A Portable VLF Magnetoelectric Antenna with High Communication Rate Based on Direct Antenna Amplitude Modulation

Very low frequency (VLF) is a characteristic frequency band in the radio spectrum. Due to its large skin depth, the signal can penetrate through water and rock, with strong anti-interference ability and long propagation distance. It is gradually applied in underwater communication and geological exploration. Mechanical antennas that driven by external mechanical driving and acoustic self-resonance have been proved to be excellent in VLF electromagnetic (EM) wave signal transmitters. However, mechanical relaxation caused by the signal modulation during resonator energy switching limits the modulation rates of ME antennas. As widely recognized, direct Antenna Modulation (DAM) can solve this problem effectively. This paper reports a portable VLF magnetoelectric (ME) antenna and a novel DAM, i.e., amplitude Shift Keying (ASK), which can eliminate the mechanical relaxation during modulation. The proposed magnetoelectric (ME) antenna is based on a three-layered acoustic resonator structure, with a length of 8 cm, (one-millionth of the EM wavelength). The amplitude switching in the proposed DAM-ASK is achieved by switching the bias magnetic field provided by the modulation coil. The measured bit rate of DAM-ASK is up to 12kbps (fmod = 6 kHz, while fcarrier= 27.68 kHz), which is 6 times higher than that of normal modulation.