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Although highly accurate relative position data can be achieved using laser tracking systems which are suitable for millimeter wave antenna characterization, a considerable gap exists in the ability to absolutely align antennas to laser tracker target coordinate systems. In particular this scenario arises in millimeter wave near-field measurements where probe antenna aperture dimensions are on the order of a millimeter, and the position of its origin must be known to better than 1/20th of a wavelength, and orientation known to fractions of a degree. The fragile nature and dimensions of such antenna negate the use of coordinated metrology measurement systems and larger touch probes typically used for accurate spatial characterization. The Antenna Metrology Laboratory at NIST in Boulder, Colorado is developing a new machine vision based technique for measuring the absolute position of small (~1 mm) millimeter wave antenna apertures relative to a laser tracker target coordinate system. A synergy with existing laser tracking systems, this approach will provide a non-contact method for determining the absolute position and orientation coordinate frame of the probe antenna aperture in all six degrees of freedom to within 30-60 microns. This alignment system technique is demonstrated using the CROMMA Facility at NIST in Boulder, CO.
The development of the Configurable Robotic Millimeter-Wave Antenna facility (CROMMA) by the antenna metrology lab at the National Institute of Standards and Technology in Boulder Colorado has brought together several important aspects of 6-degree-of-freedom robotic motion, positioning and spatial metrology useful for high frequency antenna characterization. In particular, the ability to define a unified coordinated metrology space, which includes all the motion components of the system is at the heart of this facility. We present the details of integrating robotics that have well defined kinematic models, advanced spatial metrology techniques, and millimeter wave components which make up the CROMMA facility. From this, a high level of precision, accuracy, and traceability that is requisite for performing high frequency near-field antenna pattern measurements can be achieved. Emphasis is placed on the ability to precisely characterize and model the movement patterns of the robot positioners, and probe and test antenna apertures using state-of-the-art full 6-degree-of-freedom spatial metrology, while being able to manipulate this information in a unified measurement space. The advantages of using a unified coordinated metrology space as they pertain to complex antenna alignments, scan geometry, repeatability analysis, traceability, and uncertainty analysis will be discussed. In addition we will also discuss how the high level of positioning, and orientation knowledge obtainable with the CROMMA facility can enable the implementation of sophisticated near-field position correction algorithms and precisely configurable scan geometries.
In classical probe-corrected spherical near-field measurements, one source of measurement errors, not often given sufficient consideration is the probe [1-3]. Standard near-field to far-field (NFFF) transformation software applies probe correction with the assumption that the probe pattern behaves with a µ=±1 azimuthal dependence. In reality, any physically-realizable probe is just an approximation to this ideal case. Probe excitation errors, finite manufacturing tolerances, and probe interaction with the mounting interface and absorbers are examples of errors that can lead to presence of higher-order spherical modes in the probe pattern [4-5]. This in turn leads to errors in the measurements. Although probe correction techniques for higher-order probes are feasible [6], they are highly demanding in terms of implementation complexity as well as in terms of calibration and post-processing time. Thus, probes with high azimuthal mode purity are generally preferred. Dual polarized probes for modern high-accuracy measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port isolation. As a desired feature of modern probes the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign. Consequently, the probe design is a trade-off between performance requirements and usable bandwidth. High performance, dual polarized probe rely on balanced feeding in the orthomode junction (OMJ) to achieve good performance on a wide, more than octave, bandwidth [5-7]. Excitation errors of the balanced feeding must be minimized to reduce the excitation of higher order spherical modes. Balanced feeding on a wide bandwidth has been mainly realized with external feeding network and the finite accuracy of the external components constitutes the upper limits on the achievable performance. In this paper, a new OMJ designed entirely in waveguide and capable of covering more than an octave bandwidth will be presented. The excitation purity of the balanced feeding is limited only by the manufacturing accuracy of the waveguide. The paper presents the waveguide based OMJ concept including probe design covering the bandwidth from 18-40GHz using a single and dual apertures. The experimental validation is completed with measurements on the dual aperture probe in the DTU-ESA Spherical Near-Field facility in Denmark. References: [1]Standard Test Procedures for Antennas, IEEE Std.149-1979 [2]Recommended Practice for Near-Field Antenna Measurements, IEEE 1720-2012 [3]J. E. Hansen (ed.), Spherical Near-Field Antenna Measurements, Peter Peregrinus Ltd., on behalf of IEE, London, UK, 1988 [4]L. J. Foged, A. Giacomini, R. Morbidini, J. Estrada, S. Pivnenko, “Design and experimental verification of Ka-band Near Field probe based on wideband OMJ with minimum higher order spherical mode content”, 34th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2012, Seattle, Washington, USA [5]L. J. Foged, A. Giacomini, R. Morbidini, “Probe performance limitation due to excitation errors in external beam forming network”, 33rd Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2011, Englewood, Colorado, USA [6]T. Laitinen, S. Pivnenko, J. M. Nielsen, and O. Breinbjerg, “Theory and practice of the FFT/matrix inversion technique for probe-corrected spherical near- eld antenna measurements with high-order probes,” IEEE Trans. Antennas Propag., vol. 58, no. 8, pp. 2623–2631, Aug. 2010. [7]L. J. Foged, A. Giacomini, R. Morbidini, "Wideband dual polarised open-ended waveguide probe", AMTA 2010 Symposium, October, Atlanta, Georgia, USA. [8]L. J. Foged, A. Giacomini, R. Morbidini, “ “Wideband Field Probes for Advanced Measurement Applications”, IEEE COMCAS 2011, 3rd International Conference on Microwaves, Communications, Antennas and Electronic Systems, Tel-Aviv, Israel, November 7-9, 2011.
This paper will describe newly developed mechanical and electrical alignment techniques for use with plane-polar near-field test systems. A simulation of common plane-polar alignment errors will illustrate, and quantify, the alignment accuracy tolerances required to yield high quality far-field data, as well as bounding the impact of highly repeatable systematic alignment errors. The new plane-polar electrical alignment technique comprises an adaptation of the existing, widely used, spherical near-field electrical alignment procedure [8] and can be used on small, and large, plane-polar near-field antenna test systems.
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.
A new material validation and verification standard is designed to imitate the behavior of a lossy dielectric absorber. This standard is constructed from well-characterized, low-loss materials in a manner that ensures manufacturing repeatability. The performance of this standard is verified with S-parameter and permittivity measurements in a free space focused beam system and with finite difference time domain simulations. A sensitivity analysis, based on a series of simulations, is presented to quantify the uncertainty in the measured S-parameters due to dimensional and alignment variations from the ideal design values.
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.
Far-field uncertainty due to probe errors in planar near-field measurements is analyzed for the fast irregular antenna field transformation algorithm. Results are compared with the classical technique employing two dimensional Fast Fourier Transform (2D FFT). Errors involving probe's relative pattern, alignment, transverse and longitudinal position, interaction with AUT etc. have been considered for planar measurements. The multiple reflections error originating from the interaction of the probe and the AUT tends to deteriorate the radiation pattern to a greater extent. Therefore, a novel technique which utilizes near-field measurements on two partial planes is presented to reduce the multiple reflections between the probe and the AUT.
In this paper we present an optical imaging tool, the Overlay Imaging Aligner (OIA), developed to aid in the mechanical alignment of antenna components in the mm-wave and low-THz frequency regimes (50-500 GHz) where the millimeter and sub-millimeter wavelengths pose significant challenges for alignment. The OIA uses a polarization-selective, machine-vision approach to generate two simultaneous and overlaid real-time digital images along a common axis. This allows for aligning two antenna components to within fractions of a wavelength in the mm-wave and THz frequency regimes. The overall concept, optical design, function, performance characteristics and application examples are presented. Preliminary data at specific frequencies in the WR-2.2 band are presented that compare the alignment achieved with the OIA to an electrical alignment.
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.
Quiet zone sampling is a powerful tool to evaluate the performance of a compact antenna test range. Such procedure consists of carrying out typically planar field acquisitions of the quiet zone. Acquired data yields valuable information about the measurement capabilities of the range under consideration. However, the required amount of samples increases quadratically with frequency for full 2D acquisitions. In practice, millimeter wavelength assessment of CATR facilities implies extremely time-consuming acquisition campaigns in addition to intrinsic mm-waves challenges, such as strict mechanical tolerances in construction and alignment, and high sensitivity to environmental factors. In this communication, we will focus on the CATR facility at ESTEC. A time-efficient acquisition technique has been developed and used to carry out millimeter wavelength quiet zone measurements between 46 – 50 GHz. First tests of the technique show a high level of agreement between the results obtained with the proposed method and those corresponding to a classical Nyquist acquisition approach, while a time savings factor of about 8 is achieved.
Alignment of the axis of range positioner systems can be performed using modern laser tracking equipment. An approach to the alignment of the radome measurement range at the Raytheon McKinney, Texas facility will be presented. Key words, range alignment, laser tracking, laser alignment
Wideband dual polarized probes are often used for modern high precision measurement systems. A desired feature of a good probe is that the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign [1]. This paper describes a new field probe taking full advantage of the 1: 4 bandwidth of the Ortho Mode Junction (OMJ) overcoming the aperture size problem by applying different apertures on the same field probe. The apertures are circularly symmetric so the exchange of apertures can be performed rapidly without the need to repeat calibration and alignment procedures for the full probe.
The calibration and measurement of bistatic reflectivity at short range (3.3 m) presents challenges that are significantly different from the usual test range measurements (typically monostatic at 100 to 150 m). In order to overcome this an object-free calibration procedure has been applied, eliminating crosstalk, reducing other interferences and removing errors associated with the RCS and alignment of calibration objects. It is based on calibrating the transmitter and receiver antennas as a pair by directing the antennas toward each other. The method thus requires that the antennas can be separated. Furthermore the signal level needs to be handled e.g. by the separation distance or attenuators. The bistatic reflectivity measurements were performed by placing a clutter sample on a turntable which is located at the centre of a bistatic arc. This configuration enables us to do ISAR images. Background contributions were discriminated using a combination of synthetic resolution and zero-doppler filtration. The sensitivity variation across the antenna footprint was handled by calculating an equivalent area from measured off-axis antenna sensitivities. Reflectivities have been measured for a metallic test surface and for grass. The metallic test surface had been manufactured to correspond to typical theoretical bistatic clutter models.
Millimeter-wave measurements on spherical near-field scanning systems present a number of technical challenges to be overcome to guarantee accurate measurements are achieved. This paper will focus on the affect of mechanical alignment errors of the spherical rotator system on the antenna’s measured performance. Methods of precision alignment will be reviewed. Sensitivity to induced mechanical alignment errors and their affect on various antenna parameters will be shown and discussed. Correction methods for residual alignment errors will also be described. The study includes 38 and 48 GHz data on the Alphasat EM model offset reflector antenna measured by TeS in Tito, Italy on a NSI-700S-60 Spherical Nearfield system, as well as a 40 GHz waveguide array antenna measured by NSI on a similar NSI-700S-60 Spherical Nearfield System at its factory in Torrance, CA, USA.
ESA’s Compact Antenna Test Range at ESTEC has been relocated which has given the chance to improve the alignment of the reflectors. Based on measure-ments of the reflector surfaces the best-fit positions and orientations of the reflectors have been deter-mined. It turned out that the choice of parameters to describe the reflectors and their position had impor-tant impact on the optimization process: The parame-ters shall – as far as possible – be orthogonal in the sense that a change in one parameter must not influ-ence the final value of the other parameters.
The NIST 18 Term Error Analysis Technique uses a combination of mathematical analysis, computer simulation and near-field measurements to estimate the uncertainty for near-field range results on a given antenna and frequency range. A subset of these error terms is considered for alignment accuracy of an antenna’s RF main beam. Of the 18 terms, several have no applicable influence on determining the beam pointing or the terms have a minor effect and when an RSS estimate is performed they are rendered inconsequential. The remainder become the dominant terms for identifying the alignment accuracy. There are six terms that can be evaluated to determine the main beam pointing uncertainty of an antenna with respect to dual band performance. Analysis of the near-field measurements is performed to identify the alignment uncertainty of the main beam with respect to a specified mechanical position as well as to the main beam of the second band.
The paper presents a new system, to be used in near-field antenna characterization, which (in its simplest implementation) automatically assists the alignment of the Antenna Under Test (AUT) and the definition of the uniform/non-uniform sampling and filtering procedures. The system, based on a very cheap hardware (3D, coded structured-light, digitalization device), is able to provide a complete description of the geometry of the measurement set-up and of the movements of its parts, probe included. In this paper the simplest case, the alignment of the AUT, is considered. In this case, the system determines the coordinates of the surface points of the AUT in the Laboratory Reference System (LRS), providing the position and the orientation of the AUT in the LRS. The acquisition of the geometrical data on the AUT requires only few seconds, with a negligible human intervention. The acquired data are then processed to provide the desired setting of the AUT either manually or by means of computer controlled actuators, ensuring the accuracy suited to millimetre-wave band. The geometrical information can be exploited also to make possible the correction via software, when movements of the AUT should be avoided. The accurate information on the AUT geometry can be fruitfully exploited in advanced, high-performance filtering and sampling approaches, again drastically reducing any human interaction with the system. The performance of the system is discussed by referring to a prototype working in the millimetre-wave band.
In the Flight Model (FM) of the PLANCK telescope, the feed horns are connected to either HEMTs or bolometers operating at cryogenic temperatures to detect the Cosmic Microwave Background radiometric signal. For the purpose of an overall alignment verification at ambient temperature, RCS measurements have been performed using an auxiliary feed horn that is terminated with a switching diode. This verification test has been conducted at 320 GHz, to benefit from the narrow beam and a high sensitivity to misalignment. To perform the RCS measurements, an additional “circulator” with low propagation loss and high isolation from transmit to return channel had to be developed. Besides that, the circulator also co-locates the phase centres of both Tx and Rx range antennas on the focal point of the CATR, which allows mono-static RCS measurements. Quasi-optical techniques have been used to design a circulator that meets these requirements. To test the feasibility of determining the feed location from the RCS measurements with an uncertainty of ±1 mm, a test campaign was conducted with the so called RF Qualification Model (RFQM). In this campaign, 9 feed locations with 1 mm separation were tested. With the Flight Model, the test was on the critical path of the planning and only one test could be conducted to verify the overall alignment.
Dual polarized probes for modern high precision measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port isolation. A desired feature of a good probe is that the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign. As a consequence, the probe design is a trade-off between performance requirements and the usable bandwidth of the probe. For measurement applications in circular polarization the choice is between measuring the linear polarization components separately and derive the resulting circular polarized by computation or to measure directly with a circular polarized probe. Dual polarized probes in circular polarization with high polarization purity is difficult to achieve on a wide bandwidth. Dual linear polarized probe technology has recently been developed capable of achieving as much as 1:4 bandwidth while maintaining the high performance of traditional probe designs [1–7]. This paper describes the development, manufacturing and test of dual circular polarized probes with as much as 1:2 bandwidth as shown in Figure 1.