Efficient ULF Transmission Utilizing Stacked Magnetic Pendulum Array
Foad Fereidoony, Srinivas Prasad Mysore Nagaraja, Jean Paul Santos, Yuanxun Ethan Wang
Abstract
A novel stacked magnetic pendulum array (MPA) is proposed as an antenna for efficient transmission in ultralow frequencies (ULF). The MPA structure consists of an array of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${m\times n}$ </tex-math></inline-formula> magnets stacked in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> rows (m magnets of the same magnetization per row) and a solenoid that encompasses the array. The magnetic flux generated by the solenoid couples to the magnets through a mechanical motion and creates a strong time variant magnetic field at the resonance frequency of the array. The proposed MPA is developed utilizing bearings with extremely small damping which hold the magnets, enable efficient oscillation, and guarantees a high-quality factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> -factor). A comprehensive analysis to study the performance of the MPA and a conventional solenoid will be provided and accordingly, characteristics of an arbitrary <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${m\times n}$ </tex-math></inline-formula> array in terms of field strength, resonance frequency, transmission efficiency and inductance are studied. Furthermore, a figure of merit (FOM) is introduced as the ratio of efficiency to the maximum linear dimension of the antenna. The experimental results for a fabricated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5 \times 11$ </tex-math></inline-formula> array show a resonance frequency of 715 Hz and a Q-factor of 94 while the maximum linear dimension of the antenna is just 9 cm. Moreover, measurement results show 24.9 dB improvement in the transmission efficiency of the MPA compared to the efficiency of a single solenoid of the same dimensions. The MPA has the FOM of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${1.26\times 1}{0}^{{4}}$ </tex-math></inline-formula> which is noticeably higher than that of proposed antennas in the literature. To demonstrate broadband transmission with the MPA, a binary frequency shift keying (BFSK) signal is transmitted through the use of direct antenna modulation (DAM) at a rate of 18 bps.