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A Physical optics (PO) analysis of serrated edge reflectors is presented. It is shown that to obtain the true scattered fields in the target zone, one should use PTD (physical theory of diffraction) along with the PO solution. Using PTD, scattered fields of various serrated edge reflectors are presented. From these scattered fields, one can see that by proper design of the serrations, the edge diffracted fields can be reduced in the target zone. The edge diffracted fields, however, still may be too large for certain applications.
A novel technique is presented for the determination of the quiet zone field distribution of compact range antennas with serrated edges. The main reflector has a linearly serrated rim, so that the rim projection onto the reflector aperture plane is an arbitrary polygon. Additionally, the reflector aperture field is uniform in both amplitude and phase, and can therefore be expressed as a window function. The plane wave spectrum of the aperture field can then be obtained in closed form. Next, the spectrum is expressed at a plane in the quiet zone and the field is obtained by implementing an inverse fast Fourier transform (FFT) algorithm. Quiet zone field distributions are computed for various serrated rim configurations.
This paper evaluates the RCS errors associated with measuring a large flat plate which is illuminated by a compact range reflector with significant edge diffraction stray signals. This is done by evaluating the true fields incident on the plate and then using a physical optics technique to predict the backscattered fields. Results are compared with and without the edge diffracted fields present. A simple analytic expression is developed which can approximate the size of this potential error.
Image gating or editing is often used to determine the effect of an isolated scatterer on the RCS in the frequency and aspect-angle domain. In this paper, theoretical computations indicating limits in the image-gating procedure will be presented. The process provides the image-gating capability in combination with phase-corrected (focused) imaging. Targets consisting of two-point scatterers with well-known RCS response have been used. One of the scatterers is gated out and the resulting RCS versus frequency or aspect angle is determined and compared with its theoretical value. Limits in terms of minimum bandwidth or minimum distance in resolution-cell sizes are defined. The influence of several gate shapes and windows have also been examined. Experimental investigation has been carried out in order to verify the theory.
Under many circumstances it is necessary to experimentally estimate the radar cross section of targets in a cluttered environment. A significant reduction in the clutter can be obtained when cross range filtering can be done. In this experimental RC measurement concept, scattering measurements are performed using a moving radar antenna. Thus scattering as a function of target plus clutter versus aspect angle in the near field can be measured. Next, a back projection algorithm can be used to estimate the scattering as a function of position in the neighborhood of the target. The known range to which the signal is to be focussed is used to project back to the target area. An estimate of the RCS at points along a line in the plane of the target is computed. The clutter responses can then be removed from the data, and the remaining target-only values projected forward again (possibly to the far field) to estimate the RCS of the target alone.
An RCS measurement error model, calibration procedure and correction algorithm are discussed. A distinction between frequency response reflections and range-target reflections is made. Special emphasis is placed on the selection of the gate span with time gating used with the calibration and test target measurements. Mathematical simulations and actual measurements illustrate the discussion. It is concluded that frequency response related reflections must and range-target reflections must not be included in the gate for the frequency response calibration measurement.
Images of an open box, closed box, and open and closed box on a ground plane were taken at the Hughes/Motorola Compact Range. Comparison of these images show the effect of multiple reflections in the image of an open box. A simple analytic/computer model was developed to interpret these multiple images. Data and analysis are presented on the various mechanisms that come into play in scattering from the open/closed box and the ISAR images generated as a function of the viewing angle for the box.
The reflectivity levels of the three anechoic chambers of TKK and VTT were measured at 110 GHz. The sidewall reflections were measured by the free space voltage standing wave ratio method. Typical values measured with 20 dB pyramidal horns were below -60 dB at azimuth angles less than 20 degrees and about -50 dB at angles larger than 50 degrees. When a waveguide end was used as the transmitting antenna, the reflectivity level was nearly 10 dB higher. The backwall reflections could be measured directly because the reflected field was much larger than the direct field. The maximum backwall reflection varied in the three chambers from -33 dB to -36 dB.
Serrations are used on Compact Antenna Test Range reflectors to reduce the effects of edge diffraction. It has been found that the traditional triangular shape for these serrations is not optimal and that more continuous shapes should perform better. To verify this, RCS measurements were performed on test targets consisting of strip reflectors terminated by end sections of various shapes. The RCS vs. angle data were corrected for the field irregularities caused by the measurement range and then converted to the induced current distributions on the targets, from which the fields in front of the targets were calculated using Physical Optics. These fields are equivalent to the test-zone fields of an actual Compact Range. The results are compared with theoretical data. The agreement is good.
To fulfill the future demand of highly accurate antenna-, RCS- and payload testing, MBB built a new antenna test centre at Ottobrunn (Ref. 1). This paper describes the development and qualification of the large, dual reflector Compact Range (CR) which has a plane wave zone of 5.5 x 5.0 x 6.0 m (w x h x d). It starts with the results of a detailed electrical trade-off study between different CR-concepts, followed by some mechanical/thermal construction aspects of the large, highly accurate reflectors. Finally, some qualification results are shown, covering the frequency range from 3.5 GHz up to 200 GHz (lowest frequency of operation approx. 2 GHz). The achieved plane wave performance (amplitude ripple ±5o, phase ripple ±5o, cross-polarization isolation > 40 dB) verifies the high quality overall system design.
In the theory of inverse synthetic aperture imaging of radar targets the measuring distance is ordinarily supposed to be very much larger than the dimensions of the target. If this is not the case errors are introduced. We study these errors and means to decrease their influence by computation. The result is that the maximum tolerable target dimension can be substantially increased in a plane perpendicular to the axis of rotation if error correction is used.
A 400 element phased array antenna has been constructed at the GEC-Marconi Research Centre. Each radiating element is fed from its own phase shifter. The radiation patterns of this array have been measured using a recently constructed Cylindrical Near Field Test Facility. The radiation pattern is obtained on a two dimensional grid and contains both amplitude and phase information. It is therefore possible to transform these data back to the array aperture to obtain the array excitation amplitudes and phases. The spatial resolution obtained in the aperture is a function of the angular coverage of the radiation pattern used. The effect of deliberately introduced phase errors on the calculated aperture data is shown.
Separation of the Antenna from the remainder of the system is not possible with a fully active phased array such as MESAR, since each array element has an associated electronic module which contains amplifiers (separate for transmit and receive), phase shifters, switches, etc. The "antenna" is therefore not reciprocal and it also requires a control system. As a result, the system used for pattern acquisition is considerably more complex than that used for testing conventional antennas and some of the traditional parameters are either not obtainable or require redefining. The methods used for testing the MESAR antenna are given together with details of the range equipment involved.
Actually there are several projects that involve Active Array Antenna Concept for Satellite Earth study. A very large active array for SAR proposes is being studied by ESA that includes an amount of T/R or R modules of 1960. The studies of risks of failure or variations of LNAs and HPAs gain carried out by the designer gives as a result the need of implement some type of control of these parameters, so it is necessary to study and select an Internal Calibration concept for this antenna. This subsystem allows to know and correct any variation of gain in amplitude and phase of everyone of the transmitter/ receiver (T/R) and receiver ( R) modules
The Synthetic Aperture Radar Antenna for the European Remote Sensing Satellite ERS-1 is a 10 by 1 metre deployable slotted waveguide array operating at 5.3 GHz. Electrical performance of the complete antenna is derived at the end of the environmental test programme from near field measurements on a planar NF scanner. In order to obtain very early information on electrical integrity of the flight model antenna, suitable for pre- and post-environmental comparison, a fast electrical functional test was implemented in the total test sequence. It basically consists of a 2D slot probing of a well distributed number of slots in combination with complex input impedance measurements. The paper describes the method and presents results of different test steps. The data of pre-/post-environmental measurements are compared.
The hardware and software developments undertaken to upgrade two far-field measurement facilities - a 12-m anechoic chamber and a 35-m outside range - are described. A method (termed quasi-far-field, QFF) for deriving antenna far-field patterns from a single plane scan at a distance less than the traditional distance of 2D2/? is described. The QFF technique involves pattern sample and subsequent pattern transform and reconstruction, from the easement distance to the far-field distance. A discussion of the limitations inherent in the QFF transform, including range length, is given. Experimental results for measurements made on circular-aperture antennas with both symmetric and asymmetric illumination, and on antennas with elliptical apertures, are described.
For transforming a Fresnel region pattern to a far-field pattern, we present here two methods, the "discrete beam sampling" method (DBSM) and the "displaced beam" method (DBM), which allow an accurate characterization for both linear as well as circular antenna apertures. Both methods assume a simple Fourier transform relationship between the aperture field distribution and the far-field of the antenna. The Fresnel region field is then essentially perturbed by an aperture quadratic phase error assumed to exist because of the finite distance at which the field pattern is characterized. Numerical simulation and its results are presented to show the accuracy of the reconstructed far-field data. Finally, an error analysis is performed to show the sensitivity of the above two methods.
The nonuniform sampling technique utilizes measured (or simulated) amplitude and phase far-field data at nonuniformly sampled data points and constructs the pattern from these limited number of measured data. The technique relies on the fact that the antenna far-field pattern is proportional to the Fourier transform of a function which is related to the induced current on the antenna. The application of nonuniform sampling technique becomes important in the situation for which it will be difficult (or impossible) to measure the far field at regular intervals. In this paper, the application of the nonuniform sampling technique is demonstrated for antenna pattern measurements. The foundation of the technique is first reviewed and the required mathematical steps for the implementation of the technique is summarized. Both one dimensional and two dimensional cases are reviewed with attention given to the applicability of closed form expressions for the determination of the sampling coefficients. Numerical results are presented and comparison to measurements are shown. In particular, the application of this technique to a recently proposed space-station based antenna experiment is presented.
In recent years there has been an increasing demand for antenna calibrations at millimeter wave frequencies. Because of this the National Institute of Standards and Technology (NIST) has been developing measurement capabilities at millimeter wave frequencies. The development of gain and polarization measurement capabilities have been previously reported. This paper reports on the development of the capability to measure an antenna pattern which has been achieved during the last year. Measurement accuracies of better than 4 dB have been achieved for sidelobes which are 40 dB below the mainbeam peak. NIST is now providing a new measurement service for antenna patterns in the 30-50 GHz frequency range.
A flexible near-field antenna test-facility is presented. This system gathers all that is necessary to design, to debug and to validate the high performance antennas which are made by ESD. ARAMIS has been operational since January 1988. Its applications are: - Near-field measurements (for diagrams): * planar, * cylindrical. - High speed field mapping (for default analysis): * planar radiating surface, * cylindrical radiating surface. - Generation of element excitation (active phased array testing): * planar antennas, * cylindrical antennas. - Direct far-field measurements (probes, small antennas), - Circuit measurement (S parameter). The facility features a specially designed scanner. Thanks to its six degrees of freedom, this positionner allows the differents types of measurements to be made. The instrumentation is based upon the HP 8510 B network analyzer. A single computer performs the measurements, transforms the data and presents the graphics (linear diagrams, color maps, three-dimensional colored projections). In order to grant a high scan speed, the system uses the FAST CW mode of the HP 8510 B. An external trigger is provided during the motion process of the probe. A rate of 500 measurements/sec. has been proved. This on-the-fly process is clearly depicted. Experimental results are presented which include: - Low sidelobe (-38 dB) antenna diagrams. - Default analysis through: * Amplitude mapping (leakage short-circuit in a microstrip antenna). * Phase mapping (out-of band comparison between two radiating element technologies). * Measurement of excitation laws. * 3-D transformation. - Simultaneous on-the-fly acquisition of up to three antenna outputs.