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Compact ranges have found wide application for antenna measurements, RCS measurements, and, most recently, for spacecraft payload measurements. Each of these ap plications requires certain special features of the range optics, positioning systems, electronics, and software. The system design of a compact range measurement sys tem for making all these types of measurements presents a number of challenges. This paper will discuss the system aspects of the design of a multi-purpose compact range facility. Items of inter est include the RF electronics design, the positioning sys tem design, the optimization of the reflector and feeds and the specialized software design.
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.
The ESAT-TELEMIC division at Katholieke Universiteit Leuven (KUL) has three antenna ranges: an indoor Far-Field range, an indoor planar Near-Field range and an outdoor Far-Field range. The positioning equipment is of a variety of manufacturers. The division launched an effort to modernize the range complex and add automatic measurement capabilities, while still retaining control of all three ranges from one control console and using one positioner controller, one angle readout and a single receiver to save costs. The system upgrade included some electrical refurbish ment of the positioning equipment and the replacement of all the old control and data recording equipment with Orbit Positioner Controller/Programmer, Power Control Unit and combined Near-Field and Far-Field software. Control of all three sites is achieved using a special Orbit Junction Box. With the new configuration all three ranges can be operated in fully automatic mode, one range at a time. The software package controls both Near-Field and Far Field measurements using compatible data formats and human interfaces.
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.
The RCS of extended objects measured in the near field is subject to errors induced by the spherical nature of the incident and scattered wavefields. A number of techniques have been applied to estimate far-field responses from results of monostatic near-field measurements. While the results indicate successful transformations for linear scatterers, the lack of a sound theoretical basis brings into question the appli cability to general objects. The paper explores the theoretical basis of the far-field transformation of RCS data and the consequence of the limited data obtained from monostatic measurements. The limitations of approaches reported to date [1-4] are explored from conceptual and physical con siderations with the goal of establishing reasonable expectations for practical methods. Examples using simulated and measured near-field data are presented to illustrate successes and failures of the algorithms in transforming results to far-field RCS.
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.
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.
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.
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.
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.
The radiation characteristics for an active phased array receive antenna operating at K Band were measured at the Ipswich Research Facility. On-pole and cross-pole radiation patterns were measured for several scan angles. In this paper we'll discuss the general design of the antenna and the instrumentation ensemble used to perform the far field and near field characterization of this antenna. Measurements taken on a 2600 foot far field range vs. a near field planer scanner are compared.
This paper demonstrates a high speed antenna measurement capability recently developed in the MDTI Radar Measurement Center. Originally constructed as a Radar Cross Section facility, the RMC has added the capability to measure antenna patterns on apertures up to 40-feet in length in the far field. Data will be presented to demonstrate system performance through the use of modern output formats, such as global plots and videotape presentations.
Accurate multiwavelength radiometric remote sensing of the ocean and the atmosphere from an aircraft requires antennas with the same beamwidth at the various frequencies of operation. Scientists at the National Oceanic and Atmospheric Administration designed an offset antenna with a pressure-compensating corrugated feed horn to meet this criterion. A specially designed fairing was incorporated into the antenna to optimize the aerodynamics and minimize the liquid buildup on the antenna surfaces. The antenna has two positions: the zenith (up) position and the nadir (down) position. The planar near-field facility at the National Institute of Standards and Technology was used to determine the far-field pattern of the antenna. The results show that the antenna beamwidths at 23.87 and 31.65 GHz are nearly the same as expected from the design criterion. This antenna was recently used in an ocean remote-sending experiment and performed according to expectations.
The RMS surface error predicted by holographic measurements is often smaller that the one predicted by radiometric measurements. At the FCRAO, a difference of 40% was observed for the 14-millimeter radio telescope. To find the explanation for this discrepancy, simulations and additional near-field measurements were performed. The near-field measurements were carried out at 91.9 GHz on a part of the aperture of the telescope. This paper describes the near-field measurements and presents a careful comparison between the results from holographic, radiometric and near-field surface error measurements. This comparison and simulations revealed that the main reason for the discrepancy between the radiometric and h9olographic results was the smoothing of the holographic data. The smoothing has been used for reducing the effects of the truncated far-field data in the FFT process.
Microwave holography is an important technique for analyzing electromagnetic fields in close proximity to objects such as antennas or radomes. In this paper, data measured in the far-field and near-field are transformed to the surface of a spherical radome using spherical microwave holography. Further, the fields are calculated on a sequence of spheres concentric to the spherical radome to display the spatial distribution of the fields as a function of distance from the surface of the radome out to five wavelengths from the radome, a movie. This progression demonstrates the ability to locate radome surface defects is severely limited without the use of microwave holography. Three sets of radome defects are presented and range in size from three-eights of a wavelength to three wavelengths. This paper shows that near-field measurements alone are not generally capable of locating defects directly.
Traditionally the Compact Range is not considered a viable method for conducting low frequency (VHF/UHF) antenna or RCS measurements because of the limited electrical size of the collimating reflector system. Normally, compact range measurements are conducted in the extreme near-field or the collimating system where to reflector size is on the order of 25 to 30 wavelengths minimum with at least four wavelength edge treatments. This mode of operation limits measurements to the high UHF band (800 MHz) and above for typical sized reflector systems in use today. Recent research with compact range3s indicates that acceptable VHF.UHF measurements can be conducted in the quasi far-field region of the collimating system with reflectors as small as five wavelengths and with electrically short edge treatments. A good user knowledge of this mode of operation is required to maximize its utility. A qualitative measure of acceptable quiet zone performance must also be established. This paper addresses the theory of operation, practical implementation and inherent limitations of the non-conventional use of the indoor compact range for conducting low frequency measurements.
Test zone field (TZF) compensation increases antenna pattern measurement accuracy by compensating for non-plane wave TZFs. The TZF is measured over a spherical surface encompassing the test zone using a low gain probe. The measured TZF is used in the compensation of subsequent pattern measurements. TZF compensation is demonstrated using measurements taken in an anechoic chamber, far-field range. Extraneous fields produced by reflection and scattering of the range antenna field in the chamber causes the TZF to be non-planar. The effect of these extraneous fields on pattern measurements is shown. Measured TZFs are also shown. TZF compensation results for pattern measurements using a high-gain, X-band slotted waveguide array are presented.
The Three-Antenna gain method is commonly used on far-field ranges to determine an antenna's absolute gain. This is especially true when no other calibrated antenna is available. This method has been used for years by calibration laboratories such as NIST to calibrate probes and gain standards for far and near-field ranges. In some cases, the calibration is too costly or does not meet the schedule requirements of the near-field test range. An alternative is to calibrate the probe or gain standards directly on the near-field range. In this paper we present the results of a study done to show the accuracy of the Three-Antenna gain method when used on a near-field range. An extensive error analysis is presented validating the utility of this method.
It is shown that a phase-tapered aperture may be used to produce a uniform plane wave at Fresnel zone distances. This allows one to perform antenna/RCS measurements at reduced distances relative to a far-field range, but without the illuminator complexity and cost associated with a compact range. An asymptotic expression is obtained for the Fresnel field of a circular aperture field source distribution characterized by a large quadratic phase taper. The field is shown to be equivalent to that produced by a uniform ring source and central radiator, so that the design equations for the ring source and central radiator can be applied to plane wave synthesis using a circular phase-tapered aperture. The asymptotic expression for the field as compared with a numerical evaluation obtained using aperture integration. A simple implementation of a phase-tapered aperture using a radiating source which illuminates an aperture at a distance is presented. A quiet zone field extent which is approximately 70-80% of the source aperture extent is demonstrated.
The advantages of mesh antennas include good storability and low mass for large on-board antennas over 10M in diameter. Their weak point is that surface adjustment is necessary to attain high accurate surface. Surface adjustment traditionally involves the repeated measurement of surface node position with a theodolite system and subsequent cable adjustment. These steps take much time. This paper describes a surface adjustment scheme that uses near field measurement for a modular mesh antenna composed of mesh, cable network and supporting structure. The node positions of the antenna are obtained by back projection of the far field pattern generated from the near field pattern. The cable network has low sensitivity to changes in local node position. The results of tests show that the surface accuracy needed to achieve the required RF performance can be obtained quickly without theodolite systems.