A Portable VLF Magnetoelectric Transmitter With High-Rate Phase Modulation

Very low frequency (VLF, 3–30 kHz) electromagnetic (EM) waves are widely used in areas such as mining, underwater communications, etc., because of their relatively large skin depth that enables long-distance propagation in a lossy medium. Mechanical antennas driven by acoustic oscillation or external mechanical movements have been proven to be promising candidates for low-frequency (LF) transmitters. Unfortunately, the mechanical relaxation in modulation caused by the motion inertia of mechanical antennas limits modulation rates. Here, we developed a high-rate active phase modulation (APM) scheme that can eliminate the mechanical relaxation and approach the theoretical maximal bit rate in a magnetoelectric (ME) antenna. The proposed ME antenna is based on a three-layered acoustic resonator with a length of only 8 cm and a total efficiency of
$3.59\times 10^{-14}$
, which is closer to Chu’s limit. The proposed APM is realized by the 180° phase reversal of the bias magnetic field provided by a modulating coil. The high bit rate of 18 kbps at the resonant frequency of 27.75 kHz is demonstrated by using APM, one order of magnitude higher than that of passive phase modulation (PPM) due to the effective suppression of mechanical relaxation in such 180° phase variation.