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Holographic back-projections of planar near-field measurements to a plane have been available for some time. It is also straightforward to produce a hologram from cylindrical measurements to another cylindrical surface and from spherical measurements to another spherical surface1-7. In many cases the AUT is approximately a planar structure and it is desirable to calculate the hologram on a planar surface from cylindrical or spherical near-field or far-field measurements. This paper will describe a recently developed spherical hologram calculation where the farfield pattern can be projected on any plane by specifying the normal to the plane. The resulting hologram shows details of the radiating antenna as well as the energy scattered from the supporting structure. Since the hologram is derived from pattern data over a complete hemisphere, it generally shows more detail than holograms from planar measurements made at the same separation distance.
In previous AMTA Symposia, the Air Force Research Laboratory reported on a successful effort to fabricate, measure, and predict the precise radar cross section (RCS) for various cylindrical calibration targets [1]. In this paper, we apply what we have learned about calibration cylinders to the study of a 3.048 meter ogive body of revolution. Recall that an ogive is simply the arc of a circle spun on its axis. The radar signature of this shape is extremely small in the direction of the "point", even at low frequencies. A few years ago, AFRL had the subject ogive built for an RCS inter-range comparison between AFRL and the NRTF bistatic RCS measurement system [2]. In this paper, we utilize this ogive body to assess both the quality and accuracy of VHF RCS measurements and predictions performed using multiple calculation schemes. In the end, reconciling the ogive measurements and predictions led us to reassess how composite objects are "conductively coated" to simulate a perfect electric conductor. This insight resulted in refinements in the process for measuring and predicting the ogive at low frequencies where electrical size and electromagnetic skin depth considerations are important.
Radar return data from various types of aircraft were collected and analyzed during varying flight profiles to determine the presence of consistent, dominant radar returns of point scatterers on the aircraft. These measurements were performed by integrating two separate X-band radars into one system with the ability to simultaneously track and image aircraft. Selected processed data from both radar systems were analyzed and are presented as a function of time, azimuth and elevation angle, and range. I/Q data, high-range resolution (HRR) profile data and inverse synthetic aperture range (ISAR) data are presented for selected flight profiles of helicopters, propeller aircraft, and jet aircraft.
The implementation of low observable (LO) materials and the fielding of aircraft with controlled signatures creates a new degree of difficulty for maintaining, executing prompt accurate inspections and achieving meaningful evaluations. To address this problem, Sensor Concepts, Inc (SCI) has prototyped a new radar system, (the SCI-Xe) to provide a test bed for a lighter, smaller RCS measurement and imaging system. The hardware consists of a suitcase containing RF hardware, computer and display and a hand-held or rail-mounted unit containing two X/Ku band antennas. In the rail-mounted application, imaging is followed by registration and image differencing, which allows an operator reproduce a baseline measurement geometry and evaluate RCS changes. The hand-held application forms a synthetic aperture by moving the antennas by hand. This can be used to quickly investigate an object under test.
This paper presents a first-principles algorithm for estimating a target’s far-field radar cross section (RCS) and/or far-field image from extreme near-field linear (1- D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. Wavenumber migration (WM) is an approach that was first developed for the problem of geophysical imaging and was later applied to airborne SAR imagery [1], where it is often referred to as the “Range Migration Algorithm (RMA)”[2]. It is based on rigorous inversion of the integral equation used to model SAR/ISAR imagery, and is closely related to processing techniques for near-field antenna measurements. A derivation of WM and examples of approximate farfield RCS and image reconstructions are presented for the one-dimensional (1D) case, along with a discussion of the angular extent over which the far-field estimates are valid as a function of target size, measurement standoff distance, and near-field aperture dimensions.
In order to better estimate the uncertainties in measured RCS for the Boeing 9-77 Compact Range, we study the responses from three high-quality objects, i.e., two ultraspheres of 14” and 8” in dia., plus the 4.5" squat-cylinder, each supported by strings. When calibrated against each other in pairs, the differences between measured RCS and predicted values are taken as the uncertainties for either object. Two standard-deviations from the target, reference, and background, as computed from repetitive sweeps, are taken as the respective uncertainties for the signals. Using the root-sum-squares (RSS) method, the error bars are found to be between + 0.1 to 0.2 dB for most of the frequency F, from 2 to 17.5 GHz. We also analyze the responses from a thin steel wire (dia. 0.020"), supported by fine fishing strings (dia. 0.012"), at broadside to the radar. When the ‘wire and string’ assembly is oriented vertically, the HH echo from the 3-ft metal wire alone happens to be comparable to the HH from the 30-ft dielectric strings. Varying with F4, the combined RCS in HH for the assembly spans a wide range of 38 dB from 2 to 18 GHz. The error bounds are found to bracket the measured traces even when the signals are barely above the noise floor.
We review a century of radio metrology research and development in the U.S. that paralleled the birth and evolution of radio/wireless and other electromagnetic technologies. The interplay between the scientific and technological advances and the research, measurement and standards development programs at the National Institute of Standards and Technology (formerly the National Bureau of Standards (NBS)) was a factor that facilitated both commercialization of products and implementation of systems for the public benefit.
The growing need for a mobile radar system able to conduct measurements away from fixed radar ranges has prompted System Planning Corporation (SPC) to develop a mobile MkV radar system. Planned helicopter-based SAR measurements generated a requirement for a ground-based platform to verify functionality of X-band and VHF/UHF data collection and processing systems. Accordingly, SPC developed TruckSAR, a DGPS-equipped mobile testbed to collect side-looking and normal-incidence SAR data. Interleaved step chirp data were collected at 9.0-9.3 GHz (HH polarization) and 150-450 MHz (HH, VV, HV, and VH polarization). The system is self-contained and is proving useful for applications beyond ground and foliage penetration SAR investigations. This paper describes the TruckSAR hardware and data analysis systems. Results of measurements are presented, along with observations of challenges in data interpretation. Promising extensions of this mobile ground-based radar are also discussed.
A versatile instrumentation system for automatically measuring both antennas and performing the Air sensitivity & Channel Power test. The system is capable of being easily reconfigured to perform standard FF antenna measurements using a model tower configuration which includes a dielectric mast with a rotary “head” mounted on an azimuth turntable or automated air sensitivity and channel power measurements for both GSM and CDMA mobile cellular devices. The air sensitivity test module iterates until the desired user defined frame error rate is reached at the preset scan positions and than records the data. The system also contains analysis capabilities for all modes of measurement. The paper will summarize the system configuration and the features of this integrated test system.
Waveguide simulators are widely used for low cost validation of periodic microwave designs and to perform antenna measurements. We have used measurement results of a waveguide simulator to predict both Frequency Selective Surface (FSS) and reflectarray responses. For scan angles close to normal, a suitable waveguide simulator is relatively wide and measurement results are often corrupted. This is often caused by uncontrolled multi-mode operation. The work presented here describes a waveguide simulator, which solves this problem for triple-mode operation. The triple-mode waveguide simulator has three standard waveguide ports and one triple-mode port. This device can be excited on the three standard ports. It produces each of the three propagating modes at the triple-mode port separately. Simulations and measurements on a prototype show good agreement. With our current set-up, three scan angles can be predicted instantaneously and grating lobes can be studied as well.
The system noise temperature is a fundamental parameter of performance of a satellite communications reception antenna. Traditional methods of measuring noise temperature involved the use of thermal noise standards connected to the antenna input. Indirect methods can also be used to derive noise temperature from a G/T measurement. However these traditional methods require special setups and cannot be used continuously. A noise temperature measurement system, which can be used continuously, even when tracking and receiving telemetry signals, is a valuable tool for performance monitoring. A noise adding radiometer method, which was originally design for radio astronomy applications, has been adapted for communications antenna measurement [5] and this can be used continuously. However, this method has some limitations in the degree to which telemetry signals interfere with the measurement system. A study has been undertaken under European Space Agency (ESA) contract, into the design of a noise temperature measurement system, which involves the evaluation of the bit error rate of a test signal. Changes in noise temperature result in change of bit error rate of the test signal. The test signal is spread spectrum modulated so interference between the test signal and operational signals are minimised. The study was executed in two phases. In the first phase a theoretical analysis of the test method was performed. In the second phase a prototype measurement system was developed and evaluated. The paper describes the main results of the theoretical analysis, description of the prototype system and analysis of the test results. The prototype was designed to measure the noise temperature of a standard ESTRACK S/X band ground station antenna. As a follow-on to the original study work is now in progress to produce a fully operational unit which will be installed in the ESA Perth S/X band ground station. The results of evaluation of the prototype have been used to introduce design improvements for the operational unit, which are described and discussed.
Multiple-band tests are important to demonstrate the performance of a complete system, including the ability for distinct sub-systems to simultaneously measure the same scene. This paper outlines an experiment performed under the direction of Eglin AFB. The experiment demonstrates the capability of a test range to perform a combined radio frequency and infrared (RF/IR) direction finding experiment in which the RF system cues the IR system as an object is tracked across the field of view. Such a test requires the combined performance of the test range, an RF system and an IR system, along with a procedure to integrate the three systems into a cohesive experiment. The successful demonstration of the complete system shows the strength of the individual systems, and the process of development identified some areas for advancement in future multi-spectral and multi-system tests. This report focuses on the development and performance of the RF direction finding portion of the test, with minor discussion of the other systems.
Future scientific and earth observation instruments as MASTER, PLANCK and HERSCHEL of ESA/ESTEC are working in the sub-millimeter wave range. For measurement of the instruments, a study named ADMIRALS was performed, mainly to identify the most suitable test facility, procure transmit and receive modules and perform measurements up to 500 GHz. The CCR 75/60 of Astrium GmbH, Ottobrunn, was selected for the facility calibration and the pattern verification with an Representative Test Object (RTO). The measurements were performed in three different frequency bands between 200 and 500 GHz. The mmwave transmit and receive modules were designed, manufactured and tested by Radiometer Physics GmbH (RPG). A cost efficient design was achieved by a modular concept. Within this paper, the design and realization of the modules as well as most characteristic performance parameter will be presented.
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 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).