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Ultra-miniaturized and highly power-efficient ELF (0.3-3 kHz) transmitters are desirable for underground and undersea wireless communications. This paper proposes the use of rotating permanently polarized dipoles (magnets or electrets) for such applications. A power-efficient data modulation scheme based on continuous-frequency FSK (CF-FSK) is also proposed for such electro-mechanical transmitters. Theoretical analysis and simulations show that 7-ary CF-FSK is optimal in the sense that it minimizes average mechanical torque on the mechatronic antenna for a given bit rate. Preliminary experimental results from a prototype ELF magnetic-field based transmitter based on a high-strength rare-earth (NdFeB) magnet and small brushless DC (BLDC) motor are presented. The results highlight local environment challenges such as power line interference that affect the choice of data encoding and modulation schemes. Reliable non-lineof-sight (NLOS) communication through concrete barriers at 100 Hz and ∼0.25 bit/sec is demonstrated at distances up to ∼5 m. The proposed mechatronic antenna and modulation scheme can be scaled up in both frequency and transmit power for more challenging applications in surface-to-undersea, surface-to-cave, and other conductive environments that require ELF channels. INDEX TERMS Antennas and propagation, communication systems, electromechanical devices, emergency services, magnetic devices, transmitters.

An automated instrument for improving the sensitivity of nuclear quadrupole resonance (NQR) spectroscopy is presented. The device is capable of pre-polarizing samples within a custom Halbach-type permanent magnet and then moving them into an external probe for zero-field NQR detection. Polarization transfer between protons and nitrogen (14N) nuclei in the sample occurs during demagnetization, thus increasing the amplitude of the detected NQR signals. The sample motion profile is completely programmable, thus providing a high level of control over the sample position and velocity for optimizing the polarization transfer process for various samples. Moreover, the magnet and motion controller are combined with a shielded sample probe and ultra-broadband front-end electronics (both designed in-house) to realize a complete scientific instrument for 14N NQR experiments. Compared with previous work in the field, the system is designed to be programmable, robust, and easy to use. Experimental results from several samples are also presented.