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A digital non-Foster radio approach is proposed to mitigate Wheeler-Chu limits of electrically-small antennas, with significant potential to significantly reduce energy consumption in the VHF (very high frequency) band, where radio propagation losses below 200 MHz are 100 times less than losses above 2 GHz. Operation at lower frequency could greatly extend lifetimes of small low-power Internet-of-Things devices such as battery-powered sensors operating primarily as transmitters. Unfortunately, physical size constraints and the Wheeler-Chu limit have greatly hindered utilization of VHF bands for mobile devices, where even a 200 MHz half-wave dipole is an unwieldy 0.75 m. However, recent advances in non-Foster impedance matching methods have overcome these limits. In addition, recent digital non-Foster methods are shown to closely resemble digital radio architectures, suggesting that these newer digital non-Foster methods can be readily adopted in new digital radio designs. Therefore, a novel digital non-Foster radio architecture is proposed, where digital non-Foster methods enable small devices in energy-efficient VHF bands while overcoming Wheeler-Chu antenna-size limits. 

The input impedance of recently-introduced digital impedance circuits has been discovered to be dependent on the impedance of the external signal source. To address this problem, the theory for the dependence of digital impedance on external source resistance is presented. These digital impedance circuits provide an important digitally-controlled digitally-tunable alternative approach to difficult design problems, such as design of negative capacitances for stable wideband non-Foster antennas and metamaterials. Unfortunately, undesired source-dependent variation of the digital impedance can arise in scenarios where off-the-shelf high-speed analog-to-digital and digital-to-analog converters commonly have 50 ohm impedance. Further complicating matters, the sensitivity of digital impedance on source resistance appears to also depend on other design parameters of the digital circuit. Therefore, theory and simulation results are presented to show the dependence of digital impedance on the external source resistance. Lastly, measured results for a prototype of a digital non-Foster negative capacitance confirm the theoretical results. 

It is proposed that gravitational meta-atom unit cells with gravitomagnetic moments could exhibit gravitomagnetic permeability, analogous to the magnetic permeability of materials comprised of atoms with magnetic moments. Recently, a gravitoelectromagnetic (GEM) framework was proposed to explore the possibility of a Veselago-inspired approach to gravitational metamaterials. The prospect of gravitational metamaterials motivates the consideration of candidate gravitational unit cells or gravitational meta-atoms. Although mass serves as a monopole source of a gravitoelectric field similar to positive charge, negative mass would be needed to create a gravitational analog of an electric dipole. However, moving mass is analogous to electric current, and can lead to a gravitomagnetic dipole moment analogous to magnetic dipole moments of magnetic materials and atoms. In this paper, GEM field approximations to general relativity are used to find the gravitomagnetic dipole moment of different rotating systems, ranging in scale from meters to astronomical size.