Electromechanical ELF Transmitters for Wireless Communications in Conductive Environments (ePoster)

Since the skin depth in ground or seawater is on the order of meters in the extremely low frequency (ELF) band, RF penetration through solids (e.g., into caves) and through water (e.g., to submarines) becomes feasible. This permits emergency communication for search and rescue missions and communication to submarines deep underwater. However, conventional antennas in this band are either impossibly large or highly inefficient (and thus power‐hungry). For example, the U.S. military has in the past used ELF communication to communicate with submarines via Project Sanguine, a set of 76 Hz and 45 Hz transmitters with antennas stretching 14 miles and consuming a combined 2.6 MW during transmission. The FCC only regulates frequency bands between 9 kHz and 275 GHz, in part because electrical antennas are so inefficient below this range. This leaves a conveniently unregulated frequency range below 9 kHz (in the ELF and VLF bands) for unrestricted use. Proposed applications include studies of RF penetration through the ground for the study of the earth's crust and the study of the ionosphere. Moreover, unlike regulated ham radio bands, this unregulated frequency space has no restrictions on the use of encryption. Thus, communications systems below 9 kHz could be encrypted by any means desired, making this a highly lucrative application for private communications systems. We have developed a mechanically‐based ELF antenna which replaces a conventional electrical antenna with a rotating permanent magnet. This radically different approach to wireless transmitter design allows us to take full advantage of the unique properties of the ELF band. Our design utilizes the high remanent flux density in rare earth magnet materials (e.g., NdFeB) to make ELF transmitters more power‐efficient and portable. The current prototype operates at 90‐110 Hz and supports data rates up to a few bits/sec; the next design iteration will operate at 300‐700 Hz, allowing higher transmit data rates. In this presentation we describe the theory behind mechanically‐based transmitters, describe the design of a practical transmitter, and show preliminary experimental results.