A Weak-Coupling and Compact Magnetic Pendulum Array Antenna for SLF Communication

It is always a challenge to design a portable and efficient super low frequency (SLF) transmitter. Here, we propose a compact and weakly coupled magnetic pendulum array (MPA) for efficient transmission in the SLF band. Analytic expressions for the fields generated by a swinging magnet are first established. Based on theoretical calculation and experimental results, the average power and the maximum swinging angle for an MPA unit are found to be 0.22 W and 43.84°, respectively. Then, three kinds of 1-D array structures are constructed, and both the static and the dynamic magnetic coupling between the antenna units are compared via finite element method (FEM). On this basis, the resonance frequency, the power consumption, and the radiation capability of three kinds of 1-D arrays are experimentally studied. The measured flux density at a 2 m distance in free space reaches 137 nTpk (1100 fTpk @100 m) under the driving frequency of 65 Hz for our optimized 1-D array. It has a compact dimension of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$19\times 6\times 2.2$ </tex-math></inline-formula> cm3 and the figure of merit (FOM_1) is calculated to be 5560 A2m/W. Finally, both a binary amplitude-shift keying (BASK) and a binary phase shift keying (BPSK) signal are transmitted through the optimized MPA. Reliable communication with a distance of 22 m and a data rate of 4 bit/s is experimentally demonstrated. Its FOM_2 for assessing the communication performance is as high as 169.2 b/s/(m<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}\cdot $ </tex-math></inline-formula>Hz), which is almost eight times the value of other MPAs.