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Possible mechanisms and structures for realising a dynamically adaptive radar absorbing material (DARAM) are discussed and their potential evaluated through computer simulation. Some pointers towards practical implementation are outlined and measured results for large-area DARAM panels operating over I and J bands are shown.
In order to measure the performance of microwave absorbing materials a broadband free- space measurement system constructed in INTA. The is a kind of N RL Arch that gives us the possibility of measurements in d ifferent configurations. It comprises a set of dielect ric loaded rectangular waveguide antennas, coaxial vector analyzer, sample support and a computer. A TRL calibration technique in the plate near field is developed taking advantage of the calibration functions implemented in the network analyzer and the time domain gating. We introduce the use of typical RCS calibration standards as the calibration reflect standards. It gives us the possibility of performing the near field free space calibration in the same way that it is usually done in waveguide, but for directions di fferent to the normal. This calibration allows us to check the edge diffraction behaviour of the samples in the measurement and is thought to be adecuated for thin materials.
An analytic technique recently developed at NIST [1] [2] to correct for probe position errors in planar near-field measurements has been implemented to arbitrary accuracy. The nth-order correction scheme is composed of an mth-order ordered expansion and an n - m higher-order approximation, where both n and m are arbitrary. The technique successfully removes very large probe position errors in the near-field, so the residual near-field probe position errors are substantially below levels that can be measured on a near-field range. Only the error-contaminated near-field measurements and an accurate probe position error function are needed for implementation of the correction technique. The method also requires the ability to obtain derivatives of the error-contaminated near field defined on an error-free regular grid with respect to the coordi nates. In planar geometry the derivatives are obtained using FFTs [1], giving an approximate operation count of (3 • 2=- 1 - 1 + (n - m)) N log N, where N is the number of data points. Efficient computer codes have been developed to demonstrate the technique. The results of simulations are more accurate than those obtained us ing the well-known k correction [3), which can correct for position errors in some direction in k space, but further contaminates the sidelobe levels.
A technique for estimating measurement errors in near field facilities is presented. Known mechanical and electrical errors can be accounted for in simulation and such results are presented here. Unknown factors like chamber reflection and instrumentation drift can be estimated via selective measurement and the error induced by such anomalies may be combined with the simulated findings to provide error patterns for a particular test antenna and facility. Results are shown where these patterns are used to calculate measurement error limits. The software presented here also allows the generation of parametric curves which show the impact of a parameter of interest.
We investigated two methods of probe-position error correction to determine how well the corrected results compare to the uncorrupted far field: the k-correction method and the Taylor-series method. For this investigation, we measured a 1.2 m dish at 4 GHz and a 1.2m by 0.9m phased array at 2.2 GHz. Measurements were made first without position errors and then with deliberate z-position errors. We perfonned probe position error correction using both methods and compared the results to the error-free far field. For errors up to A/4, the fifth-order implementation of the Taylor series correction was slightly better than the k-correction. For errors of ')..J2, the k-correction was better than the Taylor-series correction.
Hologram measurements are becoming more and more popular as a reliable method for identifying bad elements and the tuning of active phased array antennas. Relying on holographic data to adjust phase shifters and attenuators in these antennas can give undesired results if the accuracy of the data is poor. Often measurements can be improved if the error sources can be isolated and quantified. This paper presents an approach to producing a hologram accuracy budget based on the NIST 18-term error budget created for near-field measurements. A set of hologram accuracy terms is identified and data is presented showing the typical hologram accuracy that can be expected from a near-field scanner.
A method to transform Fresnel field data to far-field data with probe correction, based on a non linear least squares algorithm, is presented. The functional to be considered is the expression of the Fresnel field radiated by an array of isotropic sources located on the antenna aperture, and the complex excitations are the coefficients that minimize the rms error between the measured data and the functional values. The intermediate step of determining the complex excitations can be used as a diagnostic tool. Probe pattern correction has been included in the method, improving the performances of antenna measurement systems placed in small size anechoic chambers.
The UCLA hi-polar planar near-field scanner has a novel implemen tation which results in a polar sampling grid. The scanner was used to perform measuremen t comparisons using three sampl in g arrangements: polar, thinned-polar, and linear-spiral sampling. The data acquired using each was processed to the far-field for both simulated and measured near-field data. Excellent agreement was observed.
The Modular Radar System has been developed at the Danish Defence Research Establishment (DDRE) in cooperation with the Danish company CRIMP. The unique system is capable of performing nearly all types of calibrated radar measurements. The modularized highly flexible system is presented along with a number of measurement. RCS of very small targets at short ranges 400'- l 000', medium range measurements of Navy targets, aircraft and chaff at ranges from 1-10 nautical miles. The real time high resolution range profiles are used for positive identification of "hot spots" on Navy vessels leading to very efficient RCS reductions.
Superresolution techniques based on the Multiple Signal Classification (MUSIC) have recently been applied to two-dimensional (2-D) Inverse Synthetic Aperture Radar (ISAR) imaging with demonstrated results. These techniques exhibit much higher spa tial resolution than other approaches using a 2-D Fourier transform. This paper a MUSIC based superresolution algorithm for 3-D radar imaging, which is especially useful for measurements with both small frequency and aspect angle (in azimuth and elevation) spans. This algorithm models the measured 3-D data set as a sum of point source emissions plus noise. Once the positions in the 3-D space of such scattering centers are obtained using the MU SIC algorithm, the weights (or RCS) of the scattering centers are obtained through a pseudo-inverse matrix inversion computed by means of a Singular Value De composition (SYD).
The application of rapid near-field measurement systems based on the Modulated Scattering Technique (MST) and Spherical Angular Function (SAF) data processing of the measured data to extract far-zone RCS of complex targets is discussed in this paper. A first-generation Spherical near-field measurement system for efficiently determining bistatic RCS is presented.
This paper presents theory, numerical implementation and experimental validation of a new filtering technique, which exploits the band-limitation properties of radiated or scattering fields in order to significatively reduce the measurement noise due to environmental reflections.
Far-field patterns obtained from planar near-field measurements of a prototype of the AMSU-B radiometer antenna by phase retrieval at 94 GHz are presented in this paper. Comparison with results from a compact range facility show good agreement within the main beam A modified algorithm takes into account any misalignments of the two intensity data sets so that the RMS near-field error metric comparing retrieved and measured values converges to < -30 dB. Phase retrieval is revealing itself as a useful technique to be applied to electrically large antennas at frequencies extending into the millimetre and sub millimetre bands.
The feasibility of realizing a 500 GHz hologram type of compact antenna test range (CATR) for testing the 1.1 m antenna of the Odin satellite is studied. The quiet-zone field is analyzed theoretically by us ing an exact near-field aperture integration method. Due to fabrication errors the slots of the hologram are wider or narrower than in the ideal case. How ever, with reasonable value of fabrication errors the quality of the quiet-zone field is not degraded deci sively. The effect of the displacement of the different parts joined together to form a large hologram is the inclination of the amplitude and phase in the quiet zone. The analysis of the CATR feed scanning in the focal plane to avoid the need to rotate the AUT showed that at least a ±1° change of the direction of the plane wave in the quiet-zone is feasible.
Daimler-Benz Aerospace AG (Dasa) in Munich, Germany designed, developed, build and tested most of the INTELSAT VIII antennas. RF test requirements and results are presented for the Hemi/Zone antennas. These tests cover the Beam Forming Networks (BFN), the feed array in the cylindrical near field facility at ambient temperature and in a temperature range from -61 to +85 deg centigrade, and finally the complete antenna sub system, without and with satellite mock-up, in the large Compensated Compact Range. Dasa and TICRA software was used to calculate the far field results from the measured BFN coefficients and from the feed array results measured in the near field facility. Also alignment aspects are considered.
Modern low profile and conformal antennas are fre quently evaluated in the presence of a conducting surface. Antenna designers usually predict the an tenna scattering and radiation performance over an infinitely conducting ground plane. To bridge the gap between a (possibly curved) antenna host surface and the designer's infinite ground plane model, an antenna testbody is required. This testbody must possess a variety of demanding attributes, such as a very close approximation to an infinite ground plane, low testbody signature, ability to provide positional accuracy (in both azimuth and elevation), physical stability (for repeatability and background subtraction), just to name a few. The most widely regarded testbody has been the "almond" testbody [1, 2], which boasts a very low signature, and excellent fidelity when compared to an infinite ground plane. This paper addresses a new variation from the traditional "almond" testbody, in which a unique positioning design provides a phase-stationary antenna aperture center under rotation of both azimuth and elevation. This testbody will be used for a variety of antenna tests at Wright Laboratory's Radiation and Scattering Compact Antenna Laboratory (RASCAL).
Accurate mechanical-to-electrical axis alignment (boresighting), gain, and pattern testing of radar antennae requires specialized tooling/fixturing. This requirement is often taken for granted and seldom discussed in the EE community. Particularly in a production environment, where rapid change of test configurations to accommodate multiple radar platforms are required, a convenient mounting scheme is mandatory. This paper describes and illustrates a method implemented at the Warner Robins Air Logistics Center to satisfy this demand. Drawings and/or photos of a three-point Universal Adapter fixture and several UUT Specific radar mounting fixtures are discussed. The paper discusses tolerances, materials, manufacturing processes, alignment, and antenna boresight methodologies.
Antenna positioners using a single pivot joint and two linear actuators are attractive for applications requiring limited two-axis motion. Such applications include antenna and RCS measurement systems, and scanning antennas. Minimum swing clearance is required. Positioners can be light, compact and stiff. Position feedback can be independent and linear for both axes. Design and selection considerations are presented. Two examples are described
There is a need for finite ground planes to test an tennas which are normally mounted on large struc tures. These ground planes are used to simulate a large structure such as an aircraft fuselage but are limited in size based on the available target zone di mensions. For example, TCAS antennas are tested on a 4' circular ground plane based on FAA require ments. Since the conducting ground plane creates significant diffraction errors which are not present in the intended application, these ground plane tests become difficult to interpret because one can not easily separate ground plane diffraction errors from antenna characteristics. A solution to this dilemma is to attach an R-Card (resistive sheet) to a con ductor (PEC) and form an R-Card ground plane. With a properly designed resistive profile, an R-Card ground plane can greatly reduce the edge diffrac tion errors. As a result, the desired antenna charac teristics without significant ground plane corruption terms can be obtained. This paper demonstrates this new concept through calculated and measured results. Also, a Genetic Algorithm (GA) to optimize the resistive profile is presented.
Instrumentation grade, coherent, bistatic, radar cross section (RCS) measurement systems require a reliable low-noise method to link the reference, local oscillator (LO) and intermediate frequency (IF) coherent signals between the transmit and receive subsystems. One approach to this is the use of a fiber optic link (FOL). Phase noise measurements have been performed on a distributed feedback (DFB) type laser transmitter-photodiode receiver link with a delay of up to 2.26 kilometers, operating at 5 GHz, using a standard HP 3048A phase noise test measurement setup. System level tests have been performed, incorporating a FOL into a coherent bistatic instrumentation radar system local oscillator path, and performing image processing on an emulated target A first level analysis was conducted regarding the effects of the thermal noise on the radar perfonnance.