Directly Modulated Magnetic Pendulum Arrays for Efficient Ultra Low Frequency Transmission
The frequencies lying between 300 Hz to 3 kHz have been designated as Ultra Low Frequency (ULF) with corresponding wavelengths from 1000 Km to 100 Km. Although ULF has very low bandwidth it is very reliable, penetrating and difficult to jam which makes it a great choice for communication in underwater and underground environments. Small and portable ULF antennas within a diameter of 1 meter would operate under an electrical length on the order of 10^(-4) to 10^(-6) wavelengths in free space, making them very inefficient because of fundamental limits on radiation from electrically small antennas. To overcome this problem, Mechanical Antennas or ‘Mechtennas’ for Ultra Low Frequency Communications have been proposed recently. For efficient generation of ULF radiation, we propose a portable electromechanical system called Magnetic Pendulum Array (MPA) with Direct Antenna Modulation (DAM). MPAs use an array of mechanically oscillating permanent magnets to efficiently generate ULF radiation. Proof of concept demonstrations of the 1D array system and 2D array system at 1kHz and 700 Hz respectively are presented. A Direct Antenna Modulation (DAM) scheme with Binary Frequency Shift Keying is implemented achieving a data rate of 18 bps on the 2D array system. The theory and experimental results demonstrate that such a system can achieve a significantly higher quality factor than conventional coils and thus order of magnitude higher transmission efficiency. The concept can be easily scaled to the ULF range of frequencies with more complicated modulation schemes for higher data rates.