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Conventional planar microwave absorbing materials may be divided into two main types: those that employ one or more thin resistive sheets separated by dielectric spacers, such as the Salisbury screen, and those comprised of one or more lossy layers such as the Dallenbach absorber. Both types operate by absorbing incident electromagnetic energy and converting it into heat. However, an alternative approach based on the concept of phase modulation has recently been proposed [1-3], in which electromagnetic energy scattered from an object is phase modulated to produce a reflected field with a low time-averaged energy spectral density. This new type of ‘absorber’, called the phase-switched screen (PSS), consists of one or more active layers whose impedance properties are controlled electronically. Previously published work in the area has concentrated on the scattering properties of PSS at normal incidence, and has shown that single layer screens exhibit similar characteristics to those of a Salisbury screen. More interestingly however, multi-layer PSS can be configured to provide an active scatterer with dynamic reflectivity null tuning properties [4]. In this contribution we extend the analysis to consider the characteristics of PSS at oblique incidence and present results to compare the performance of active PSS to those of conventional passive designs.
This paper will discuss the design and performance of a small step-frequency homodyne monopulse radar. The radar is designed to sit on the ground and penetrate weedy foliage to observe moving vehicles. It operates with horizontal polarization near 3.3 GHz with approximately 500 MHz bandwidth. Only 8 dBm power is needed. We will show the results of tests done with a corner reflector and with a walking human. Tracking performance in both range and azimuth will be shown.
This paper describes Dr. Nikola Tesla’s contributions to electrical engineering, focusing mainly on his work in Colorado from 1890 to 1900. Tesla’s patented polyphase AC system of alternators, motors, and electric power transmission, led the way for the first commercial power plant (in the world), to be installed near Telluride, Colorado, in 1890–91. A collaborative effort between L. L. Nunn, George Westinghouse, and Nikola Tesla, the successful installation of the Ames power station, five miles south of Telluride, marked the beginning of the modern age of electricity. In 1899 Tesla erected an experimental station at Colorado Springs, to advance his pioneering work in very high voltage/high-frequency techniques and machinery. There he discovered the phenomena of standing waves, ground-wave propagation, generated artificial ball lightning, and powerful high frequency electrical discharges over 100 feet long. His chief aim was to investigate the feasibility of sending electrical power without wires for future commercial applications.
The work described in this paper is devoted to measurement and analysis techniques for performing electromagnetic (EM) test environment assessments and error compensations for antenna performance testing and RCS testing at indoor and outdoor test sites. This paper is focused primarily on test articles and test facilities that are physically and/or electrically large and difficult to handle by conventional measurement and analysis techniques. The approaches discussed herein are based on the combined use of 1) arrays of EM field probes to rapidly measure the test zone fields, and 2) specialized EM spectral analysis techniques including the MUSIC high resolution imaging technique and the Spherical Angular Function (SAF) integral formulation of EM coupling and scattering.
The OSU/ESL has been developing a broadband, dual-polarized dielectric horn antenna (DHA). This antenna has some attractive characteristics such as dual-polarization, good antenna isolation and stable beamwidth. By adjusting the geometry, the beamwidth of the E- and H-plane patterns can also be controlled independently. Critical design issues that affect the DHA performance include launch structure, lateral-wave elimination and dielectric constant will be addressed. A design example will be provided with a prototype DHA antenna constructed and tested for 2~14 GHz frequency range with a 110o beamwidth in both Eand H-planes. The antenna isolation was found to be greater than 30 dB for this prototype. The new DHA antenna could have wide applications in which broadband, dual-polarization operation, independent E- and H-plane beamwidths are desired.
An inflatable communication antenna is a subject of current space research because of its potential for enabling high-bit-rates. However, a significant problem associated with inflatable technology is the “all-or-nothing” scenario, where success of the mission depends on successful deployment of the antenna. For this reason, few satellite programs are willing to take the risk of using an inflatable unless it is mission enabling. The Hybrid Inflatable Antenna, a concept developed by the Johns Hopkins Applied Physics Laboratory and ILC Dover, addresses the risk by providing a backup capability within the inflatable dish. This system combines a fixed parabolic dish with an inflatable reflector annulus that greatly increases antenna area. For example a 1-meter diameter dish can be increased to 4-meter resulting in a 16X improvement in reflector surface. A prototype Hybrid Inflatable Antenna has been successfully fabricated and tested. This scale model demonstrates that a highly accurate reflector surface can be formed via inflation.
Linear dipoles are universally employed as low-gain metrology antennas. At shorter wavelengths it becomes difficult to implement linear antennas for which the feed regions comprise an insignificant fraction of the entire structure. Thus, at shorter wavelengths, radiation from the feed region itself and base loading are important issues. Moreover, if an open balun is employed, radiation from the balun can occur. The combination of a linear dipole with a detachable, shielded balun having a coaxial input and two coaxial output ports as called for in CISPR 16-1 has become the preferred approach for site attenuation and path loss measurements below 1000 MHz. Such a design can effectively eliminate the possibility of radiation from the balun. However, the Roberts dipole with its integral Marchand balun can provide superior balance to that obtained using most commercial off-the-shelf 180-degree hybrid networks. Furthermore, at shorter wavelengths, it is difficult to implement a detachable, shielded balun small enough to ensure scattering from the balun housing is negligible. Thus, for metrology dipoles at higher frequencies, the Roberts dipole topology is most appropriate. Because the balun in this antenna is open, it can contribute to the radiation from the antenna. Radiation from the balun distorts the radiation pattern and can displace the phase center of the antenna. However, radiation from the balun can be minimized through careful design. The guidelines for minimizing radiation from the balun differ from those previously published concerning maximizing bandwidth. Here we present gain and radiation pattern measurements for two sets (representing two different design approaches) of linear dipoles employing Marchand baluns. While the experimental effort in this paper focuses on a 900 MHz implementation of the dipoles, the design concept has been shown to work well over a frequency range of 400 MHz to 2.5 GHz. We show how radiation from the balun and feed region can be minimized to provide a dipole with performance very close to that of an idealized linear dipole.
The use of multimode antennas to aid problems of direction finding (DF) has been examined and shown to provide benefit over standard interferometric techniques [3, 5]. In this work, we consider the issue of managing the configuration of multimode antennas on a standard platform to optimize the robustness of the system DF capability. The Fisher Information Matrix (FIM) and corresponding Cramér Rao Lower Bound (CRLB) for a given antenna steering vector size leads to a normsquared maximization problem for steering vector optimization. This, in turn, drives the array configuration for a given element pattern vector. The optimization is developed based on desired performance, using a cost function over elevation. An optimized design is found using both theoretic element pattern calculations and measurements collected at the Radiation and Scattering Compact Antenna Laboratory (RASCAL).
We introduce a very efficient method for extracting from RCS measurements the cutoff frequency of modes propagating in a waveguide of arbitrary cross section. Based on a model of propagating mode, it offers the capability of identification of mode and it gives also an information about the frequency evolution of the mode excitation amplitude. The effectiveness of this method is illustrated by the analysis of measurements of different shapes of waveguide. The results obtained show that this representation widely improved the performance of time-frequency distributions usually used to analyze this kind of dispersive structure.
Simulated data is presented for a planar array to demonstrate the limitations of planar near-field back projections. It is well known that the result obtained in this way is of limited resolution and accuracy and these limitations are further illustrated through the data presented here. The impact of probe to AUT separation distance is shown as well as the correspondence between array excitation perturbations and that detected through the back projection technique. Results are shown for a simple iterative array excitation adjustment process. The purpose of this paper is to provide guidelines for the application of the planar near-field back projection technique.
This paper presents an accurate method for diagnosis of element failures in large phased arrays. The method is based on the reconstruction of the excitation from measured near-field data by solving the linear system relating the excitation coefficients to the field at measurement points. The dimension of the linear system is reduced by adopting sampling strategies with minimal redundance. The strongly ill-conditioned system is solved using an iterative generalized Landweber algorithm. Numerical simulations on a 2225 elements planar array confirm the effectiveness of the approach.
Analysis of in-orbit phased array antenna patterns measured from earth station requires a considerable examination of the in-orbit antenna operation. The antenna analysis should take into account the constant change of both observation angles and scan angles. The in-orbit phased array antenna pattern characteristics are mathematically analyzed. The coordinate transformation technique to calculate the time-varying trajectory of the observation angle in the antenna coordinate system is presented. The technique also encompasses the satellite track angle calculation as seen from the ground antenna. Data processing procedure of the dynamic antenna patterns and several test issues are discussed.
The electrical phase center locations of the elements in a two-dimensional phased array are examined. A technique is described for precisely locating the element phase centers when the elements are offset from the center of rotation. The element phase centers are shown to be significantly displaced from the physical element locations. The displacement causes a systematic beam pointing bias, which can be predicted and measured.
In this paper, the authors propose an improved Rotatingelement Electric-field Vector (REV) method taking into account amplitude and phase error of phase shifters in order to achieve more precise calibration. The conventional REV method has been used in order to determine and/or adjust amplitude and phase of electrical field radiated from each antenna element -element fieldin phased array antennas. However, amplitude and phase deviations due to phase shifter errors, and so on, reduce the measurement accuracy because the conventional REV method assumes no deviation. On the other hand, the proposed REV method can evaluate element fields without error and error electrical fields -error fields- due to phase shifter errors in each bit, by measuring both amplitude and phase value of array composite electrical field. In a simulation for a 31- element array with 5-bit phase shifter, the evaluated element fields and error fields agree well with the expected values. This result shows that the proposed method allows the phased arrays to be calibrated more accurately as considering phase shifter errors.
A thermal technique for the remote calibration of phased array radar antennas is proposed in this paper. The technique is based on infrared (IR) measurements of the heat patterns produced in a thin planar detector screen placed near the antenna. The magnitude of the field can be measured by capturing an isothermal image (IR thermogram) of the field with an IR imagining camera. The phase of the field can be measured by creating a thermal interference pattern (IR/microwave hologram) between the phased array antenna and a known reference source. This thermal imaging technique has the advantages of speed and portability over existing hard-wired probe methods and can be used in-the-field to remotely measure the magnitude and the phase of the field radiated by the antenna. This information can be used to calibrate the individual elements controlling the radiation pattern of the array.
Although the original reason for creating the Boulder Laboratories of NBS (later known as NIST) was to accommodate the radio program, as described by Dennis Friday and Allen Newell at this conference, the first major program in Boulder was in Cryogenics, which was created in response to a perceived emergency in national security and went on to provide basic cryogenic data to serve the national space program. It was also the origin of programs continuing to the present in the reliability of materials, the thermophysical properties of fluids, and cryoelectronics. The early work at NBS on radio propagation led to the development of new tools for meteorology and became an essential part of the newly formed National Oceanic and Atmospheric Administration. The Joint Institute for Laboratory Astrophysics was formed in collaboration with the University of Colorado and quickly exceeded the field of activity suggested by its name. The Time and Frequency program created a series of radical innovations in frequency standards and the dissemination of time, including a new definition of the meter and an experimental frequency standard based on a single mercury ion.
In practice, accurate VHF Antenna radiation patterns are usually difficult to achieve due to high level multipath present in the measurement test range. Special range geometry’s and source arrangements have been devised over the years [1] to mitigate the measurement errors produced by test range multipath. In this paper we will describe a new illumination source method designed to accurately control the influence of ground path illumination and in turn reduce quiet-zone amplitude ripple. An array of VHF elements with adaptive complex weights will be used to produce a controlled illumination line source for a given range geometry. Simulated quietzone performance will be shown.
This paper describes a High Speed (HS) channel switching capability for an “on-the-fly” data collection scheme primarily intended for use with dual-port probes. A program configures hardware switching of independent channels based on user selections. The switching selects a channel of an antenna (usually a dual-port probe) that is subjected to a continuous switching rate. This rate is carefully linked to the velocity of the scanning axis and the number of sweep points on a Network Analyzer (NWA). Design measures were implemented to take advantage of one of the fastest modes of an Analyzer: a CW sweep. Therefore, precise timing was necessary to ensure that the system is gathering the right datum at the correct time.
We have developed a methodology to determine the quality of a EMC test facility using equipment that may be generally available to RF testing services. By utilizing both the timeand frequency-domains, and accurate picture of the scattering and modal properties of the facility can be determined. This gives a much more information of the facility performance than a traditional scalar, frequency sweep of the facility. While the same frequency information is available with this dual-domain method, the causes of the irregularities can now be determined without guesswork and remediation to the facility can be preformed with more confidence.
Current methods of measuring the electromagnetic properties of materials (i.e., RF shielding effectiveness) include an ASTM (American Society for Testing and Materials) standard test cell and a stripline field applicator, among others. This paper outlines the basic theory of operation of each measurement setup, and compares measured data of similar samples from each setup.