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An examination of mutual coupling effects in a linear phased array is presented. The approach derives mutual coupling coefficients from array element patterns measured in the Fresnel region, at R/D=3. The technique allows edge diffraction effects and mutual coupling effects to be identified and separated. The results are compared with conventional mutual coupling measurements and mutual coupling coefficients determined by numerical integration. The technique is used for far-field pattern reconstruction, and for pattern optimization which corrects mutual coupling effects to the maximum extend possible.
Nowadays, the standard facility for accurate satellite antenna testing is the Compensated Compact Range (CCR). In order to increase measurement accuracy several techniques can be applied, which are based on antenna pattern comparison. The theory of these techniques together with experimental results have been described in several papers in the past [1][2][3]. This paper presents how pattern comparison techniques are applied for space programs and is another step to official qualification of the Advanced Antenna Pattern Comparison (AAPC) method at Dornier Satellitensysteme (DSS).
This paper presents several enhancement tools that were developed to improve the Georgia Tech Spherical Far-Field/ Near-Field Antenna Measurement Range. Measurement amplitude and phase drift was quantified by sampling an antenna measurement signal over long time intervals while leaving the AUT rotation positioners fixed. A return-to-point drift correction tool was implemented to correct for the long-term drift component for spherical surface measurements. Temperature sensitive components of the receiver were moved from an area with severe temperature variations to a temperature stable area to reduce the phase variation. A software tool was developed to display a histogram of the variation in repeated spherical scan measurements. Histogram vales show that drift correction improves the repeatability of an antenna pattern measurement. The shapes of the histograms have been helpful in identifying random and deterministic variations.
The use of global positioning satellite (GPS) signals for automotive navigation and this on-vehicle GPS antennas has become more common recently. As the number of users increases the cost of the highly integrated receiver is predicted to come down to less than $50. It is possible to measure the antenna patterns of GPS antennas as installed on vehicles, but it is important to make sure that the parameters measured are valid for the GPS environment. In this case, sky coverage and polarization are more important than the directive pattern, for example. This paper shows a method of comparing a number of antennas by using the actual GPS satellite signals as test signals.
New windows which allow the user to select the level of sidelobe suppression near the DFT resolution limit are reported. By a parametric study, we identify the truncated Lorentzian and Gaussian functions as better choices compared with the popular Hann windows.
Both Near-field Antenna Measurement Technology and Beam Waveguide Antenna techology have been in existence for some time. This paper describes a measurement combining both of these technologies. During an internal study of beam waveguide implementation, a near-field antenna measurement was made of a development model. The model and techniques of measurement are described herein.
At high frequencies, the scattered fields from a radar target can be modeled as a sum of contri butions from a finite number of scattering centers. We use a parametric model based on the Geometric Theory of Diffraction (GTD) to estimate the location and type of scattering centers present in a frequency domain data set. The parameters of the model are estimated using a modified MUSIC algorithm that incorporates the GTD model. A new spatial smoothing algorithm is also introduced.
The imaging of radar targets is typically accom plished by measuring the radar cross section (RCS) of the target as a function of frequency and az imuth angle. We measure a third dimension of the RCS by tilting the target and collecting data for conical cuts of the RCS pattern. This third dimension of data provides the ability to estimate the three-dimensional location of scattering centers on the target. Three algorithms are developed in order to process the three-dimensional RCS data.
RCS data measured under near-field conditions is corrected to the far-field. The algorithm uses the HUYGEN's principle approach. The processing technique is describes and validates using anechoic chamber data and simulations taken on flat plate target at a distance from the radar R << 2D2/A, where D is the target cross range extend and A the wavelength. Good agreement with the theoretically predicted far-field RCS patterns is obtained.
This paper describes an adaptive array that was designed to improve the carrier-to-interference ratio (C/I) delivered to base station radios by 6 dB in U.S. 800 MHz analog cellular networks. The C/I performance of this kind of system is difficult to verify, because it cannot be characterized in terms of traditional antenna specifications such as beamwidth and directivity. This paper describes a simple C/I measurement strategy in which the antenna under test and a collocated reference antenna are placed into simultaneous operation in an actual cellular network. Relative C/I performance can then be deduced from a statistical analysis of the antenna outputs. This method is particularly well-suited to software radio based systems, because no special test equipment is required to gather the necessary data.
In this paper, we present a new technique for measuring the input impedance of balanced antenna systems. The process uses standard two-port scattering parameters for balanced antennas, feeding each of the balanced input ports as the port of a two-port. The scattering-parameters will be related to the designed input impedance which may be obtained by post-processing the data. In addition, the scattering-parameters may be used to check for the assumed balance of the system. Both experimental and simulated results will be presented to validate the technique.
The amplitude of a point target observed in an ISAR image is equal to their free space RCS when effective sidelobe windowing is used. Likewise, its location in the image is identical to its actual location. The interpretation of observed amplitude and dimension of area targets is not as easy. The ISAR image of a rectangular flat plate formed by rotating it around its longer axis is significantly different from an ISAR image of the same plate rotated about its shorter axis. Both the amplitude and the size of the plate's image are different. In this paper, the theory of physical optics is reviewed in conjunction with the principles of ISAR processing to explain these differences.
ERIM is currently investigating several near-field to far-field transfonnations (NFFFfs) for predicting the far-field RCS of targets from monostatic near-field measurements. Each of the techniques uses approximate tions and/or supporting information to overcome the need for the bistatic near-field data which is required to rigorously transfonn a target's scattered field from the near zone to the far zone. Our focus has been on spheri cal near-field scanning, since this type of collection geometry is most compatible with existing RCS ranges. One particular NFFFT is based on the reflectivity approximation commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is the most computationally efficient technique under con sideration, because, despite its theoretical underpinnings, it does not explicitly require image fonnation as part of its implementation. This paper presents an efficient discrete implementation of the image-based NFFFT, along with numerically-simulated examples of its perfonnance. The advantages and limitations of the technique will be discussed. A simplified version which applies to high aspect ratio (length-to-height) targets and requires only a single great circle (waterline) data in the near field is also summarized.
Due to inherent cost, safety and logistical advan tages over dynamic measurements, Ground-to-Ground (G2G, aircraft and radar on tarmac) diagnostic radar measurements may be the preferred method of assessing aircraft RCS for signature maintenance. However, some challenging complications can occur when interpreting SAR imagery from these systems. For example, the effect of ground induced multi-path often results in the measurement of a significantly different image based RCS than would have been obtained by a comparable Ground-to-Air (G2A) or Air-to-Air (A2A) system. Although conventional 2-D SAR images are useful in determining the physical source (down-range/cross range) of scatterers, it is difficult at best to deduce whether an image pixel is a result of direct (desired) or ground induced multi-path (undesired) scattering. ERIM and MRC recently completed an experiment testing the utility of collecting and processing interfero metric (2-antenna) SAR radar data. This effort produced not only high resolution SAR imagery, but also a com panion data set, derived from interferometric phase, which helps to isolate the source (direct or multi-path) of all scattering within the SAR image. Additionally, the data set gives a measure of the physical height of direct scatterers on the target. This paper outlines the experiment performed on a RCS enhanced F-4 aircraft using a van mounted radar. Conventional high resolution imagery (down-range/ cross-range/intensity) will be shown along with down range/height/intensity and cross-range/height/intensity images. The paper will also describe the processing pro cedure and present analysis on the interferometric results. The unique motion compensation processing technique combining prominent point and motion mea surement instrumentation data, eliminates the need for a tightly controlled collection path (e.g. bulky rail sys tems). This allows data to be collected with the van driven somewhat arbitrarily around the target with side mounted antennas taking measurements at desired aspects.
This paper examines the possibility of increasing the speed of Near-Field measurement of an Antenna, by reducing the number of measurement points and by determining the degree of truncation permissible while maintaining a prescribed degree of precision of the reconstructed far-field. The Near-Field of a planar radiating array is analysed in depth. A formulation and a procedure to correct the spectral domain of the field are established. It is shown that correction in the spectral domain can improve the accuracy of the Far-Field while using the same amount of Near-Field data. The technique has a good potential to be applied to Near Field data of large radiating Antennas leading to new information about the accuracy and speed of measurement achievable.
Several approaches are known for the identification of noncooperative air-borne targets with radar. Assuming that the tar get can be tracked during a certain flight path, observations from different aspect angles will be obtained. High-resolution radar (HRR) systems use these observations to create one-dimensional range profiles. With Inverse Synthetic Aperture Radar (ISAR) the data from all observed aspect angles are combined to obtain two-dimensional images. In recent years, techniques for resolution enhancement have been developed for both techniques. The choice for one of the two approaches should depend on the applicability of the target representation for identification. ISAR is the most suitable for reproduction on a display and identification by human observers. In case of identification by a machine, for example an algorithm on a computer, the choice is not straight forward. In this paper an overview of the influence of several errors on the performance of HRR and ISAR will be given. The error sources that will be evaluated are: • uncertainty of the absolute distance of the target; • errors in the mutual alignment of observations; • additive noise. The errors are generated numerically and applied to data from simulations and low-noise measurements. The influence of the bandwidth and angular span on the quality of the target reconstruction will be regarded as well as the performance of some high-resolution techniques. Finally, conclusions are drawn concerning the applicability of ISAR and HRR.
In an earlier paper ("System Engineering for a Radome Test System," John R. Jones, et al, AMTA, October 1994) the system level design of a compact range enhancement for the testing of the Triband Radome was presented. This paper will discuss the installation and testing of the radome measurement system in the compact range. The purpose of the radome measurement system is to determine (within close tolerances) boresight shift, transmission loss, antenna pattern changes and polarization effects caused by the radome. Unique features include novel coordinate transformation and correction by means of a laser autocollimator and data reduction algorithms. Also featured is the tracking subsystem which consists of a specially designed two-axis track pedestal, an autotrack controller, and three five-horn compact range feed arrays operating at X, K, and Q-bands. The performance of the triband radome measurement system in the compact range setting will be presented.
The development* of a real time electronic system to accurately measure the pattern of high gain, ultralow sidelobe level antennas in the presence of multipath scatterers is described. Antenna test ranges contain objects that scatter the signal from the transmitting antenna into the main beam of a receiving antenna under test (AUT), thereby creating a multipath channel. Large measurement errors of low sidelobes can result. The design and computer simulation of an Antimultipath System (AMPS) is complete. Fabrication of a feasibility demonstration model AMPS to operate with rotated AUTs to suppress indirect (scattered) components and permit accurate pattern measurements is almost done. Results to date show the likelihood of measuring sidelobe levels 60 dB below the main beam. * This project is sponsored in part by the Air Force Material Command under Rome Laboratory Contract Nos. F30602-92-C-0009, Fl9628-92-C-0130 and F 19628-93-C-02 l4.
Due to rapid growth in the RF commercial market, new thinking is required in antenna measurement techniques. Certain customers, such as those designing cellular base station antennas, have unique requirements. One example of this is accurate front to-back ratio measurements. This is a difficult measurement to make inside an anechoic chamber, particularly at the currently used commercial frequencies. This paper focuses on a technique for measuring front-to-back ratio, which involves averaging patterns collected at different test antenna positions in order to resolve the chamber back wall reflection from the antenna back lobe measurement.
This paper reports on the results of computer simulations of planar near-field scanning and its ability to achieve an high accuracy test-zone field over a wide range of pattern angles. An quality test-zone field was defined for this study to have less than 0.2 dB peak-to-peak amplitude variation and less than 1.5 peak-topeak phase variation. This investigation sought the minimum scan length, for a given critical angle, ec and separation, S. The minimum scan length determined from this investigation is given by: L = D + 2S(tan(0c)) + 20/cos(0c). This scan length is approximately 60),, larger, for a critical angle of 70 degrees, than previously accepted. It is suggested that the maximum practical value of Sc is between 60 and 70 degrees. The use of raised cosine amplitude and/or quadratic phase windows to the edges of the measurement plane is shown to provide test-zone field quality improvement and/or allow scan lengths approximately 10),, smaller.