Eric J. Kuster

GTRI reconfigurable aperture design

Researchers at the Georgia Tech Research Institute (GTRI) have been investigating a reconfigurable aperture concept by which the current distribution on an antenna can be controlled for the purpose of changing the apertures function during operation. The geometry of the particular aperture being developed is shown. It consists of electrically small metallic pads connected by switches (FETs or MEMS) with a single feed point on the aperture. To illustrate the use of the switches to configure the aperture, certain switches are closed to form a bowtie pattern. GTRI anticipates that such an aperture will enable the following adaptations: (1) switching between broadband, low-gain operation and narrow-band, high-gain operation; (2) steering of the pattern from broadside; and (3) switching between omni-directional and directive horizontal patterns. This paper discusses the technical challenges in making this aperture a reality, how these challenges might be overcome, and the performance of the prototypes.

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.

The GTRI prototype reconfigurable aperture antenna

In this paper, we describe a prototype antenna resulting from a three-year effort to design, build, and test a reconfigurable aperture (RECAP). The antenna consists of a planar array of electrically small, metallic patches that are interconnected by switches. The antenna can be reconfigured to meet different performance goals, e.g., bandwidth requirements and steering, 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, with a full FDTD simulation. In previous papers, we presented the basic concept for the GTRI RECAP antenna along with several possible methods for controlling the switches. Here, we give the details for a prototype antenna that uses FET-based electronic switches with optical control.

Near-field probe measurements of microwave scattering from discontinuities in planar surfaces

In microwave scattering, nonradiating fields may contribute to radiating fields by local perturbations such as geometric discontinuities or variations in impedance or electromagnetic properties. Near-field measurements of scattering bodies provide insight into these scattering mechanisms by measuring both radiated and nonradiated fields. In this research, an H-field probe measured scattering from simple discontinuities in planar bodies at frequencies between 2 and 10 GHz. Illumination of the test-body was furnished by a focused lens system with a Gaussian-like tapered beam that locally illuminated inhomogeneities on the body. Measured data and model calculations are presented for scattered H-fields near canonical discontinuities (e.g. gaps and edges in conducting planes). Calculations of the plane wave spectrum of the measured and modeled data were used to distinguish specular reflected components from surface modes. A focused beam was simulated in a finite-difference time-domain (FDTD) model with a weighted sum of plane waves. FDTD results agreed with the measured near-field data.

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.

Design, fabrication and testing of well-matched microwave array antennas with greater than 2.5:1 bandwidth

A connected array antenna architecture composed of multiple fragmented metallic surfaces over a ground plane has been used for a number of ultra-wideband applications. In this paper we apply design procedures developed for ultra-wideband operation but emphasize extremely low impedance mismatch to enable operation at high power levels in the Ultra High Frequency (UHF) range. The design evaluation includes simulations to assess robustness to manufacturing tolerances. An S-band prototype test article was fabricated with dummy elements terminated in matched loads made of etched resistive material. Measurements of the prototype confirmed suspicions that impedance variability in the resistive layer limits the ability of the test coupons to validate the designs. A comprehensive test plan to mitigate risk is developed that will culminate in demonstration of a fully populated array. The paper concludes with a summary of plans for future tests.

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.

Free-space evaluation of well-matched periodic structures

In early design stages of connected array radiators, passive evaluation using reflected illumination of test coupons without feed electronics provides a quick and inexpensive validation of design models. Inherent in the measurement setup are assumptions about array antenna behavior and acceptable approximations. Particular questions include the minimum number of unit cells necessary to adequately reproduce array characteristics and appropriate illumination functions. In this paper, those assumptions are examined and the implications for performing such measurements are discussed.

FDTD simulations of electromagnetic coupling to internal resonance modes of metallic and resistive casings of electronic equipment

This paper examines coupling to electromagnetic modes found within cylindrical (NATO cylinder) and rectangular volumes with metallic and/or resistive surfaces using finite difference time domain (FDTD) simulations. Stripline geometries are positioned within the volume to simulate internal circuitry. External fields from sources in the near and far fields are coupled to internal modes via apertures of various dimensions in the external surfaces. The results have implications for protection of circuitry from high-power, ultra wideband excitation. Several parameters are considered. These include: volume and dimensions of cylindrical and rectangular casings; casing conductivity; aperture placement and size; electromagnetic frequency; and positioning of the simulated circuit and loads placed on that circuit. Calculations to date have demonstrated that intense internal fields can be excited via the casing apertures. Maximum coupling appears near the frequencies of characteristic modes of the casing geometries. Peak coupled field magnitude at the location of the simulated circuit can be 30 dB above the directly radiated value. External excitation from a high power RF source could excite amplified local internal fields that result in nonlinear responses on internal circuitry. Thus, shielding protection offered by the casings may be limited. Frequencies of greatest amplification are shown to be functions of cavity and aperture geometry, and casing conductivity. Impact of intrinsic loss of casings is shown to be minimal, but additional treatment with RF absorbers can be an effective mitigation strategy.