Morris P. Kesler

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.

Switched fragmented aperture antennas

This paper introduces a reconfigurable aperture concept derived from fragmented aperture design where the configuration of the fragmented aperture may be switched by the user to obtain different functionalities. A fragmented aperture antenna is a patchwork of discrete conducting and dielectric units distributed over the specified aperture. The arrangement of the units is determined using an efficient, multistage procedure that incorporates the genetic algorithm for optimization and the finite-difference time-domain method for the electromagnetic computation. Typically, the criterion for optimum performance has been broadband gain at a particular angle. The resulting antennas contain isolated scattering structures that are fragmented in appearance.

A system for unobtrusive measurement of surface currents

The authors demonstrate an approach that is an effective, noninvasive means of measuring surface current distributions. Some guidelines for making the measurements are given, and results from an actual measurement are presented. For example, it is shown that the magnetic field components have better measurement characteristics than the electric field components. The number of spatial samples per wavelength can be estimated from the spatial sharpness of the current distribution. The size of the measurement window, which is directly proportional to the measurement distance, must be sufficiently large to capture the majority of the spatial field distribution and to prevent the edges of the window from interfering with the results. In addition, the extraction of sub-wavelength features or sharp edges requires the measurement plane to be close to the surface current, a high SNR, close spacing of measurement samples, and a small field probe.

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.

Localized Short-Pulse Scattering from Coated Cylindrical Objects: Experimental Measurements and Numerical Models

This paper presents some results of our investigations into transient scattering from finite cylindrical targets. One unique feature of this work is that the vector field measurements were performed with non-planar excitation in the near-field of the target (localized measurements). Two electromagnetic models were developed to predict the transient near-field scattering and were validated by extensive comparison to measurements. One was a finite-difference time-domain (FDTD) model, while the other was based on the method of moments technique. The complexity of the field behavior in the near-field region provided a challenge for the model validation efforts. In this paper we discuss the measurement techniques, the electromagnetic models, and present the results of representative scattering measurements to highlight some of the interesting behavior which was observed.

Incorporating lossy materials and PML into the split-field update FDTD method

Periodic structures, such as frequency selective surfaces, photonic bandgap structures, and antenna arrays, are being more widely used in electromagnetic systems. To make the numerical modeling of a periodic structure more practical, usually a single unit cell of the structure is modeled and periodic boundary conditions are used to incorporate the periodic nature. Traditionally, frequency-domain techniques are used to numerically model such structures because of the simple way in which the boundary condition can be applied. However, when broadband data is needed, a time-domain approach is desirable. Unfortunately, periodic boundary conditions in the time-domain introduce the need for time-advanced data, obviously a problem for a time-domain approach. To get around this, a field transformation can be applied which simplifies the boundary conditions, but results in a more complex set of equations to be solved. The FDTD algorithm can be applied to the transformed field equations; however, additional variables must be introduced because of stability considerations. One technique that has been developed for discretizing and solving the transformed field equations is the split-field method. This paper extends the method to include general anisotropic dielectric and magnetic media.