Christos Mias

A Varactor-Tunable High Impedance Surface With a Resistive-Lumped-Element Biasing Grid

A high impedance surface consisting of metallic square patches electrically connected through vias to the ground plane beneath them is made tunable. Tunability is achieved by connecting adjacent patches with varactor diodes thus altering the capacitance between the patches and hence the surfaces resonance frequency. The varactor diodes are biased with the aid of a resistive grid. The grid is made resistive using surface mount resistors. Using an approximate equivalent circuit the effect of the varactor diode resistance is investigated for normal plane wave incidence. It is shown that at resonance, a small varactor resistance may lead to a significant absorption. The potential use of a waveguide simulator to characterize approximately the performance of the proposed metamaterial structure is investigated.

Varactor-tunable frequency selective surface with resistive-lumped-element biasing grids

A novel tunable frequency selective surface (FSS) is presented consisting of vertically and horizontally positioned convoluted dipole FSS elements. Each of the latter incorporates a varactor diode, which is biased via novel resistive-lumped-element biasing grids that suppress undesirable resonances and improve the filters low-pass response. The proposed design is shown, numerically and experimentally, to perform well. It is expected that this letter will assist research work in low-cost, large-scale tunable FSS.

Molecular Versus Electromagnetic Wave Propagation Loss in Macro-Scale Environments

Molecular communications (MC) has been studied as a bio-inspired information carrier for micro-scale and nano-scale environments. On the macro-scale, it can also be considered as an alternative to electromagnetic (EM) wave based systems, especially in environments where there is significant attenuation to EM wave power. This paper goes beyond the unbounded free space propagation to examine three macro-scale environments: the pipe, the knife edge, and the mesh channel. Approximate analytical expressions shown in this paper demonstrate that MC has an advantage over EM wave communications when: 1) the EM frequency is below the cut-off frequency for the pipe channel, 2) the EM wavelength is considerably larger than the mesh period, and 3) when the receiver is in the high diffraction loss region of an obstacle.

Implementation of an exact modal absorbing boundary termination condition for the application of the finite-element time-domain technique to discontinuity problems in closed homogeneous waveguides

This paper proposes an exact and mesh-efficient modal absorbing boundary termination condition (MABTC) to the Galerkin weighted residual finite element time domain (FETD) modeling technique. The developed boundary-condition formulation is specific to closed homogeneous waveguides. Two-dimensional parallel-plate waveguide problems are considered for which the MABTC, at the waveguide ports, is obtained by the convolution of the modal characteristic impedances with the modal profiles of the waveguide modes following an inverse Laplace transform. A time-truncated approximate version of the MABTC is also presented, which results in a more efficient memory and time FETD implementation (compared with the FETD code without the approximation) for the exact electromagnetic analysis of microwave circuits. Very good agreement between FETD-MABTC and finite-element frequency-domain results is demonstrated.

Hybrid finite-element analysis of electromagnetic plane wave scattering from axially periodic cylindrical structures

In many antenna systems the primary feed or the subreflector will most often be supported by struts which obstruct the aperture and therefore cause a reduction in the directivity and an increase in sidelobe levels. So as to be able to design new structures which avoid these drawbacks, the problem of plane wave scattering from an infinite axially periodic cylinder of arbitrary geometric and material parameters is analyzed by a hybrid finite element/boundary element method. Covariant-projection edge elements are employed in the inhomogeneous region of the unit cell and the scattered field is expanded in terms of cylindrical Floquet harmonics. The resulting practical numerical procedure has been tested to ensure that power conservation rules are obeyed and checked satisfactorily against both analytical results and measurements on periodically loaded struts.

Electronic problem based learning of electromagnetics through software development

A computer‐assisted problem‐based learning (PBL) project is proposed to assist undergraduate student learning of electromagnetics through software development. The project employs the finite difference time domain (FDTD) method to model transmission line performance. The usefulness of this approach is evaluated from direct observation and formative assessment.

Fast Computation of the Nonlocal Boundary Condition in Finite Difference Parabolic Equation Radiowave Propagation Simulations

Finite difference parabolic equation method (FD-PEM) codes using a nonlocal boundary condition to model radiowave propagation over electrically large domains, require the computation of time consuming spatial convolution integrals. For the first time, we propose the use of recursive convolution (RC) with vector fitting (VF) to reduce this computational burden. RC is based on the ability to express functions as a sum of exponential terms. This is achieved using the VF method. Details of the RC formulation applied in a two-dimensional (2D) Wide-angle FD-PEM (WA-FD-PEM) are presented together with 2D simulations which demonstrate the computational speed and accuracy of the synthesized RC-WA-FD-PEM code.

Fast Finite Element Time Domain – Floquet Modal Absorbing Boundary Condition Modelling of Periodic Structures Using Recursive Convolution

Finite element time domain (FETD) codes using a Floquet modal absorbing boundary condition to model scattering from periodic structures require the computation of time consuming convolution integrals. In this paper, we propose, for the first time, to reduce this computational burden using recursive convolution. Recursive convolution is based on the ability to accurately approximate functions, over the entire computation time, using a summation of exponential functions. A novel approach, based on the exponential curve fitting facility of the commercially available software MATLAB, is employed. The time efficiency of the developed FETD code based on recursive convolution is demonstrated by comparing its computational speed with that of an FETD code employing standard convolution when modelling plane wave scattering from two-dimensional singly-periodic structures.

Generalized Wait-Hill formulation analysis of lumped element periodically-loaded orthogonal wire grid generic frequency selective surfaces

By combining the work of J. R. Wait on a periodically loaded vertical wire grid and the work of D. A. Hill and J. R. Wait on a wire mesh, a novel generalized formulation, the Wait-Hill formulation, is obtained for the analysis of lumped-element periodically-loaded orthogonal wire grid generic frequency selective surfaces. The Wait- Hill formulation is simple and not restricted by the miniaturization assumption of current approximate simple methods for the analysis of loaded and unloaded wire grids. The results of the Wait-Hill formulation are shown to agree well with those of a commercial software.

Finite-Element Time-Domain Modelling of Broadside Radiation from a 2D Parallel-Plate Waveguide Antenna Array

In this paper, a wideband infinite parallel-plate waveguide antenna array radiating along the broadside direction is modelled using the finite-element time-domain (FETD) method. Based on Floquets theorem, only a single unit cell is analysed. At, both, the radiating finite element port and the waveguide finite element port of the unit cell exact boundary conditions are employed. The FETD formulation results are compared with those of a finite-element frequency-domain (FEFD)