Paul Friederich

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.

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.

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.

Comparison of Techniques for Microwave Characterization of Percolating Dielectric -Metallic Media and Resolution of Discrepancies in Measured Data

The measured microwave effective dielectric properties of metal-dielectric composites show discrepancies when data from free space, resonant cavity or transmission line measurements are compared. Discrepancies are especially evident for materials where the metallic concentration is near the percolation threshold. This paper presents theory and measured data which highlight and resolve these discrepancies. Electrical correlation length is the relevant parameter which must be considered in choice of measurement technique. Agreement between effective medium models and measurement are best when focussed beam measurement techniques are utilized so as to produce planar wave-fronts whose extent is 3–4 freespace wavelengths when incident on the sample.

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.

Moving target emulators for ultra wide band signals: Electrically modulated fragmented surfaces

MTI radar systems and signal processing algorithms may be made more efficient if a wide range of perfect and background distorted Doppler signatures could be made available to test system hardware and algorithms. This paper addresses emulators of moving vehicle Doppler signatures. Measured Doppler spectra of tracked and wheeled vehicles are used to guide the development of moving target emulators made from ldquoswitchablerdquo reflectivity panels\. The geometry of the panels are electrically percolating surfaces The percolating patterns provide 10:1 bandwidths and thus satisfy UWB criteria for frequency response and they use minimal modulating elements. A potential problem is the number of modulating elements on a panel that are required to synthesize the complex Doppler waveform measured on a ground vehicle.