In this paper, we present an electromagnetic (EM) Lagrangian approach to quantify the reactive energy density distribution around generic antenna systems operating in the time-harmonic regime. First, we introduce the ‘period-averaged EM Lagrangian density’ for radiating EM fields and present its intrinsic relations with the complex Poynting vector, total EM energy density, complex Helicity density, antenna reactive energy and radiation Q-factor. Furthermore, utilising full-wave MoM (method-of-moments) based MATLAB Antenna Toolbox, we compute contour plots of period-averaged EM Lagrangian density around thin-strip dipoles as well as printed microstrip patch antennas. It is demonstrated that such contour plots can not only distinguish between the capacitive and inductive nature of reactive energy density around multiple antenna systems, but also assist in the visualization of the links of inter-element mutual coupling via reactive fields. Such spatial maps of period-averaged EM Lagrangian density can be potentially useful for engineers working in the design of antenna arrays, MIMO (multiple-input multiple-output) and full-duplex systems, as well as EMI/EMC (Electromagnetic Interference/Compatibility) compliant devices.

In this paper a multi-functional metasurface is proposed which can work as a narrowband transmissive/reflective frequency selective surface (FSS) and an on-air frequency mixer based on its switching response. The metasurface is made up of unit cells with square and circular metallic loops connected by PIN diodes that are controlled by a biasing source. In contrast to typical wideband FSSs, the structure provides 0.55 GHz of narrow stopband (a fractional bandwidth of 22\%) at 2 GHz in the OFF state bias. The bandstop response can be adjusted by varying the reverse bias voltage. The metasurface alternates between its functionalities when in forward bias by providing a passband at the operational frequency. The structure is compact and operates as a transmissive/reflective surface under two different bias conditions (ON and OFF). The design is angularly stable and polarization-insensitive for both TE and TM polarisation. A prototype of the designed structure is developed and the measured results correlate well with the simulated responses. On-air frequency mixing for a wave propagating through the metasurface is experimentally demonstrated and effects of different parameters affecting the mixing are parametrically studied.

In this letter, we provide new insights into the mutual coupling between antennas, using spatial distribution of near-field reactive Helicity and period-averaged EM Lagrangian density. First, we analytically show that reactive Helicity (given by the real part of complex Helicity) in frequency domain always becomes zero in the far-field region of generic linearly and circularly polarized antennas. Furthermore, we introduce the concept of ‘instantaneous Helicity’ (analogous to instantaneous Poynting vector) to calculate reactive Helicity for antennas excited by arbitrary time-domain signals (eg. short-duration pulses). We analytically demonstrate the fundamental connection between this instantaneous Helicity, EM inertia density and EM Lagrangian density. Finally, using MoM-based MATLAB Antenna Toolbox, we compute the distribution (contour-plots) of normalized reactive Helicity and period-averaged Lagrangian density around a two-port system of thin-wire dipoles in frequency domain, revealing complex near-field inter-element interactions. Such visualization of mutual coupling via reactive Helicity and period-averaged EM Lagrangian density can enable antenna array designers to go beyond the traditional S-parameter approach, and identify regions containing near-zone non-TEM fields and non-propagating energy density, which may contribute constructively/destructively to the port-to-port mutual coupling.

In this letter, we propose the designs of 1-bit and 2-bit digitally coded metasurfaces, which achieve anomalous reflection-based beam-steering. First, we demonstrate the systematic design flow of coded metasurface using phase gradient digital unit cells to achieve anomalous reflection in the preferred direction for a normally incident plane wave. Initially, the design procedure is analytically implemented  to get the phase profiles for 1-bit and 2-bit digitally coded metasurfaces having equal sizes of $10\lambda_0 \times 10\lambda_0$ (where $\lambda_0$ is the free space wavelength at $5.9$ GHz) for a specific reflection angle, $(\theta_r=30^0,\phi_r=0^0)$, and subsequently, the far-field plots are generated. Furthermore, these phase profiles are used to generate 3D-CAD models of the 1-bit and 2-bit metasurfaces using CST, and the desired far-field patterns are obtained having half-power beamwidth (HPBW) of $5^0$. It is observed that the 1-bit coded metasurface produces additional side-lobe levels, which are minimized by use of 2-bit coding. Finally, the proposed 1-bit and 2-bit coded metasurfaces are fabricated, and the anomalously reflected far-field beam is detected by the received power at desired reflection angle and comparing the results with a perfect electric conductor (PEC). The proposed design is suitable for V2X communications and can be scaled to other frequencies.

In this paper, we propose a parasitic patch-based method to improve the power transfer efficiency of radiative wireless power transfer (WPT) systems involving implanted biomedical devices. The proposed WPT system uses circularly polarised antennas, having a four-element sequentially rotated antenna array, as the Tx and a miniaturized circularly polarized antenna implanted inside a body mimicking gel as the Rx with a parasitic patch placed in close vicinity. The design guidelines for the dimensions of the parasitic patch and its distance from the implanted antenna are discussed in-depth based on rigorous parametric studies and analysis. The proposed parasitic patch-based method improves the implantable antenna gain by 7.83 dBic, thereby increasing the power transfer efficiency at 2.4 GHz operating frequency. Improvement in power transfer is experimentally demonstrated with the help of a low-cost experimental setup having a software-defined radio (SDR) and a spectrum analyzer. Further to analyze the effect of interfering signals on the performance of the WPT system, power received by the implantable antenna is measured in presence of a secondary transmitter.

In this paper, MATLAB based computational approaches for the design and analysis of time-varying capacitors using the finite-difference time-domain (FDTD) technique and the Simulink design environment are presented. The FDTD formulation for multiple lumped capacitors loaded in series on a transmission line is discussed and extended to include time variation of capacitance. The design methodology for the same is also discussed using MATLAB’s Simulink using the RF Blockset Library. The developed FDTD formulation and the Simulink method are then used to design a mixer with time-varying capacitors loaded transmission line.

In this paper, we develop a formalism based on electromagnetic Lagrangian which provides new insights about the near-field reactive energy density around generic antennas for arbitrary spatio-temporal excitation signals. Using electric and magnetic fields calculated via FDTD technique and interpolation routines, we compute the EM Lagrangian density around antennas (thin-wire dipoles and Yagi-Uda Arrays). Spatial maps of time-integrated EM Lagrangian density sheds light onto the capacitive/inductive nature of reactive energy density distribution around antennas and highlight its inherent connection with inter-element mutual coupling.