Glenn S. Smith

A fully three-dimensional simulation of a ground-penetrating radar: FDTD theory compared with experiment

A fully three-dimensional, finite-difference time-domain (FDTD) model of a ground-penetrating radar is described. The FDTD simulation completely models the transmitting and receiving antennas, the antenna feeds, the dispersive Earth, and the buried object. Results of scattering from three different buried cylindrical pipes are compared to previously measured results for a one-third size scale model of an actual radar and are shown to be in good agreement.

A reconfigurable aperture antenna based on switched links between electrically small metallic patches

A reconfigurable aperture (RECAP) antenna is described in which a planar array of electrically small, metallic patches are interconnected by switches. The antenna can be reconfigured to meet different performance goals by changing the switches that are open and closed. The switch configuration for a particular goal is determined using an optimizer, such as the genetic algorithm. First, the basic concept for the RECAP antenna is verified by comparing theoretical results with measurements for configurations in which the switches are simply wires connecting the patches. Next, details are given for a prototype antenna in which field-effect transistor based electronic switches are used with optical control. Theoretical results for the prototype antenna are then compared with measurements for cases in which electronic reconfiguration is used to change the bandwidth of operation or steer the pattern of the antenna. Finally, an overview of alternate switch/control strategies, some of which were tested, is given along with suggestions for improving the next generation of this antenna.

Optimization of bow-tie antennas for pulse radiation

Resistively loaded bow-tie antennas are considered as radiators for temporally short, broad-bandwidth pulses. Analysis is by the finite-difference time-domain (FDTD) method. The geometrical details of the antenna and the resistive loading along the antenna are chosen to optimize this antenna for pulse radiation. Theoretical results for the reflected voltage in the coaxial feed line and the time-varying radiated electric field are compared throughout with experimental measurements. The optimized, resistive bow-tie antenna is shown to radiate a pulse that more closely resembles that of the excitation than is radiated by a metallic bow tie of comparable size. Issues involving the use of the FDTD method for modeling fully three-dimensional antennas are also discussed. These issues include the use of a simple feed model and the staircasing of the edges of the antenna. >

The use of surface impedance concepts in the finite-difference time-domain method

Surface impedance concepts are introduced into the finite-difference time-domain (FDTD) method. Lossy conductors are replaced by surface impedance boundary conditions (SIBC), reducing the solution space and producing significant computational savings. Specifically, a SIBC is developed to replace a lossy dielectric half-space. An efficient implementation of this FDTD-SIBC based on the recursive properties of convolution with exponentials is presented. Finally, three problems are studied to illustrate the accuracy of the FDTD-SIBC formulation: a plane wave incident on a lossy dielectric half-space, a line current over a lossy dielectric half-space, and wave propagation in a parallel-plate waveguide with lossy walls. >

The efficient modeling of thin material sheets in the finite-difference time-domain (FDTD) method

A subcell model is presented for including thin material sheets in the finite-difference time-domain method. The subcell model removes the normal restriction which sets the spatial grid increment at least as small as the smallest physical feature in the solution space. Removing this restriction leads to greatly reduced storage requirements and a corresponding reduction in the number of time steps needed. The subcell model is verified by comparison with the exact results for the loss and phase shift for a parallel plate waveguide loaded with a thin material sheet. Specifically, thin conducting as well as thin dielectric sheets are investigated for both TEM and TM/sub 1/ time-harmonic excitations of the waveguide. The FDTD results are in very good agreement with the exact results. Finally, the subcell model is used in the analysis of a practical problem-a resistively loaded monopole antenna formed from a thin-walled conducting tube. The FDTD results are compared with accurate measurements for this antenna, and, again, the two are in very good agreement. >

A simple FDTD model for transient excitation of antennas by transmission lines

A simple FDTD model is developed for use with antennas that are fed from transmission lines. The model is especially designed for use with transient excitations, where the incident and reflected waveforms within the transmission line are of interest, and the latter is determined directly in the FDTD calculation. The model is verified for both transmission and reception of transient waveforms by comparison with measured results for a cylindrical monopole antenna with a plane reflector. >

Land mine detection using a ground-penetrating radar based on resistively loaded Vee dipoles

Resistively loaded Vee dipoles are considered for use in a short-pulse ground penetrating radar (GPR) used to detect buried antipersonnel land mines. First, a study is made to select a short pulse to radiate that is most appropriate for the problem. A simple one-dimensional (1-D) analysis of some representative soils and a land mine is used to select a radiated pulse similar in shape to a differentiated Gaussian pulse with a spectral peak at 4 GHz. Based on previous studies, the conductivity of the arms of the Vee dipole is linearly tapered from the feed to the open ends. A fully three-dimensional (3-D) finite-difference time domain (FDTD) model is developed and used to simulate the GPR land mine detection problem. Using this model, a resistively loaded Vee dipole is selected and evaluated. Parametric studies related to the problem are conducted including: varying the height of the Vee above the ground, varying the position of the land mine both laterally and in depth, and examining the effects of the geometry of the land mine on the received signal. Environmental conditions are examined including signal returns from rocks and variations in the shape of the surface of the ground. The FDTD results are validated by comparisons with experimental data. These studies demonstrate that resistively loaded Vee dipoles can greatly reduce clutter related to the antenna, making the task of distinguishing land mines (targets) much easier.

A direct derivation of a single-antenna reciprocity relation for the time domain

In this paper, a single-antenna reciprocity relation is derived for the time domain. First, the antenna is considered on transmission; next, the same antenna is considered when it is receiving an incident plane wave. The two states, transmission and reception, are related by the application of a modified form of the reciprocity theorem for electromagnetic fields with general time dependence due to Cheo. The derivation of the reciprocity relation for the antenna makes use of simple geometric arguments to evaluate the spatial/temporal integrals that occur in the theorem. A few extensions of the reciprocity relation are also described.

Antenna design with the use of photonic band‐gap materials as all‐dielectric planar reflectors

A finite thickness slab of two-dimensional photonic band-gap (PBG) material is analyzed to determine the plane-wave reflection and transmission coefficients as functions of the angle of incidence. It is shown that within the band-gap, where there is total reflection, the reflection is equivalent to that from a plane surface located within the PBG material (the reflection plane concept). The two-dimensional PBG material is used as an all-dielectric reflector for an electric dipole antenna. Field patterns computed with the use of the reflection plane concept are compared with measurements made on an experimental model and are found to be in good agreement. A three-dimensional PBG material with a BCC lattice is also used as a reflector for an electric dipole antenna. Field patterns calculated for this structure using the finite-difference-time-domain (FDTD) method are also in good agreement with measurements.

A study of pulse radiation from several broad-band loaded monopoles

Five previously proposed designs for broad-band monopole antennas are evaluated for pulse radiation. These designs use continuous resistive loading and/or discrete capacitive loading to increase the bandwidth over that of a simple, metallic monopole. The parameters for each of the designs are scaled so that the designs can be compared on a common basis (frequency range). Each of the antennas is analyzed numerically, and quantities characteristic of their pulsed performance are computed. These quantities include the reflected voltage in the feeding transmission line, radiated electric field, radiating efficiency, time-domain gain, fidelity, and symmetry when the monopoles are excited by a differentiated Gaussian pulse. In addition, the input reflection coefficient and gain at broadside for monochromatic excitation are shown. Explanations are provided for the differences in performance for these designs.