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Compensated compact ranges offer accurate testing techniques for large devices under test. The quiet zone field performance is affected by diffracted field components from the sub and main reflector edges even though they are equipped with serrations in order to reduce this effect. The size, shape, and alignment of the serrations have a strong influence on the range performance and are important design parameters. For performance estimation and optimization, detailed EM simulation models are required. Integral equation methods like the Method of Moments (MoM) with Multilevel Fast Multipole (MLFMM) acceleration promise accurate simulation results. However, the memory requirements limit simulations nowadays to lower frequencies due to the electrical size of the compact range reflectors. For example, the main reflector of Astrium's Compensated Compact Range CCR 120/100 including serrations is 1860 ? by 1600 ? in size at 40 GHz. Asymptotic methods are suitable for objects of this size, however, the accuracy has to be investigated and is related to the degree of detail in the model. A detailed simulation model based on the Physical Optics (PO) / Physical Theory of Diffraction (PTD) method is developed in GRASP. Each serration is realized as an individual scatterer and can thus be modeled with arbitrary shape and orientation. Different modeling techniques have been applied in order to realize an accurate simulation model with acceptable runtime. In this paper, the simulation model will be described in detail and a comparison of the quiet zone fields will be drawn with the MoM / MLFMM tool Feko as well as with quiet zone probing measurements.
Compact range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signature. This paper investigates strategies for improving compact range measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the range source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational finite-difference time-domain (FDTD) model at VHF/UHF frequencies to determine baseline performance and develop trade rules for more advanced designs. Physical optics (PO) models were used to analyze the final design at the frequencies of interest.
The 3D reconstruction algorithm is applied to a slotted waveguide array measured at the DTU-ESA Spherical Near-Field Antenna Test Facility. One slot of the array is covered by conductive tape and an error is present in the array excitation. Results show the accuracy obtainable by the 3D reconstruction algorithm. Considerations on the measurement sampling, the obtainable spatial resolution, and the possibility of taking full advantage of the reconstruction geometry are provided.
The Antenna Metrology Lab at the National Institute of Standards and Technology in Boulder Colorado has developed a robotically controlled near-field pattern range for measuring antennas and quasi-optical components from 50 GHz to 500 GHz. This range is intended to address the need for highly accurate antenna pattern measurements above 100 GHz for a variety of applications including remote sensing, communications and imaging. A new concept in near-field range systems, this system incorporates the positioning repeatability of a precision industrial six-axes robot, six-axes parallel kinematic hexapod, and high precision rotation stage, integrated with a highly accurate laser tracking system. Programmable robot positioning allows the system geometry to be configured for spherical, planar, and cylindrical scans, as well as gain extrapolation measurements. Variable scan volume accommodates different test antenna sizes. Positioning accuracy better than 10 µm is predicted. Specifics of the system design, operating specifications and configurability will be presented.
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed in 2007 at CEA for indoor near field monostatic RCS assessment. This experimental layout was composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target was located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allowed full 3D near field monostatic RCS characterization. A new study was conducted in 2011 in order to achieve a more accurate positioning of the measurement antenna. The main objective is to enhance the RCS measurement performances, especially the environment subtraction directly related to the positioning repeatability of the measurement antenna. This new mechanical design has therefore been optimized to allow a +/-100° azimuth range with an angular positioning repeatability of less than 1/1000°. To achieve this level of accuracy, several keys design elements were considered: robust mechanical design, position control system… This paper describes the new experimental layout and the results of a positioning accuracy assessment campaign conducted using a laser tracker.
Numerous applications for antenna, radome and RCS measurements require a very accurate positioning capability to properly characterize the product being tested. Testing of weapons (missiles), guidance systems, and satellites, among other applications, require multi-axis position accuracies of a few thousandths of an inch or degree. For global positioning, spherical error volumes can be extremely small having diameters of .002 inches to .005 inches. This paper addresses the issues that must be resolved when highly accurate mechanical positioning is required. Many factors such as thermal stability, axis configuration, bearing runout and mechanical alignment can adversely affect the overall system accuracy. Additionally, when examined from a global positioning system perspective, the accuracy of the entire system is further degraded as the number of axes increases. Successful system implementation requires carefully examining and addressing the most dominant error factors. The paper will cover current tools and techniques available to characterize and correct the contributing errors in order to achieve the highest possible system level accuracy. A recently delivered 4 ft radius SNF arch scanner, which achieved ± .0043° global positioning accuracy, will provide insight into these methods and show how the dominant factors were addressed.
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed in 2007 at CEA for indoor near field monostatic RCS assessment. This experimental layout was composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target was located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allowed full 3D near field monostatic RCS characterization. A new study was conducted in 2011 in order to achieve a more accurate positioning of the measurement antenna. The main objective is to enhance the RCS measurement performances, especially the environment subtraction directly related to the positioning repeatability of the measurement antenna. This new mechanical design has therefore been optimized to allow a +/-100° azimuth range with an angular positioning repeatability of less than 1/1000°. To achieve this level of accuracy, several keys design elements were considered: robust mechanical design, position control system… This paper describes the new experimental layout and the results of a positioning accuracy assessment campaign conducted using a laser tracker.
Antenna measurement systems employ mechanical positioners to spatially orient antennas, vehicles, and a variety of other test articles. These mechanical devices exhibit native positioning accuracy in varying degrees based on their design and position feedback technology. Even the most precise positioning systems have insufficient native accuracy for some specific applications. As the limits of economical positioning accuracy are approached, a new error correction technique developed by MI Technologies satisfies these higher accuracy requirements without resorting to extreme measures in positioner design. The new technique allows real-time correction of repeatable positioning errors. This is accomplished by (1) performing a finely grained measurement of positioner accuracy, (2) creating a map of the errors in both spatial and spatial frequency domains, (3) separating the errors into their various components, and (4) applying correction filters to algorithmically perform error correction within the positioner control system. The technique may be used to achieve extreme positioning accuracy with positioners of high native accuracy. It may also be applied to conventional (synchro feedback) positioners to achieve impressive results with no modifications at all to the positioner. The following paper discusses the new error correction technique in detail.
The paper presents in-house developed antenna positioner capable to acquire radiation pattern in the full spherical format for wearable and textile antennas. The positioner features remarkable advantages and mitigates troublesome to measurement accuracy shadowing by the positioner structure. Furthermore, development methodology of the human phantom without heavy liquids is proposed. Since evaluation of wearable and textile antennas must put considerations to major variations in antenna performance during antenna operation, we have found an urgent need to define new engineering measure that will help in quick evaluation of such antennas.
The improved calibration model proposed in this paper is based on the traditional capacitance model which suffers from errors caused by the assumption that the capacitance is independent of frequency and the permittivity of the ambient medium under test. By analyzing the near-zone field of the coaxial opening, we introduce the new near-field capacitance to account for the dependency on the external permittivity. Simulation results show that the calibration error is significant reduced for low and moderate loss medium. And the calibration of the unknown coefficients simply requires the premeasurement of three known material including air, which provides convenience for the real field measurement. Measurement results obtained by a novel wideband in-situ coaxial probe are included to prove the accuracy improvement improved calibration model. by using this
This paper describes the development of a method based on measurements of the radar cross section (RCS) in the forward direction to determine the extinction cross section for the 2.5-38GHz frequency range using the optical theorem. Forward RCS measurements are technically complicated due to that the direct signal has to be subtracted from the total signal at the receiving antenna in order to extract the forward RCS. The efficiency of this subtraction as a function of time is evaluated. A traditional calibration method using a calibration target and a second method that does not require a calibration target are investigated and compared. The accuracy of the forward RCS measurements is determined using small spheres of different sizes. The spheres have a forward RCS that is straightforward to calculate with good accuracy. The method is also extended to polarimetric measurements on a small helix that are compared to theoretical calculations.
As part of a target simulator [1], a linearly polarized signal was required with a variable tilt angle that could be controlled electronically and changed at a 1 kHz rate. However, microwave components available in the 33.4 – 36 GHz operating range were inadequate to achieve the desired performance. A novel approach was developed to downconvert the input signal to a lower frequency range and use vector modulators available in this band to produce the appropriate phase and amplitude changes in each path, then upconvert back to the desired operating frequency to drive an orthomode transducer. A calibration and measurement procedure was developed to determine the vector modulator input settings that produced the most accurate tilt angles and best cross-polarization performance. By iteratively measuring cross-polarization and tilt angle, then adjusting the vector modulator controls, a tilt angle accuracy of +/-1 degree was achieved with a crosspolarization of -25 dB, exceeding the required performance. This paper provides an overview of the concept, a block diagram of the design, discussion of the calibration and measurement procedure, and a summary of the results achieved.
The use of adaptive acquisition techniques to reduce the overall test time in planar near-field antenna measurements is described. A decision function is used to track the accuracy of a measurement as the data acquisition proceeds, and to halt such acquisition when this is considered sufficient for the measured quantity of importance. Possible decision functions are defined and compared. Several test cases are presented to show that significant test time reduction is possible when compared to traditional acquisition schemes.
This paper describes the behavior of the antenna radiation pattern for different planar near-field measurement errors superimposed on the near-field data. The disturbed radiating near field is processed using multilevel plane wave based near-field far-field transformation to determine the far-field. Errors like scan area truncation, transverse and longitudinal position inaccuracy of measurement points, and irregular sample spacing are analyzed for an electrically large parabolic reflector at 40 GHz. The error behavior is then compared with the standard transformation technique employing 2D Fast Fourier Transform (FFT) using the same near-field data. In order to exclude the effect of any other measurement or environmental error, electric dipoles with appropriate magnitude profile and geometrical arrangement are used to model the test antenna.
The 2D full wave finite difference frequency domain (FDFD) method can provide propagation constant and eigenmode information for various guided wave structures. In addition, the mode information is well suited for exciting the waveguides and extracting the modal S-parameters in 3D FDTD simulation. However, most 3D full wave FDTD solvers have used conformal techniques to improve the accuracy and efficiency. To match these two methods and take advantage of the conformal features, it is necessary to apply the conformal techniques to the 2D FDFD method. In this paper, a conformal 2D FDFD eigenmode method is derived for solving arbitrarily shaped waveguides or transmission lines. The numerical results showed that the propagation constants obtained by the proposed method agree well with those obtained by the analytical solutions and commercial circuit solvers. The eigenmodes obtained by the developed conformal 2D FDFD eigenmode solver can be used to excite various transmission lines and to extract the modal S-parameters in 3D conformal FDTD simulation. Some examples such as horn antennas, circular waveguide filters and differential pairs are presented to show the capabilities of the developed conformal 2D FDFD eigenmode solver. The simulation results are also verified by some measurement results.
Large phased arrays have stringent beam-pointing accuracy requirements over their scan volume. Measuring the beam-pointing accuracy of a phased array with a planar near-field scanner is convenient but can lead to erroneous results if the near-field sampling grid is not well controlled. This paper describes numerical experiments that were carried out to assess the impact of various types of grid errors on the measurement of beam-pointing accuracy. The types of grid errors considered include skewing and curvature in the plane of the grid. The numerical experiments use infinitesimal dipoles as the radiating elements and assume an ideal probe. It is shown that beam-pointing errors induced by grid errors that can be described by an affine transformation can be estimated in closed form. For more complicated grid errors, the model is shown to be a useful tool in estimating their impact on measuring beam-pointing error. Finally, the amount of over-scan required for accurate beam-pointing measurements over a large scan volume is examined.
The radiation pattern of an antenna can be determined accurately by near-field measurement and transformation techniques. Low numerical complexity, full probe correction capabilities, and high accuracy of the transformed far-field pattern are important features of near-field transformation algorithms. The multilevel plane wave based near-field transformation algorithm achieves an efficient full probe correction by plane wave representations of antenna and field probe and realizes the low numerical complexity by hierarchical grouping of measurement points. Field translations are carried out to the boxes on the coarsest level and are further processed to the measurement points by disaggregation and anterpolation. Disaggregation is a simple phase shift of the plane waves and anterpolation reduces the sampling rate corresponding to the spectral content of the plane wave spectra on the various levels. The accuracy of the transformation is influenced by several variables where the number of buffer boxes between antenna and measurement point groups is crucial. A higher accuracy due to more buffer boxes can be achieved at the cost of increased computation time. Adaptive field translations structure the measurement setup such that individual groups are transformed with the required accuracy at lowest costs. A detailed investigation for a planar near-field measurement will be shown.
This paper compares 3 sets of far-field patterns of an S-Band Open Ended Waveguide (OEWG). The sources for the data are measurements from NPL, an EM-Model and the commonly used NIST analytical model. Both co-polarized (co-pol) and cross-polarized (x-pol) patterns are compared. Results indicate that accuracy improvements are possible by utilizing an EM-Model in certain applications. These applications as well as the pros and cons of doing this are discussed. Understanding the differences between these 3 independent sets of data enables near-field range engineers to better understand the directional dependence of probe correction accuracies over the majority of the forward hemisphere. Information and insight gained from this comparison, along with specific AUT requirements, better equips the near-field range user to address probe correction concerns and ultimately to determine if a calibrated probe solution is required for their unique testing scenario.
The use of computational electromagnetics (CEM) prediction codes in the analysis and interpretation of RCS measurements has become increasingly prevalent. This is in large part due to rapid advances in computing capability over the last several years, particularly for rigorous techniques such as the method of moments (MoM). In many instances, however, these codes are still limited to plane waves and/or elementary dipoles as the sources of target illumination. Modeling of the illumination from an arbitrary antenna therefore requires meshing and solution of the combined antenna-target geometry for each frequency and aspect angle, with an associated increase in the computational complexity of the problem, even if the interactions between the antenna and the target are negligible. In this paper, we describe a method by which measurements or predictions of the antenna pattern are used to develop an equivalent representation of the antenna in terms of an array of non-interacting elementary dipole current sources in a MoM code that uses RWG basis functions. The representation can then be used to efficiently derive the antenna’s illumination on the target as a function of frequency and aspect angle with only a minor increase in the computational burden relative to plane wave illumination. Results are presented using antenna pattern predictions for an ETS-Lindgren 3164-01quad-ridged VHF antenna which illustrate the accuracy and efficiency of the technique.
Radar antennas are typically required to operate in transmit and receive modes, and may or may not support both CW and pulsed signal operation. In active antenna applications, these modes may require different operating parameters, which currently dictate testing the antenna independently in transmit and receive using different test system configurations. In testing highly-integrated active arrays, electrical and thermal considerations make it preferable to test the antenna in its nominal Tx/Rx (Transmit/Receive) operating mode as opposed to transmit-only or receive-only. An extension to the NSI Panther 9100 RF measurement system has been developed to support multiplexed transmit and receive, pulse-mode measurements with different measurement parameters during the course of a single data acquisition. This capability allows pulsed transmit and receive tests to be interleaved using a single measurement setup, reducing overall test time and improving the real-world accuracy of the test results.