A Partial Rotation Formulation of the Circular Near Field-to-Far Field Transformation (CNFFFT)
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.
High Accuracy Boresight Referencing Method in a Horizontal Planar Near Field Satellite Antenna Test Range
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.
Characterization of Compact Antenna Test Ranges from Amplitude-Only Data
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.
Optimization of Large Compact Range Reflector Installation and Verification Methodology
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.
Nonlinear Interpolation Technique for Generating 3D Antenna Radiation Patterns
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.
Demonstration of an Inverted Steward Platform Target Suspension System using Lightweight, High Tensile Strings
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.
Simplified Spherical Near-field Accuracy Assessment
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.
Characterization of the PLANCK Radio Frequency Qualification Model and Preparations for Flight Model Tests
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 Correction Technique
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.
Complex Antenna Transfer Function Measurements with Emphasis on High Positional Resolution
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  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.
Measurement of Circular Polarized Antennas
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.
An Extended Method for Measuring Time Delay Behavior of Small Antennas
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.
The device of the embedded control of parameters of the microwave feeder of airborne radar
A device and algorithm of measuring of microwave airborne radar antenna impedance and input power level are presented. A compact five-port microwave reflectometer, p-i-n diodes switch, single microwave detector are used. The output detector signal is processed. All of that results in decreasing of the cost of equipment, elimination of instrument components non-ideality and reaching of high equipment accuracy.
Assessment of a Planar Near-Field Range for Quiet-Zone Measurements at 650 GHz
Planar near-field probing is used in the optimisation of the quiet-zone of a hologram-based compact antenna test range (CATR). In this paper, the measurement instrumentation for 650 GHz operation is introduced and the potential measurement errors in the quiet-zone measurements are identified. Applicable error correction and compensation methods are discussed and the total measurement accuracy is calculated.
Amplitude and direction evaluation of very small stray signals in compact range
This paper presents a novel method to evaluate very small stray signals in compact range. The ripples of signals probed by an omni-directional antenna along the orthogonal direction of the bore sight could be treated as signals in time domain. Transforming the probed data with fast Fourier transform (FFT), the direction and amplitude (relative to the test signal) of each stray signal could be obtained. To improve the accuracy, time domain software gating should also be used in calibrating the measurement error of amplitude and phase. The presented method has the ability to measure very small stray signals with good angle resolution. The method has been tested by both simulation using MATLAB and experiment in the compensated compact range CCR120/100 in CAST using a monopole antenna centered on a circular ground plane as a probe. Good results were obtained.
VHF/UHF High Performance Absorbing Material Measurements in a Coaxial Line Using Time-Gating Techniques: Validation & Error Analysis
This paper describes the Rectangular Coaxial 40’ long measurement system recently designed and installed at AEMI with the primary purpose of measuring the reflectivity of its high performance VHF/UHF absorbing materials in the frequency range 30 – 510 MHz. The basic principles of the system are described in detail in  and are based on S11 – measurements of absorbing material reflectivity by a Vector Network Analyzer (VNA). In order to improve the system productivity and measurement accuracy it was enhanced by the time-gating software option – the standard option of ORBIT/FR Spectrum 959 automated measurement software package .The measurement system performance was thoroughly evaluated and validated by a number of tests performed in the “empty” coaxial line, and in the line loaded by absorbing materials. The list of RF uncertainties – various measurement error sources - was generated, the main measurement error contributors were identified, the corresponding errors – estimated and the overall RSS measurement errors were calculated for the absorber reflectivity varying in the range of -30dB to – 40dB.
NFâ€“FF TRANSFORMATION WITH PLANAR SPIRAL SCAN: AN EFFECTIVE SOURCE MODELLING FOR QUASI-PLANAR ANTENNAS
ABSTRACT A new probe compensated near-field – far-field transformation technique with planar spiral scanning is here proposed. It is tailored for quasi planar antennas, since an oblate ellipsoid instead of a sphere is considered as surface enclosing the antenna under test. Such an ellipsoidal modelling is quite general (containing the spherical one as particular case) and allows one to consider measurement planes at a distance smaller than one half the maximum source size, thus reducing the error related to the truncation of the scanning surface. Moreover, it reduces significantly the number of the needed near-field data when dealing with quasi planar antennas. Numerical tests are reported for demonstrating the accuracy of the far-field reconstruction process and its stability with respect to random errors affecting the data.
Implementation of a "Cam" as an RCS Dual-Cal Standard
The 2004 AMTA paper entitled “The “Cam” RCS Dual-Cal Standard” introduced the theoretical concept of the “cam,” a new calibration standard geometry for use in a static RCS measurement system that could simultaneously offer multiple “exact” RCS values based on simple azimuth rotation of the object. Since that publication, we have constructed a “cam” to further explore its utility. The device was fabricated to strict tolerances and its as-built physical geometry meticulously measured. Utilizing these characteristics and moment-method analysis, a high-accuracy computational electromagnetic (CEM) “exact” file required for calibration was produced. Finally, the “cam” was evaluated for its efficacy as a single device that could be utilized as a dual-cal standard. This development was conducted with a particular focus on the hypothesized improvements offered by the new standard, such as the elimination of frequency nulls exhibited by other resonant-sized calibration devices, and improved operational efficiency. In this follow-on paper, we present the advantages to and challenges involved in making the “cam” a viable RCS dual-cal standard by describing the fabrication, modeling and performance characterization.
A Compact but Highly Flexible 5-axis Positioner
ACC has developed for the ESA-ESTEC CATR a compact but highly versatile 5-axis positioner. It is composed of a roll axis, upper azimuth, elevation, translation and lower azimuth axis. The clearance between the floor and the translation stage is designed to pass over a 12” walkway absorber while the roll axis height is only 155 cm (~5 feet). The standard configuration for medium or high gain antennas is the roll-over-azimuth or elevation-overazimuth configuration with a vertical interface for the AUT. For omni-directional antennas and RCS measurements, the positioner can be configured as a low profile azimuth positioner with a horizontal interface without a blocking structure behind the AUT. The positioner can also be configured for bistatic RCS measurements and Spherical Near Field. With the addition of a linear scanner, the Quiet Zone can be scanned in a polar way but also planar scanning is possible. Other key parameters are: angular accuracy: 0.01°, accuracy of the translation axis: 0.01 mm, load capacity 100 kg.
Position Correction using a Multi-Axis Controller for High-Accuracy Measurements
Current means to improve position accuracy in antenna ranges are often expensive, consume important CPU time, and/or limit data acquisition speed. By taking advantage of axes with good repeatability, higher multi-dimensional positioning accuracy can be achieved directly by a controller to ease complexity and achieve real-time position correction. A product family of controllers brings this capability to fruition. Comparison analysis of field data demonstrates improved accuracy with no measurement speed degradation. Results indicated a considerable accuracy improvement limited by axis repeatability. Existing and new antenna ranges can benefit from this simple cost-effective approach to improved position accuracy.