Achieving non-reciprocity through unconventional methods without the use of magnetic material has recently become a subject of great interest. Towards this goal a time switching strategy known as the Sequentially-Switched Delay Line (SSDL) is proposed. The essential SSDL configuration consists of six transmission lines of equal length, along with five switches. Each switch is turned on and off sequentially to distribute and route the propagating electromagnetic wave, allowing for simultaneous transmission and receiving of signals through the device. Preliminary experimental results with commercial off the shelf parts are presented which demonstrated non-reciprocal behavior with greater than 40 dB isolation from 200 KHz to 200 MHz. The theory and experimental results demonstrated that the SSDL concept may lead to future on-chip circulators over multi-octaves of frequency.

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 a Magnetic Pendulum Array (MPA). A proof of concept demonstration of the system at 1.03 kHz is presented. 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.

An unconditionally stable finite-difference time-domain (FDTD) algorithm is proposed to predict and understand the complex response in strain-mediated multiferroic RF devices such as antennas. A system of three coupled sets of governing equations is solved simultaneously: (1) Maxwell's equations for electromagnetic (EM) wave propagation, (2) the Landau-Lifshitz-Gilbert (LLG) equation for magnetic spin response, and (3) Newton's law for acoustic behavior. The formulation of this algorithm is elaborated on in detail followed by a demonstration of its capability through the simulation of a 1.3 μm thick bulk-acoustic-wave (BAW) based strain-mediated multiferroic antenna. Analysis of the far-field radiation efficiency of this antenna as a function of magnetic DC bias demonstrates the importance of aligning the ferromagnetic resonance (FMR) and the mechanical BAW resonance to enhance EM radiation performance. Results also show that reducing the magnetic loss, or in other words reducing the FMR linewidth, represents the dominating feature to achieve higher radiation efficiencies in these antennas.

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 ${m\times n}$ magnets stacked in $n$ 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 ( $Q$ -factor). A comprehensive analysis to study the performance of the MPA and a conventional solenoid will be provided and accordingly, characteristics of an arbitrary ${m\times n}$ 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 $5 \times 11$ 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 ${1.26\times 1}{0}^{{4}}$ 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.

A physics-based nonlinear circuit model is developed for frequency-selective limiters (FSLs) based on thin-film magnetic materials. The equivalent circuit model is structured and its parameters are determined rigorously from the fundamental physics of electromagnetic waves and spin waves. The spin motions as well as the ferromagnetic resonance (FMR) are modeled by RLC parallel circuits with parameters derived from Polder's tensor and Kittel's equations. The exchange coupling between spins is modeled by an inductor added between adjacent RLC circuits based on quantum spin theory. The nonlinear crossfrequency coupling from signal at ω to spin waves at ω/2 is represented by a nonlinear coupled inductor model that follows the mathematics of a pendulum motion to represent the parametric oscillations of spins. The circuit units are cascaded to describe the spin-wave propagation inside the magnetic material, and transmission line parameters are added finally to describe the electromagnetic wave propagation. An FSL device described in the literature is used as an example to validate the circuit model, and the model successfully predicts the power-dependent insertion loss as well as the threshold power level, time delay, and frequency selectivity of the power limiting effect.

A rigorous yet computationally efficient 3-D numerical method has been proposed based on modified alternating-direction-implicit finite-difference time-domain (ADI FDTD)methods. It has the capability of modeling the anisotropic and dispersive properties of magnetic material. The proposed algorithm solves Maxwell's equations and Landau-Lifshitz-Gilbert equation jointly and simultaneously, requiring only tridiagonal matrix inversion as in ADI FDTD. The accuracy of the modeling has been validated by: 1) the nonreciprocity of an X-band ferrite resonance isolator; 2) the attenuation constant of a magnetically tunable waveguide filter; and 3) the dispersive permeability of a 2-μm-thick magnetic thin film. Time steps that are up to 5000 times larger than the Courant-Friedrichs-Lewy limit have been used in these simulations without encountering stability issues. The simulation results agree with the predictions made from the theory, the commercial software or the experiments. Moreover, the algorithm has been applied to predict the effect of high permeability thin films in platform effect reduction. An electric current sheet close to a perfect electrically conducting plane coated with a 2-μm-thick magnetic thin film is simulated, which exhibits an enhanced surface resistance by three orders of magnitude higher than that without the magnetic thin film.