Martin Norgren

A deep parametric study of resistor-loaded bow-tie antennas for ground-penetrating radar applications using FDTD

Resistor-loaded bow-tie antennas are analyzed thoroughly to find out their performance on ground-penetrating radar (GPR) applications. The analysis is done with the finite-difference time-domain (FDTD) technique. The antenna is pulse driven and enclosed in a rectangular conducting cavity. The ability to detect a buried conducting sheet using two such identical antennas for transmitting and receiving is investigated. Simulations are carried out for various antenna parameters like end resistor values, flare angle, and antenna length. The gap between the two antennas and their height above the ground are also varied. Moreover, the results are obtained for different sizes, depths, and positions of the buried sheet. It is studied how the broadband impedance characteristics and better target discrimination with low clutter can be achieved by optimally selecting these antenna parameters. Also, it is shown that apart from the total parallel end resistance, the individual end resistor values and the number of resistors connected have no significant effect on the input impedance and the received signal.

A COMPLETE FDTD SIMULATION OF A REAL GPR ANTENNA SYSTEM OPERATING ABOVE LOSSY AND DISPERSIVE GROUNDS

Ground penetrating radar (GPR) systems are increasingly being used for the detection and location of buried objects within the upper regions of the earth’s surface. The antenna is the most critical component of such a system. This thesis presents a comprehensive study of resistor-loaded planar dipole antennas for GPR applications using both theory and experiments. The theoretical analysis is performed using the finite difference time domain (FDTD) technique.The analysis starts with the most popular planar dipole, the bow-tie. A parametric study is done to find out how the flare angle, length, and lumped resistors of the antenna should be selected to achieve broadband properties and good target detection with less clutter. The screening of the antenna and the position of transmitting and receiving antennas with respect to each other and ground surface are also studied. A number of other planar geometrical shapes are considered and compared with the bow-tie in order to find what geometrical shape gives the best performance. The FDTD simulations are carried out for both lossless and lossy, dispersive grounds. Also simulations are carried out including surface roughness and natural clutter like rocks and twigs to make the modeling more realistic.Finally, a pair of resistor-loaded bow-tie antennas is constructed and both indoor and outdoor measurements are carried out to validate the simulation results.

An optimization approach to the frequency-domain inverse problem for a nonuniform LCRG transmission line

The inverse problem for a nonuniform LCRG transmission line is considered in the frequency domain. Imbedding equations for the reflection and transmission coefficients are derived through the concept of wave-splitting. An optimization approach is applied to reconstruct the line parameters as functions of the position using band-limited reflection and/or transmission data. Exact and explicit expressions for the gradients are derived, and the reconstruction algorithm (based upon a conjugate gradient method) is tested with both clean and noisy data, The problem of the nonuniqueness is also discussed.

Reconstruction of the constitutive parameters for an /spl Omega/ material in a rectangular waveguide

The inverse problem of determining the constitutive parameters of an /spl Omega/ material is considered. The dispersive bianisotropic /spl Omega/ sample is placed in a rectangular waveguide. All the constitutive parameters except one are reconstructed using the reflection and transmission data for some TE/sub m0/ and TE/sub 0n/ modes. The remaining one can be obtained, e.g., from reflection of normally incident plane waves. Numerical results for the reconstruction are presented. >

On the Possibility of Reflectionless Coating of a Homogeneous Bianisotropic Layer on a Perfect Conductor

ABSTRACT The possibility of eliminating the electromagnetic reflection of a plane wave from a perfect conductor by coating a homogeneous layer of a bianisotropic medium is studied. It is shown that the uniaxial chiral-omega medium is a natural choice for such a reflectionless coating at a fixed frequency in the normal incident case. The condition for zero reflection is derived. Numerical results show that a uniaxial omega coating can give more effective anti-reflection results than an isotropic conducting anti-reflection coating and an isotropic dielectric (or magnetic) anti-reflection coating.

Scattering from a thin magnetic layer with a periodic lateral magnetization: application to electromagnetic absorbers

A magnetized thin layer mounted on a PEC surface is considered as an alternative for an absorbing layer. The magnetic material is modeled with the Landau-Lifshitz-Gilbert equation, with a lateral static magnetization having a periodic variation along one lateral direction. The scattering problem is solved by means of an expansion into Floquet-modes, a propagator formalism and wave- splitting. Numerical results are presented, and for parameter values close to the typical values for ferro- or ferrimagnetic media, reflection coefficients below −20 dB can be achieved for the fundamental mode over the frequency range 1-4 GHz, for both polarizations. It is found that the periodicity of the medium makes the reflection properties for the fundamental mode almost independent of the azimuthal direction of incidence, for both normally and obliquely incident waves.

Electromagnetic reflection and tramsission for a dielectric-Ω interface and an Ω slab

A time-harmonic electromagnetic plane wave obliquely incident on a half-space or a slab consisting of a so-called Ω medium is considered. The up- and down-going eigenmodes in the Ω medium are derived and used to calculate the reflection and transmission coefficients for TE and TM modes. The Brewster angles for an Ω half-space are computed. Numerical results for the co- and cross-polarized reflection and transmission coefficients for an Ω slab are presented.

Lossy gradient index metamaterial with sinusoidal periodicity of refractive index : case of constant impedance throughout the structure

We used an exact analytical approach to investigate the electromagnetic wave propagation across an isotropic metamaterial composite with i. a sinusoidally periodic gradient of the real parts of the effective permittivity and permeability, ii. spatially uniform imaginary parts of the effective permittivity and permeability, and iii. spatially uniform impedance. The real part of the effective refractive index can be positive and negative along the direction of nonhomogeneity. A remarkably simple direct solution for the field distribution was obtained.

Comparison between frequency domain and time domain methods for parameter reconstruction on nonuniform dispersive transmission lines - Abstract

In this paper, we present two methods for the inverse problem of reconstructing a parameter profile of a nonuniform and dispersive transmission line - one frequency domain and one time domain method. Both methods are based on the wave splitting technique, but apart from that the methods are mathematically very different. The time domain analysis leads to hyperbolic partial differential equations and an inverse method based on solving implicit equations. The frequency domain analysis leads instead to Riccati differential equations and an inverse method based on optimization. The two methods are compared numerically by simulating a reconstruction of a soil moisture profile along a flat band cable. A heuristic model of the dispersion characteristics of a flat band cable in moist sand is derived. We also simulate the effect parasitic capacitances at the cable ends has on the reconstructions. The comparison shows that neither method outperforms the other. The time domain method is numerically much faster whereas the frequency domain method is much faster to implement. An important conclusion is also that it is crucial to model the connector parasitic capacitances correctly - especially if there are impedance mismatches at the connectors.

An Investigation of Some Geometrical Shapes and Selection of Shielding and Lumped Resistors of Planar Dipole Antennas for GPR Applications Using FDTD

A ground-penetrating radar (GPR) antenna system is modeled using the three-dimensional finite-difference time-domain technique. This paper investigates some basic geometrical shapes for planar dipoles to find what shape gives the best performance for GPR applications. The antenna is resistor loaded and shielded by a rectangular conducting cavity to suit the application. The effect of adding a wave-absorbing coat to the shield is also studied. Furthermore, a genetic algorithm is used to optimize the cavity height and the resistor values