indoor communication experiments with a code rate of 12.5 bps in the ULF band.