James G. Maloney

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.

Optimization of a conical antenna for pulse radiation: an efficient design using resistive loading

The conical monopole antenna with a section of continuous resistive loading is considered as a radiator for temporally short, broad-bandwidth pulses. The geometrical details of the coaxial feed and the resistive loading are varied to optimize this structure for pulse radiation. Compared with the perfectly conducting cone, the optimized resistive cone radiates a better reproduction of the pulse excitation with no loss in amplitude, and has internal reflections that are much smaller in amplitude. Graphical displays of the field surrounding the antenna are used to give insight into the physical processes for transient radiation from this antenna. Experimental models were constructed to verify the optimization and demonstrate the practicality of the design. Measurements of both the reflected voltage in the feed line and the time-varying radiated field are in excellent agreement with the theoretical calculations. >

TEM HORN ANTENNA FOR PULSE RADIATION : AN IMPROVED DESIGN

The finite-difference-time-domain (FDTD) method is used in an interactive mode to improve the performance of a TEM horn antenna for pulse radiation. The new design is shown to yield a small reflected voltage in the feeding transmission line, to radiate a far-zone field in the axial direction that is proportional to the derivative of the incident voltage, and to be fairly directive. Experimental results are in very good agreement with those predicted by the theory.

GPU based FDTD solver with CPML boundaries

The use of graphical processing units has been recently documented for the implementation of the FDTD technique; however, little has been reported about the necessary additions to three dimensional GPU based FDTD codes to make the technique more useful for EM analysis and antenna design. This paper will detail the practical addition of a convolutional perfectly matched layer absorbing boundary to a three dimensional GPU accelerated FDTD code and present results of simulations with dielectric and conducting objects in the computational domain.

Wide scan, integrated printed circuit board, fragmented aperture array antennas

Over the last decade, fragmented aperture antennas have been developed to support bandwidths up to 33∶1 and higher. Largely, these extreme bandwidth apertures have been developed to support receive applications. In this paper, we present a new design approach to increasing the scan volume to beyond 60 deg, a simplified printed circuit board fabrication approach, and a sample whole x-band (8–12 GHz) array antenna that demonstrates the new design and fabrication approach.

Photonic band structures of finite thickness: theory and experiment

There has been a great deal of interest in photonic band structures (PBS). In the literature, the PBSs considered are usually infinite periodic dielectric lattices. These lattice structures exhibit frequency bands over which propagation can not occur for any direction. The type of structures which have been theoretically analysed to date have been infinite, two- and three dimensional periodic dielectric structures. The analysis technique has been an expansion of the wave equation in plane waves and its solution obtained through eigenvalue techniques [Plihal and Maradudin, 1991]. However, there has been little attention given to the problem of PBSs with finite thickness. The present authors provide a new analysis based on the FDTD method and experimental measurements of a physically realizable, finite thickness PBS. These results reveal the existence of evanescent modes in the bandgaps as well as a passband ripple.<<ETX>>

On the characteristic impedance of TEM horn antennas

We have built several TEM horn antennas, measured their performance and compared this to calculations made using the finite-difference time-domain (FDTD) method. All of the horns were of the image equivalent design shown, were driven from a coaxial line (APC-7) with the characteristic impedance Z/sub 0/=50/spl Omega/, and had the angle /spl beta//2=160/spl deg/. During the course of these investigations, we noticed that Carrels (1958) formula and graph did not agree with our measurements or FDTD calculations; this observation prompted the study which we present.

FDTD in curvilinear coordinates using a rectangular FDTD formulation

In the simplest formulation, the FDTD algorithm requires that objects follow the rectangular grid. For curved surfaces, this is a severe limitation. In this paper, an approach to modify an existing rectangular FDTD code to model structures more naturally described in another coordinate system is demonstrated. The approach is a modification to the update coefficients and does not require significant changes to an existing piece of software.

Determination of surface currents by backpropagation of field measurements

Knowledge of surface currents is useful for evaluating antenna performance and tailoring the scattering characteristics of objects. For surfaces where these currents cannot be easily calculated, it is useful to have a technique for surface current measurement that does not require modification of the object. Previously, we described a measurement system for this purpose and simulated its performance using the finite-difference time-domain (FDTD) method. In this system, the magnetic field is measured on a planar surface a distance s in front of the object supporting the currents. These measurements are back propagated using a plane wave spectrum approach to obtain an estimate of the current. Parametric studies were performed to determine the effects of spacing s, sampling density, scan area, probe size, and signal to noise ratio (SNR) on the performance of the system. Only preliminary measurements were available at that time. Since then, more extensive measurements have been performed using an improved probe design, namely, a shielded loop with two feed points. This paper presents the results of the new measurements as well as details of a novel, local backpropagation approach.

RCS Determination from Localized Short-Pulse Scattering Measurements: Theory and Experiment

The Radar Cross Section (RCS) of a target has become an important metric for the characterization of electromagnetic performance. As a result, much effort has gone into developing techniques to accurately measure RCS. By its definition, RCS is a plane-wave concept, i.e., it is determined by the far-field scattering of an object when illuminated by a plane-wave. Most RCS measurement techniques involve illuminating the object under test with an approximation to a plane-wave (one exception is the near-field scanning technique). Specialized facilities, such as outdoor and compact ranges, are currently used to measure RCS; these facilities are generally very large in terms of the electromagnetic wavelength and are often located at remote sites. In this paper, we present a new RCS measurement technique that can be applied in much smaller spaces, and is potentially transportable.