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Diagnostic tools for the determination of the excitation coefficients of a multifeed antenna based on pattern measurements are extremely useful during a spacecraft antenna design. Due to the complexity of state of the art multifeed antennas, it is not straight forward to trace back to the location of possible error sources, if deficiencies or non-compliance's are detected during an antenna measurement campaign. Therefore a method was developed and tested at DSS which directly determines all effective excitation coefficients from pattern measurements. The method approximates the measured composite array pattern a set of computed element beam pattern, weighted by a set of unknown excitation coefficients. The resulting equation system is solved using the Method of Moments (MoM). The tool was extensively tested at DSS. The accuracy obtained for the calculations of the coefficients was in the 2% range beeing compareable to the accuracy of Beam Forming Network (BFN} measurements using a network analyser. In this paper the theoretical background of the method as well as some application cases will be described.
This paper examines antenna measurement errors attributable to instrumentation, and their effect on measurement uncertainty.
A sophisticated software package FARANA (FAR-field ANAiysis) is presented for transforming planar near-field test data to far-field antenna patterns, including enhanced analysis of far-field results. FARANA is coded in MATLAB version 5.0. MATLAB (MATrix LABoratory) is an interactive mathematical modelling tool based on matrix solutions without dimensioning. Using MATLAB, numerical engineering problems can be solved in a fraction of time of time required by programs coded in FORTRAN or C. FARANA operates with a state-of-the-art graphical user's-interface, is intuitive to use and features high speed and accuracy. This paper addresses an assessment of the program, discusses its use and enhanced far-field analysis capabilities.
Cellular and PCS basestation antennas are basically arrays with highly directive elevation patterns and broad azimuth patterns. This causes measurement problems because they are large but not directive in both principal planes. As a result, the pattern measurements of these antennas that have been performed outside have been unreliable in many cases because they are very receptive to interference and range clutter. Thus, one wants to move inside but the antenna size can significantly impact the overall range cost. This paper describes a very practical solution to this problem. Since basestation antennas are long and narrow, one can use a near field scanner approach to deal with the length. In fact by using a sectorial horn probe, the narrow dimension of the antenna-under-test is illuminated by a cylindrical wave. Thus, the scanner need only probe the field along the antenna length. This linear scan data can then be transformed to generate the desired far field elevation pattern. The details of this novel design will be described as well as the results, to illustrate the system capability and accuracy.
A technique was developed to recover the near-field function on a larger data set than the one that is measured. It requires the preliminary determination of functions containing the information relating the two data sets. The simplest way of obtaining such a function is to measure the near-field function on the larger and the smaller data set. This seems to be a drawback to the technique. However. after making one such pair of measurement it is therefore non necessary to do so again and the field of the antenna can be obtained, from the smaller data set measurement, with comparable accuracy. The technique is somewhat different when compensating for a sampling rate reduction. However, in both cases an analytical extension is required to fill the desired domain of definition, followed by a division. In the case of the sampling area the division of the spectral functions f2 by f1 is made in the spectral domain while in the case of the sampling rate the division of the near-field functions E' by E is made in the near-field domain. An experiment was performed to demonstrate the applicability of the above technique. A full near-field measurement of a linear array antenna was performed and processed, then after displacement of the antenna, measurements were done, in one case, on a truncated smaller scan area, and in another case with a larger sampling interval. The technique was applied to recover the complete far-field characteristics of the antenna from the smaller data set. The far-field characteristics of the antenna obtained by this technique were shown to be very similar to the results obtained from a more complete near field measurement.
A method is presented for computing far field antenna patterns from spherical near field antenna measurement data. The new method utilizes a novel Uniform Geometrical Theory of Diffraction (UTD) based transformation of spherically scanned antenna tangential electric (or magnetic) near field measured values to more efficiently obtain the antenna far field. Examples illustrating the accuracy and speed of UTD based spherical near to far field transformations for large to moderately large antennas are presented.
The far-field parameters of an antenna are obtained from near-Field measurement with an accuracy that is limited by the sampling area and the sampling rate used to collect the measurement data. It is therefore important to know the relation between the far-field parameters and the sampling parameters. A parametric study of the far field parameters accuracy versus the sampling parameters was made. In order to determine the optimal choice of the sampling parameters to achieve the desired far-field accuracy, planar near-field measurements of a linear array were performed in an anechoid chamber at the Canadian Space Agency. A program performing Fast-Fourier Transform was used to process the data and to obtain spectral domain and reconstruct the far field patterns. A methodology developed in [1] was used to compare different spectral and far field patterns obtained from different sampling conditions. Parametric curves were developed for the far-field parameters such as gain, beam pointing, beam width, sidelobes, etc.
The image-based near-field to far-field transformation is based on a reflectivity approximation that is commonly used in ISAR imaging. It is a limited but computationally efficient transform whose accuracy, for appropriate targets, rivals that of computationally more intense transforms. Previous results include applications of the transform to lOA. long wire and lOA. long conesphere numerical data. Here, 1-D and 2-D versions of the transform are applied to conesphere near-field measurements data and the results are compared to corresponding far-field measurements data. Transform errors obtained for these data are compared to corresponding results obtained using newly generated near-field and far-field numerical data. The image-based transform is believed to be especially applicable to the far-field correction of near-field measurements of complicated targets like aircraft or vehicles that are too large or too poorly defined to be simulated numerically.
Far-field range testing has been the standard at the Southwest China Research Institute of Electronic Equipment (SWIEE) and at other facilities in mainland China. SWIEE has recently commissioned a new spherical near-field measurement system from Nearfield Systems Inc. (NSI) and Hewlett Packard (HP) to improve its antenna measurement capability. The near-field system provides significant advantages over the older far-field testing including elimination of weather problems with outdoor range testing, complete characterization of the antenna, and improved accuracy. This paper will discuss the antenna types at SWIEE tested with the NSl/HP near-field system, and the results being achieved.
The measurement of the characteristic antenna data by means of conventional far-field ranges in frequencies up to 200 GHz requires measurement distances of some kilometers. The high atmospherical attenuation and the low available transmit power limit the dynamic range of the measurements considerably. The DASA Compensated Compact Range (CCR) /1/ is a high precision test facility; which avoids these disadvantages and allow measurements with considerably higher accuracy under controlled environmental conditions. The precision reflectors have an extremely high surface accuracy of 25 µm RMS, which allow their use even in the mm-wave range. For the frequency band of about 200 GHz, the relative roughness is in the order of N/60. This results in considerably lower degradation for the DASA CCR compared to the typical degradation on far-field ranges (N/16). For mm-wave application the test facility is equipped with broadband transmit and receive moduls, which covers the frequency range from 75 to 220 GHz. The basic transmit frequency is generated in a tunable Gunn oscillator, which is phaselocked to an externally supplied I 0 MHz reference signal. This optimized concept allows measurements with a dynamic range of more than 60 dB at 200 GHz. For a cost efficient solution the complete equipment for the transmit and receive moduls consists of commercial components. Keywords: MM-Wave Antenna Measurement, Compensated Compact Range, MM-Wave Transmit Module Tracking Converter
The DTU-ESA Spherical Near Field Antenna Test Facility in Lyngby, Denmark, which is operated in a cooperation between the Danish Technical University (DTU) and the European Space Agency (ESA), has for an ex tensive period of time been used for calibration of Standard Gain Horns (SGHs). A calibration of a SGH is performed as a spherical scanning of its near field with a subsequent near-field to far-field (NF-FF) transformation. Next, the peak directivity is determined and the gain is found by subtracting the loss from the directivity. The loss of the SGH is determined theoretically. During a recent investigation of errors in the measurement setup, we discovered that the alignment of the antenna positioner can have an extreme influence on the measurement accuracy. Using a numerical model for a SGH we will in this paper investigate the influence of some mechanical and electrical errors. Some of the results are verified using measurements. An alternative mounting of the SGH on the positioner which makes the measurements less sensitive to alignment errors is discussed.
For frequencies above 30 GHz, RCS reference target method is, in general, more accurate than scanning the field by a probe. Application of mechanically calibrated targets with a surface accuracy of 0.01 mm means that the phase distribution can be reconstructed accurately within approximately 1.2 degrees across the entire test zone at 100 GHz. Furthermore, since the same result can be obtained for both azimuth and elevation patterns, all data is available for the characterization of the entire test zone. In fact, due to the fact that the reference target has a well known radar cross-section, important indication of errors in positioning can be obtained directly from angular data as well. In the first place the data can be used in order to recognize the first order effects (+/- 5 degrees in all directions). Applying this data, defocussing of the system reflector or transverse and longitudinal CATR feed alignment can be recognized directly. Furthermore, mutual coupling can be measured and all other unwanted stray radiation incident from larger angles can be recognized and localized directly (using timedomain transformation techniques). Inmost cases even a limited rotation of +/- 25 degrees in azimuth and +/- 10 degrees in elevation will provide sufficient data for analysis of the range characteristics. Finally, it will be shown that sufficient accuracy can be realized for frequencies above 100 GHz with this method.
This paper discusses the efforts of an on-going research program which has been exploring the use of expert systems (artificial intelligence) techniques to support automated analysis of wideband radar scattering data. The primary objective of this research is to explore and demonstrate the applicability of expert system techniques to the analysis of diagnostic radar images. There are two modes which are being explored. The first is an automated system that would allow lesser skilled (in radar imaging science) individuals to do the work of highly skilled engineers and analysts. The second mode would aid the highly skilled worker with the application and correct implementation of software tools, interpretation of phenomenology, and data quality assessment. In both cases, the expert system should allow for the increase through-put and accuracy of data being analyzed. A software prototype is being developed and tested with real data to demonstrate the feasibility and potential accuracy of such as system.
Instrumentation Radar systems evolution includes improved stability. Metrologists know frequency within Hertz. Amplitude and Phase variations are low. Ranges check drift with reference systems. Still, with increased capability, expectations of accuracy have increased. Todays instrumentation makes analysis of stability factors practical. This study analyzes Radar Cross Section (RCS) return of a stable target under controlled conditions. Methodology will be an analysis of a constant RCS target return. The target is a stable object at a typical measurement site. Data points are at several discrete frequencies in bands between S and Ku. This study sample is a set of data taken over a 87 hour span with several duty factors. Duty factors will range from minimal 0.1% to 1.5%, near the 2% maximum for the output amplifiers. Acquisition times for data sets are chosen for outdoor temperatures ranging from hot -- desert afternoon -- through cool in the early morning. This data will be analyzed statistically. If statistical correlations exist, analysis will quantify factor contributions with multiple linear regression. Hypothesis: Drift does not correlate to variables such as duty factor, & temperature.
This paper addresses some of the practical considerations and numerical consequences of using the Advanced Antenna Pattern Comparison (AAPC) method to improve the accuracy of antenna measurements in compact ranges. Two main issues are of particular importance: 1. Appropriateness of circle-fitting algorithm results to the measured data. 2. Ambiguous circles due to the crowding of data. These issues deal specifically with Kasa’s circle-fitting procedure—an essential part of the AAPC method—and provides useful checks for conditions commonly met with the use of this technique. In addition, we consider the problem of data distribution along the fitted circle, another important element of the AAPC method. Simulation results are submitted in support of the proposed methods.
Gain calibration of circular horns and radiation pattern integration applying patterns in two principle planes only is accurate and does not require large computational or measurement effort. This technique is thus more practical than the integration over the entire angular domain, required in case of rectangular horns. However, for many types of AUT’s, additional errors may occur due to the differences in aperture size of the AUT and standard gain horn. The AUT will in many cases have physically larger aperture dimensions. Consequently, unknown test-zone field variations across this aperture can result in additional errors in gain determination. The new method uses a flat plate as a reference target. An RCS measurement of the flat plate is used to derive test-zone field characteristics over the same physical area as the AUT. Combined with the accurate gain calibration described above, field information is available over the entire area of interest and the accuracy in gain determination is increased. In this paper, experimental results and practical considerations of the method will be presented.
This paper reports on the results of computer simulations of planar near-field test-zone-fields. Techniques for the improvement of the quality of the test fields are presented and demonstrated. These techniques include the use of larger scan areas and the use of window functions applied to the measured near-field data. Test-zone-field quality is measured by the angular spectrum of the error of the test-zone-field as compared to an ideal plane wave test-zone-field. This investigation sought the minimum scan length, L, for a given critical angle, ?c and separation, S. It is shown that significant improvements in test-zone-field quality can be realized if the test zone is extended from the standard length, Ls=D+2S(tan(?c)) by an amount 20?/cos(?c). This scan length is approximately 30? larger, for a critical angle of 50 degree and 60? larger, for a critical angle of 70 degrees, than the standard length. A raised cosine amplitude/quadratic phase window applied to the measured near-field data can significantly reduce scan length requirement while maintaining the increased accuracy of the extended scan length. The recommended scan length with window is given by Lw=D+2S(tan(?c))+2W, where W is the length of the window applied to each end of the scan measurements. The window description and required length are presented.
So far, Azimuth-over-Elevation rotators on RCS pylon tips were of large size (typically 10” for 500 lb. load, over 2’ for a 6000 lb. load). Therefore, RCS measurements of small but heavy targets were very difficult if not impossible to perform. The new design supports loads of 5,000 lb. with an Azimuth turntable diameter of only 136 mm, close to the pylon’s maximum width. The Azimuth and Elevation axes mechanisms are hidden inside the pylon body. The Azimmuth rotator is mounted on the top surface of the elevation main plate. The Elevation plate is attached to the pylon tip on one side and on the other side to the actuator, which is attached to the base of the tip. The actuator drives the Elevation plate to the required rotation angle. Even with its small size, the new design does not compromise on performance. The Azimuth axis moves a full 360° continuous motion at 22 deg/min with 0.03° accuracy, 0.03° backlash and 0.01° repeatability. The Elevation axis moves in a 0°-40° sector at 1.5 deg/min with 0.05° accuracy, 0.05° backlash and 0.01° repeatability.
For indoor antenna measurements, compact ranges or near-field/far-field techniques are most frequently used. One of the major problems is the handling of physically large antennas. Compact ranges will in general provide test-zone sizes up to approximately 5 meters in diameter. Applying the planar NF/FF technique, even larger test-zone sizes can be realized for certain applications. On the other hand, requirement of real-time capability, for instance in production testing, will exclude NF/FF techniques. It has been shown previously that single-plane collimators have a pseudo real-time capability which makes these devices comparable to compact ranges. Furthermore, the physical test-zone sizes which can be realized when compared to compact ranges are approximately 2-3 times larger for the same size of the anechoic chamber. Finally, it will be shown that the accuracy in sidelobe level determination, gain and cross polarization is considerable higher than with other indoor techniques, even at frequencies below 1 GHz.
Radar cross section (RCS) range characterization and certification are essential to improve the quality and accuracy of RCS measurements by establishing consistent standards and practices throughout the RCS industry. Comprehensive characterization and certification programs (to be recommended as standards) are being developed at the National Institute of Standards and Technology (NIST) together with the Government Radar Cross Section Measurement Working Group (RCSMWG). We discuss in detail the long term technical program and the well-defined technical criteria intended to ensure RCS measurement integrity. The determination of significant sources of errors, and a quantitative assessment of their impact on measurement uncertainty is emphasized. We briefly describe ongoing technical work and present some results in the areas of system integrity checks, dynamic and static sphere calibrations, noise and clutter reduction in polarimetric calibrations, quiet-zone evaluation and overall uncertainty analysis of RCS measurement systems.