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Range resolution of a radar image can be obtained by use of wide-band signal (linear FM or chirp waveform) and cross-range resolution by object rotation which synthesized a large antenna aperture (the so called ISAR method, refer [1]). Although both cross-range profiles can be resolved by rotation of the abject about two mutually orthogonal axes, however, the data manipulation would be quite cumbersome and the measurement implementation would require a mechanical support system by which the objet [sic] can be independently tilted and rotated relative to the radar axis. In this paper, the algebraic reconstruction technique (ART)[2] for tomography is used to resolve the vertical cross-range profile (along the axis normal to the ground) while the horizontal cross-range profile still resolved by ISAR method. Applications of the ART to a simple circular pattern and a complicated emblem pattern of the CSIST show that ART is a suitable approach and easier than ISAR method to obtain the second cross-range resolution.
Martin Marietta designed and brought on-line an indoor far-field chamber used for radar cross section (RCS) evaluation. The range has conductive walls on all sides except for the pyramidal absorber covered back wall. The chamber was designed such that wall/floor/ceiling interactions occur with a distance (time) delay allowing for their isolation from the test region. Software gating techniques are used to remove these unwanted signals. This paper presents an analysis of the conductive chamber using Geometrical Optics (GO). The objective was to analyze and evaluate the plane wave quality in the chamber test region. The evaluation of the plane wave was performed using the angle transform technique. The measured results were compared to analytical results and measured antenna patterns.
The properties of a focused scalar horn-lens antenna are presented. The behavior of the field from the lens to the far field is determined from electromagnetic principles and measured antenna patterns at the focal distance are shown.
Recently an antenna pattern measurement technique has been developed which eliminates the effects of the finite ground plane on which the test antenna is mounted. The scattered fields from the edge of the ground plane can often cause perturbations in the total fields, and thus, result in significant differences in the measured patterns as compared the theoretical predictions. This technique consists of the measurement of the edge diffracted fields and their subsequent subtraction from the original pattern. A simple theoretical model is developed to introduce the subtraction technique, and comparisons are made which show the excellent agreement between theoretical (obtained assuming an infinite ground plane) and “corrected” experimental antenna patterns. Experimental results are given from an open-ended waveguide opening into both circular and square ground planes.
A novel differential phase measurement method is developed. No flexible cables or rotary joints are needed in this method. Phase center positions and phase patterns of two corrugated horns are measured at 105-115 GHz and 176-190 GHz by using this method. Good agreement between the measured values and theoretical values, calculated with the modal matching technique, is obtained. Also a new phase error correction method is introduced. This method makes possible to measure the phase error in the cable and then to remove the error numerically from the results. The accuracy of the phase error correction is limited by the phase measurement device in the system. Experimentally this method is verified at 10 GHz.
The aperture opening design of the subreflector chamber for a dual-chamber Gregorian compact range system is presented in this paper. The subreflector is a serrated edge ellipsoidal reflector. The performance of the subreflector chamber and absorber aperture opening has been evaluated in terms of pattern measurements and by cross-range diagnostic techniques. The results of this evaluation have been used to further improve the design of the aperture opening of the subreflector chamber.
A new approach has been developed to achieve an octave bandwidth, reduced size feed fot compact range reflectors. It can provide highly isolate, orthogonal polarizations with a minimal size, suitable for operation at frequencies down to 500 MHz and below. Its construction is relatively simple, with only a few specific dimensions. The beam-width is compatible with compact range reflector feed requirements. The method uses crossed dipoles over a small circular ground plane, with a rim to equalize the E- and H- plane patterns. Parasitic elements are employed to extend the bandwidth with matching provided via a section built into the feed line. The design was optimized using the Numerical Electromagnetics Code (NEC) computer program.
The effects of non-systematic receiver instrumentation errors on precision antenna measurements are investigated. A simple uncertainty model relating dynamic range to random perturbation effects on amplitude measurements is proposed. Examples of measurement uncertainty versus both input level and measurement speed are presented using data taken on modern measurement receivers. Dara are compared with the model to estimate measurement uncertainty at various pattern levels and acquisition speeds. Equivalent dynamic range specifications are deduced from the measures data.
The utility of wideband RCS data for characterizing scattering mechanisms of complex objects has been established by wide-spread applications. The fundamental data from which the final products are derived consist of calibrated scattered fields measured coherently as a function of frequency and aspect angle. By processing these data, one-dimensional range or cross-range reflectivity profiles can be derived; by further processing, two-dimensional images can be derived. Modern RCS instrumentation systems capable of rapidly measuring and processing wideband data provide more object information than is conveyed by the RCS pattern, which has been the traditional descriptor of scattering behavior. The procedures of one- or two-dimensional imaging inherently involve integration processes, constituting many-to-one mappings in which data from a large set are collapsed to produce an individual pixel of the image. For example, a particular pixel of a range response is derived from the total object response “integrated” over a band of frequencies; similarly, a pixel of a two-dimensional image is derived from the object response “integrated” over frequency and angle. The exposure of a local feature of the object signature, obtained by collapsing the fundamental data, comes at the cost of obscuring the global descriptor. This paper explores techniques for presenting large amounts of information on single displays which retain both global and local features of the scattering process. These tools provide to the RCS analyst options for extracting and interpreting significant information from the measured data without arbitrary degrees of integration which can mask essential details represented in the data. The display methods utilize color coding to increase the amount of information conveyed by a single plot. Because color reproduction is not available for the proceedings, the paper is to be distributed at the conference.
This paper examines antenna pattern measurements of RF frequency antennas (300 kHz-3 GHs) using an integrated source/receiver and measurement control software. Current microwave measurement systems provide sufficient measurement capability but are often too expensive to be used on ranges which require test frequencies of less than 3 GHz such as aircraft communications, cellular radio, GPS, and satellite telemetry antenna. Several system block diagrams based on the HP 8753 network analyzer will be examined with respect to system performance, measurement accuracy, and cost. System considerations for outdoor RF ranges such as RFI susceptibility will also be addressed.
A new implementation of the planar near-field back projection technique for phased array testing and aperture imaging is described. In the alignment of phased arrays, the aperture field is treated as a continuous distribution rather than using idealized array concepts. The continuous field is then sampled to obtain element excitations. In this way, nonrectangular arrays can easily be accommodated. The method also produces highly interpolated images of apertures that can offer much insight into their nature. Also, any polarization of the aperture field may be obtained if the probe pattern has been characterized. The technique uses large FFTs which are computed very quickly by a workstation located in the facility. Results from an iterative phase alignment of a 12x18 phased array are presented, as well as highly interpolated images of apertures and results which demonstrate the polarization selection.
A technique for aligning a phased array is described. Array element attenuation and phase commands are derived from far-field patterns measured without calibrations. The technique is based on iteratively forming mulls in the antenna pattern in the directions specified by a uniform array illumination. It may be applied in situations where array elements are not individually accessible, or where an array contains no build-in calibration capacity. The alignment technique was evaluated on a far-field range with a linear, 32-element array operating at L-band. The array containing transmit/receive modules with 12-bit amplitude and phase control. Insertion attenuation and phase measurements were comparable to those obtained by conventional techniques. However, the alignment procedure tends to compensate for the effects of nonuniform element patterns and range multipath. Thus, when used to implement other excitation functions, the array sidelobe performance with adaptive calibrations was substantially better.
To enhance the performance of existing near field techniques the new idea of far field pattern determination from only amplitude distributions of the near field is proposed. In this way the difficulties related to phase measurements are overcome. Some different algorithms are introduced and discussed. In particular, after recalling results for the planar geometry, cylindrical scanning surfaces are considered. The feasibility and the performances of the introduced algorithms are shown through numerical examples.
The reflective properties of a flat circular plate and a long thin wire are discussed in connection with the quality and calibration of the quiet zone (QZ) of a compact antenna test range. (CATR). The flat plate has several applications in the CATR. The first is simple pattern analysis, which indicated errors as function of angle in the QZ, the second uses the plate as a standard gain device. The third application makes use of the narrow reflected beam of the plate to determine the direction of the incident field. The vertical wire has been used to calibrate the direction of the polarization vector. The setup of an optical reference with a theodolite and a porro prism in relation to the propagation direction of the incident field is presented as well.
A method for obtaining the individual element excitations of an array antenna from measured radiation patterns is presented. Applications include element failure diagnosis, phased array antenna calibration, and pattern extrapolation. The measured far-field information is restricted to visible space which does not always contain the entire Fourier domain. A typical example is phased array antennas designed for large scan angles. A similar problem arises during near-field testing of planar antennas in which case the significant far-field domain is restricted by the scanning limitations of the near-field test facility. An iterative procedure is then used which is found to converge to the required solution. The validity of the approach has been checked both using the theoretical radiation patterns and real test cases. Good results have been obtained.
The Dassault Electronique flexible near-field antenna test facility, ARAMIS, has been used for test and calibration of state-of-the-art active phased-array antennas which were designed for military SATCOM operation. The 14-month successful program dramatically emphasized the benefits of a flexible antenna test facility such as ARAMIS. These benefits are the following: • Flexibility o Far-field mode (test of radiating elements and modules) o Planar near-field mode (test of sub-arrays and complete antenna) o High-resolution field mapping mode o Array Element testing • Speed: quick mode switching, “on the fly” multiplexed acquisition • Versatility: calibration of a module, a sub-array and the antenna; radiation patterns; gain; faulty element detection • Productivity: a single indoor facility performing different types of measurements, integrated software Test results gathered during this program and showing the ARAMIS contribution are presented.
Accurate near-field cross-polarization measurements on circularly polarized (CP) antennas at millimeter-wave frequencies require well-characterized probes with low axial ratios. We have recently obtained and calibrated dual-port CP horns for use as near-field probes at frequencies of 40-50 GHz. These horns have axial ratios which are 0.3 dB or less over a 10% frequency bandwidth. With these good axial ratios the difference between vector and scalar probe correction is usually small. Additional advantages of the dual-port probes are the need for only a single alignment, more accurate knowledge of the relative phase between two ports of the same probe, and the ability to obtain both main and cross polarized data during one scan. The axial ratios of the dual port CP probes are also better than those of single-port CP Probes. In this paper we present some gain, axial ratio, and pattern measurements for these probes and show that they give accurate cross-polarization measurements.
The near field technique has grown from experimental systems of the early 1960s to sophisticated accepted means of testing antennas. Several schemes have been employed, namely planar, cylindrical and spherical scanning. The spherical scanning system chosen for one of the near field ranges at GE Aerospace is different from most near field systems in that the test antenna remains stationary while the probe is made to scan over a surface of an imaginary sphere surrounding it. The sampled field is corrected for positional, phase and amplitude errors and transformed to the far field. Radiation patterns, gain, EIRP, group delay and amplitude response were measured for a shaped beam communications antenna.
Antenna engineers recognize that the planar near-field method for calibrating antennas provide accurate pattern and gain measurements. Bothe the pattern and gain measurements require some degree of probe position accuracy in order to achieve accurate results. This degree of accuracy increases for antennas that have structured near-field patterns. These are antennas in which the amplitude and phase change rapidly over a very small position change in the near-field scan plane. The National Institute of Standards and Technology (NIST) has recently measured an antenna with a very structured near-field pattern. This measurement was performed using a new probe positioning system developed at NIST. This measurement will be discussed and results will be presented showing how slight probe position errors alter the antenna pattern and gain.
The extraneous signals that perturb antenna patterns can be found and identified by a method known as “longitudinal translation at selected points”. The method is usually applied to certain selected angular points on the antenna pattern. With this technique the composite pattern – consisting of the direct-path signal and the reflection signal – is measured at a series of translation distances along the axis of the antenna range. By utilizing both the amplitude and phase of the received signal, one can remove the signal that results from stray reflection and retain the desired direct path signal. The result is an improved and more accurate version of the pattern. In this presentation I review this technique as specifically applied to compact range antenna measurements, and apply it to several patterns, to demonstrate the method.