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Accuracy

Precision Motion in Highly Accurate Mechanical Positioning
Tim Schwartz,MI Technologies, November 2012

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.

Achieved mechanical Accuracy of a 3D RCS spherical near field Arch Positioning System
Pierre MASSALOUX,CEA, November 2012

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.

An Innovative Technique for Positioner Error Correction
Roger Dygert,MI Technologies, November 2012

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.

Major challenges to wearable and textile antenna measurements in the spherical format
Pawel Kabacik,Wroclaw University of Technology, November 2012

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.

An Improved Capacitance Model for Permittivity Measurement
Ming Chen,ElectroScience Lab, The Ohio State University, November 2012

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

Wideband Measurements Of The Forward Rcs And The Extinction Cross Section
Christer Larsson and Mats Gustafsson, November 2012

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.

Electronically Controlled Tilt Angle Of A Linearly Polarized Signal At Ka-Band
Steven R. Nichols, November 2012

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.

Adaptive Acquisition Techniques For Planar Near-Field Antenna Measurements
D. Janse van Rensburg,D. McNamara, G. Parsons, November 2011

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.

Planar Near-Field Measurement Error Analysis for Multilevel Plane Wave Based Near-Field Far-Field Transformation
M.A. Qureshi,C. Schmidt, E. Thomas, November 2011

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.

A Conformal 2D FDFD Eigenmode Method for Wave Port Excitation and S-parameter Extraction in 3D FDTD Simulation
Y. Wang,S. Langdon, November 2011

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.

A Study of Near-Field Sampling Grid Errors and Their Effect on Phased Array Beam-pointing Error
J. Bowen, November 2011

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.

Evaluation of Multilevel Plane Wave Based Near-Field Far-Field Transformation Employing Adaptive Field Translations
C. Schmidt,T. Eibert, November 2011

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.

OEWG Probe Pattern Comparisons between NPL Measurements, EM-Model and Analytical Model
C. Dempsey, November 2011

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.

Efficient Method for Representing Antenna Pattern Illumination in Method of Moments (MoM) Radar Cross-Section (RCS) Predictions
I. LaHaie,M. Blischke, November 2011

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.

Multiplexed Pulsed Transmit and Receive RF Measurement System for Active Phased Array Testing
K. Hassett,B. Williams, November 2011

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.

A "Two–Level GTD" Anechoic Chamber  for VHF/UHF Antenna Measurements:  Design and Experimental Validation
J. Aubin,M. Winebrand, V. Vinogradov, November 2011

Recently ORBIT/FR Inc. has introduced a far – field antenna measurement anechoic chamber design method called “ Two Level GTD “ , which combines shaped chamber walls with a specific absorber layout intended to achieve a better level of reflectivity in the test zone [1-3]. The sidewalls may have the shape of an “inverted open book “, while the back wall may be a pyramidal shape with a small subtended angle at the base. A wedge type foam absorber with a variable orientation of the wedge tips can treat the sidewalls in a “fishbone” layout, while the back wall may be treated by using conventional foam based pyramids. The’ fishbone’ like layout is intended to adverting the reflected waves by the sidewalls out of the test zone, while the back wall pyramid layout is applied to utilize both: the optimum pyramid reflectivity at almost normal incidence; the back wall shape diverting the reflected incident plane or quasi - plane wave out of the test zone. It well known that GO and GTD principles are widely applicable to electrically large structures, delivering a high quality simulation accuracy and good correlation with measurement results. Therefore, the application of the “Two –Level GTD “ is expected to deliver well predicted improvement in the reflectivity of anechoic chambers operating at relatively high frequencies , where the chamber sidewall characteristic dimensions may reach 30. where . is the wavelength at lowest operating frequency. The key question to be answered is - Can the method be successfully applied to cases where the chamber sidewall characteristic dimensions are only – 2-3.? This represents a typical situation in anechoic chambers designed for operation at VHF/UHF frequency bands. In order to answer the question, a full wave 3D simulation has been performed on two anechoic chambers having similar dimensions: 20’ x 20’ x 33’ (L). The two cases are a conventional anechoic chamber and a shaped wall chamber designed based on the “Two – Level GTD” principle. The simulation results were compared, and the “Two - Level GTD” has shown superior performance. Based on these encouraging results, the anechoic chamber was constructed and measurements were performed in the tests zone at a number of frequencies down to 100 MHz. The chamber construction, simulation and measurement results are discussed in the paper below.

Wideband in-situ Soil Permittivity Probe and Novel Iterative Permittivity Calibration Method
M. Chen,C. Chen, November 2011

A novel probe design for measuring complex permittivity of soils in-situ from 10 to 1000 MHz without taking soil samples is presented. The dielectric constant and conductivity of soil is derived from step-frequency reflection taken inside a small freshly bored hole. As a result, permittivity at various depths with in-situ moisture content and soil texture can be obtained in the fields. A novel calibration method was developed to account for the frequency- and material-dependent geometrical factor which causes bias errors in conventional calibration methods. Experimental measurement results and simulation results are used to prove the efficiency and accuracy of this method.

Practical Gain Measurements
M. Baggett, November 2011

Collecting accurate gain measurements on antennas is one of the primary tasks for our community. One of the primary concerns in making gain measurements is choosing one of the well known gain measurement techniques to make the measurement. Each gain measurement technique has an inherent accuracy limit based on the measurements made, the measurement environment and the equipment required. The frequency band of interest may also have an impact on the gain measurement scheme employed. In addition, each technique can affect the throughput of the range in question. Balancing the cost of obtaining the gain versus the required accuracy of the gain measurement is a difficult task. This paper will discuss the basic accuracy limitations for several of the standard gain measurement techniques and will catalog the accuracy limitations of the various gain measurement techniques versus the cost associated with obtaining that quality of measurement.

A Highly Accurate Spherical Near-Field Arch Positioning System
J. Fordham,T. Schwarz, G. Cawthon, Y. Netzov, S. McBride, M. Awadalla, D. Wayne, November 2011

Highly accurate spherical near-field measurement systems require precise alignment of the probe antenna to the measurement surface. MI Technologies has designed and constructed a new spherical near field arch positioner with a 1.5 meter radius to support measurements requiring accurate knowledge of the probe phase center to within .0064 cm throughout its range of travel. To achieve this level of accuracy, several key design elements were considered. First, a highly robust mechanical design was considered and implemented. Second, a tracking laser interferometer system was included in the system for characterization of residual errors in the position of the probe. Third, a position control system was implemented that would automatically correct for the residual errors. The scanner includes a two position automated probe changer for automated measurements of multi-band antennas and a high accuracy azimuth axis. The azimuth axis includes an algorithm for correcting residual, repeatable positioning errors. This paper defines the spherical near-field system and relation of each axis to the global coordinate system, discusses their associated error sources and the effect on global positioning and presents achieved highly accurate results.

Implementation and Analysis of an Improved Accuracy Microwave Measurement Method for Low Loss Dielectric Materials
M. Scott,J. Schultz, D. Reid, S. Blalock, B. Cieszynski, November 2011

A free space transmission line measurement method for dielectric constant and loss tangent determination in low-loss dielectric materials has been analyzed and implemented. This method utilizes dielectric materials with thicknesses greater than half the wavelength in the material to obtain greater sensitivity for determining intrinsic dielectric properties. An analysis of the process sensitivities and experimental measurements has been utilized to estimate the accuracy and lower limits of the dielectric property extractions from the reflection loss magnitude.







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