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In this paper a circular planar near-field scan region is considered as an alternative to the commonly used rectangular boundary. It is shown how the selection of this alternative boundary can reduce test time and also to what extent the alternative truncation boundary will affect far-field accuracy. It is also shown how well known single dimensional filter functions can be applied over a two-dimensional region of test and how these attenuate the truncation effect. The boundary and filter functions are applied to measured data sets, acquisition time reduction is demonstrated and the impact on far-field radiation pattern integrity in assessed.
We use a rotating dihedral to determine the cross-polarization ratios of radar cross section measurement systems. Even a small amplitude drift can severely degrade the calibration accuracy, since the calibration relies on accurate determination of polarimetric data over a large dynamic range. We show analytically how drift introduces errors into the system parameters, and outline an analytic procedure to minimize the in.uence of drift to estimate system parameters with greater accuracy. We show that only very limited information about the drift is needed to provide measured system parameters accurate to second order in the error-free parameters. Higher-order accuracies can be achieved by using more detailed information about the drift. We use simulations to explain and illustrate the analytic development of this theory. We also show that, using cross-polarimetric measurements on a cylinder, we can recover the exact system parameters. These .ndings show that we can now calibrate polarimetric radar cross section systems without the large uncertainties that can be introduced by drift.
For many years now, General Dynamics has described the development, characterization, and performance of an image-based circular near-field-to-far-field transformation (CNFFFT) for predicting far-field radar cross-section (RCS) from near-field measurements collected on a circular path around the target. In this paper, we consider the CNFFFT algorithm as an azimuthal filtering process and develop a formulation capable of transforming data that is not measured over a full 360º. Such a formulation has applications in measurement scenarios where collection of a complete rotation is not practical. As part of the development, we provide guidelines for the near-field data support required to achieve a desired accuracy in the sub-360º CNFFFT result. Numerical simulations are provided to demonstrate that the results of this partial-rotation formulation are consistent with the full-circle CNFFFT results presented in past papers.
In a FF antenna range the DUT mechanical Boresight can be aligned with the Range Boresight simply by using a Boresight scope to transfer the DUT mechanical Boresight to the Range coordinate system. This is not applicable to a PNF Range; hence, another transfer device and different transfer methods are required. This paper describes the development, testing and referencing of an existing PNF range to a reference optical cube that serves as the coordinate system transfer device. The optical measurement system employs an automated total station Theodolite system, incorporating true 3D positioning of the NF probe along defined axes of movement. The data collected is processed to best fit a straight line defining the vector representing the axis. The scanning PNF plane is defined with high accuracy, by a geometrical representation of two (or more) axes in that plane. Thus, the scan plane coordinate system was transferred by auto collimation methods to the reference optical cube. A second optical cube must be placed on the AUT to be used as a reference for its mechanical Boresight. When the AUT is set up for testing, the coordinate systems are transferred from each cube to the other by means of co-collimation using a temporarily positioned Theodolite combination.
A new algorithm for the amplitude-only characterization of Compact Antenna Test Ranges (CATRs) is presented. The algorithm applies a successful strategy to retrieve the missing phase of the field in the quiet zone. Particular care is devoted to facing the issue of the typically large electrical dimensions of CATRs and to obtaining the necessary accuracy by the use of an “efficient” representation of the radiated field. This is accomplished through a Jacobi-Bessel expansion of the aperture field which allows to keep low the overall number of unknowns and to improve the accuracy and the reliability of the algorithm. The presented numerical analysis, based on realistic CATR simulations by means of GRASP8-SE, shows the feasibility of the algorithm to estimate amplitude and phase of the quiet zone field within an acceptable accuracy.
A large rolled edge compact range system featuring a 12’H x 16’W quiet zone has been designed, fabricated, installed, and tested in a large aerospace test facility. During the program, a high precision alignment methodology was utilized in conjunction with electromagnetic prediction capability to verify both mechanical and electrical performance while still under trial assembly conditions at the factory. A coherent laser radar (CLR) was utilized to measure the reflector surface on a very fine grid, and the electromagnetic (EM) quiet zone performance was calculated from the raw CLR data using a Physical Optics (PO) model. Despite extremely high surface accuracy of the panels, this evaluation methodology highlighted systematic alignment errors in the reflector system, and guided the process of correcting these errors to achieve a final factory verification assembly for the entire 20’H x 24’W reflector system of better than 0.001” over the quiet zone section of the reflector, and 0.004” rms over the entire reflector. This procedure was also utilized for the on-site installation to achieve alignment of the reflector to an AUT positioning system using the CLR, as the positioning system and chamber were already existing and operational. Thus, it was required to align the reflector to the positioning system, and not the positioning system to the reflector as is usually the case. A unique vertical carousel feed system was also aligned using this procedure. Predicted EM results were again used to finalize alignment on site prior to quiet zone field probe evaluation. This paper summarizes the overall alignment and EM evaluation process, and presents results for the installed compact range reflector system.
This paper presents a generalized nonlinear interpolation technique for generating 3D antenna radiation patterns from 2D cross sections. The motivation for this work is that most of the patterns provided by antenna manufacturers are only available as vertical and horizontal cross sections. Accurate propagation calculations, however, require gain values at arbitrary orientations, corresponding to points on a 3D gain surface. After reviewing the current methods of generating such a gain surface, we find that linear interpolation algorithms seem the most promising, even though they can often lead to pronounced mathematical artifacts. To overcome these shortcomings a new nonlinear algorithm is proposed. The new approach mitigates, and in most cases eliminates, the artifacts produced by linear interpolation weights. The new method is fast, yields smooth, more realistic surfaces that are consistent with the vertical and horizontal cuts, exhibits diminished mathematical artifacts, and improves the accuracy of propagation calculations of radio frequency signals. Representative examples from the application of the new algorithm to cellular base station antenna patterns will be presented.
This paper presents the design, development and testing of an inverted Stewart platform for suspending and positioning targets during RF antenna and signature testing. Previous string target support systems use multiple string attachment point configurations that do not allow the target roll or pitch to be modified during the azimuthal data collection. This presentation will discuss an in-house development of a scale model target support system that allows for high accuracy simultaneous target roll and pitch positioning. The inverted Stewart platform also offers unique stability of the target by damping out the torsional pendulum motion typically encountered in conventional string support systems. In this paper we will also discuss the advantages and disadvantages of the string support concepts and provide design guidance for a building an inverted Stewart platform support system. If possible, a simple squat calibration standard will be measured to assess the quality and precision of this novel support system.
Spherical near-field measurements have become a common way to assess performance of a wide variety of antennas. Published reports on range error assessments for spherical near-field ranges however are not very common. This is likely due to the perceived additional complexity of the spherical near-field measurement process as compared to planar or cylindrical measurement techniques. This paper will establish and demonstrate a simple procedure for characterizing the performance of a spherical near-field range. The measurement steps and reporting can be largely automated with careful attention to the test process. We will summarize the process and document the accuracy of a spherical near-field test range at NSI using the same NIST 18 terms commonly used for planar near-field measurements.
The measurement of the radiation patterns of the PLANCK Radio Frequency Qualification Model (RFQM) is one of the most important elements of the verification of the PLANCK telescope. PLANCK is one of the scientific missions of the European Space Agency and is devoted to observe the Cosmic Microwave Background radiation, with unprecedented accuracy. The satellite payload consists of two state-of-the-art, cryogenically cooled instruments sharing a dual reflector telescope with 1.5 m aperture and covering the frequency range from 27 GHz to 1000 GHz. As a key part of the telescope verification logic, the radiation patterns of the RFQM has been measured in the Alcatel Alenia Space Compact Antenna Test Range (CATR) at four frequencies (30, 70, 100 and 320 GHz) using representative flight feed horns of the focal plane unit. This paper presents the test logic, the measured radiation patterns, the custom-made instrumentation set-up, the correction techniques used and the final link to the Flight Model verification.
Adaptive array based antenna pattern comparison technique is presented in this paper. In the method, the antenna pattern of the antenna under test (AUT) is measured several times at different positions in the quiet-zone. The corrected antenna pattern is obtained by taking a weighted average of the measured patterns. An array synthesis algorithm is used to obtain averaging weights at the different rotation angles of the AUT. In addition, the weights are adapted specifically for the AUT. The adaptive array correction technique is demonstrated in a hologram based compact antenna test range (CATR) at 310 GHz. The demonstration is based partly on the measurements and partly on the simulations. For verification, the accuracy provided by the method is compared to the accuracy provided by the uniform weighting.
Hand-made scale models in antenna measurements have been used since the late 1940s. Today, aircraft models are fabricated using a stereolithography (SLA) process and the Computer Aid Design (CAD) for manufacturing the full size aircraft. This is the fabrication method used for the V-22 1/15th scale model. Once the SLA machine is programmed, these models are very inexpensive to produce. In this paper, antenna patterns measured on the V-22 scale model are compared with antenna patterns measured on the aircraft in-flight. Comparison of the patterns shows high correlation. Figure 1 V-22 Aircraft
Position uncertainty in antenna measurements is unavoidable. This is due in part to mechanical inaccuracy in the fixturing and positioning equipment. For many classes of antenna, there is also not an obvious choice of reference point, due to lack of a well-defined phase center. It has been shown [1] that a UWB transfer function measurement, taken either in the time or frequency domain, is highly sensitive indicator of antenna displacement. Extraction of the linear phase from the transfer function data results in a uniquely defined distance for any given pair of antennas in a given orientation. When a two- or three-antenna measurement using identical antennas is performed, the result is a unique reference plane for the antenna. Unlike the phase center, is not tied to a particular frequency. Here, using frequency domain measurements of monopoles, ridged horns, and an end-fed biconical antenna, we show that distances can be extracted with a high degree of repeatability. Resolution on the order of 1 part in 5,000 can be obtained in a 4-meter chamber with measurements extending to 20 GHz. Thus, variation in the extracted distance should be a highly sensitive indicator of positional inaccuracy.
In antenna measurements, the orientation of the antenna under test (AUT) is very important. The orientation here refers to the antenna placement in a plane perpendicular to the incident wavefront. For a linear polarized antenna, the antenna should be oriented parallel to the co-polarized component of the incident fields. A small error in the orientation can lead to a drop in the measured gain and an increase in the measured cross-polarization level. In the case of a circularly polarized antenna, it is not obvious how the antenna should be oriented. If the quiet zone fields (incident wavefront) have no cross-polarized component, then the orientation does not affect the measured data. However, when the quiet zone fields have a cross-polarized component, which is true for almost all test ranges, the measured gain and cross-polarized level can vary significantly with the antenna orientation. In this paper, the measured data is used to show the effects of antenna orientation on a circularly polarized antenna. The reason for the variations in the measured data with antenna orientation is discussed. A simple method to improve the measurement accuracy is presented.
The time delay behavior of antennas is of high importance for high accuracy localization and navigation systems. Next to the investigation of the receiving antennas, the transmitting antennas are of substantial interest, too. In the application envisioned these antennas are small dipoles integrated in a battery powered miniaturized transmitter system. The method described in this paper is based on the measurement of the time difference of arrival of a broadband signal in a synchronized setup. This setup consists of the transmitter under test which transmits a bursted sequence of the localisation waveform. The receiving side of the measurement system consists of two antennas, where one works as a reference antenna (with fixed position in relation to the transmitter) and the other works as “classical” probe antenna. Two synchronized tuners and data acquisition systems determine the time difference of arrival of the signal. Detailed measurements of different transmitters have been performed in the 2.45 GHz ISM band and will be presented.
This paper presents measurements of the CloudSat Collimating Antenna (CA) as fabricated for the 94.05 GHz CloudSat radar, which is to be used to measure moisture profiles in the atmosphere. The CloudSat CA is a 3 reflector system consisting of the 3 "final" (relative to the transmitted energy) reflecting surfaces of the CloudSat instrument. This assembly was fed by a horn designed to approximate the illumination from a Quasi-Optical Transmission Line (QOTL). This same horn was employed as a "standard" for measurement of the CA gain via substitution, and its patterns were also measured (this substitution represents a departure from the standard insertion loss technique in the near field range). The CloudSat CA presented a substantial measurement challenge because of the frequency and the electrical size of the aperture is approximately 600 wavelengths in diameter, with a nominal beamwidth of 0.11 degrees. In addition, very high accuracy was needed to characterize the lower sidelobe levels of this antenna. The CA measurements were performed on a 3122-ft outdoor range (this distance was 41% of the far field requirement), which were immediately followed by measurements in an indoor cylindrical Near Field (NF) range. The instrumentation challenges, electrical, mechanical, and environmental are described. Comparison of the outdoor vs. indoor pattern data is presented, as well as the effect of the application of tie-scans to the near field data.
The performance of modern Satellites Antennas and Payloads is characterized by physical parameters like e.g. Antenna Pattern and Gain; EIRP, Flux Density, G/T and the overall PIM-performance. The available time frame for measurement of these parameters is getting constantly shorter. The EADS Astrium GmbH Compensated Compact Range (CCR) allows a time efficient measurement of all payload parameters with high accuracy under controlled environmental conditions. In addition to an efficient measurement facility high-performance measurement equipment is required. The economical budgets of most space programs demand the application of well-known measurement techniques in a cost efficient way. EADS Astrium GmbH supported by Agilent Technologies GmbH has developed an easy to handle and therefore cost optimized measurement platform for Satellite Payload Measurements. This platform consists mainly of a generic Agilent switch matrix operating up to 40GHz which can be connected to a wide range of measurement equipment. The matrix allows a highly flexible routing of the RF uplink and downlink signals including reference paths. Integrated and/or external RF components, like amplifiers, attenuators, and hybrids can be added to the paths, depending on the required test configuration. Starting from a minimum configuration the system can be modularly upgraded to satisfy any further test requirements. The software interface utilizes standard protocols and can be therefore easily addressed by any user specific measurement software. The EADS Astrium GmbH Advanced Antenna Measurement System (AAMS) includes an optional payload toolbox which provides a modular concept expandable for additional test functions.
DSO National Laboratories (DSO) has commissioned a state-of-the-art combined near-field and far-field antenna test facility in 2004. This facility supports highly accurate measurement of a wide range of antenna types over 1–18 GHz. The overall system accuracy allows for future extensions to 40GHz and higher. The 11.0m x 5.5m x 4.0m (L x W x H) shielded facility houses the anechoic chamber and the control room. As the proffered location for this indoor facility is on top of an existing complex instead of the ground floor, antenna pickup is facilitated by a specialized loading platform accompanied by a heavy-duty state of the art fully automated 2.0m x 3.0m (W x H) sliding door, as well as an overhead crane that spans the entire chamber width. Absorber layout comprises 8-inch, 12-inch, 18-inch and 24-inch pyramidal absorbers. The positioning system is a heavy-duty high precision 3.6m x 2.9m (W x H) T-type planar scanner and AUT positioner. The AUT positioner system is configured as roll over upper slide over azimuth over lower slide system. This positioning system configuration allows for planar, cylindrical and spherical near-field measurements. A rapidly rotating roll positioner is mounted on a specialized alignment fixture behind the scanner to facilitate far-field measurements. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 4.0 package. A Real-Time Controller (RTC), accompanied by an 8-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.
Precise mechanical alignment of motion axes of both cylindrical and spherical near-field systems is critical to producing accurate data. Until recently the only way to align these types of systems was to employ traditional optical tooling (i.e. jig transits, theodolites). Alignment by these methods is difficult, time consuming, and requires specialized training. More recently, laser trackers have been used for this type of alignment. Unfortunately, these devices are expensive and demand an even higher level of operator training. This paper describes the use of low cost alignment tools and techniques that have been developed by Nearfield Systems, Inc. (NSI) that greatly simplify the alignment process. Setup and alignment can be performed in a very short period of time by technicians that have been given minimal training. Suitable optical alignment procedures when followed by the use of electrical alignment techniques [7] yield sufficient alignment accuracy to permit testing up to Ku-band.
In previous work [1], we presented an antenna pattern compensation technique for linearly-scanned near field measurements. In this paper, we present a similar technique to mitigate the errors from uncompensated azimuthal antenna pattern effects in circular near-field monostatic radar measurements. The antenna pattern co mpensation is implemented as part of an improved algorithm for transforming the near-field measurements to the far-field RCS. A description of this improved circular near field-to-far field transformation CNFFFT technique for isotropic antennas is presented in a companion paper [2]. In this paper, we formulate the near-field signal model in the presence of an azimuthal antenna pattern under the same scattering approximation used in the isotropic CNFFFT. Using this model, we derive a modified version of the CNFFFT that includes antenna pattern compensation. Numerical simulations are presented that demonstrate the ability of the technique to remove antenna pattern errors and improve the accuracy of the far field RCS patterns and sector statistics.