AMTA Paper Archive
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Experimental Verification of the Focal Plane APC Method with the VAST-12 Antenna
Boumans  has introduced an alternative to the classical (Advanced) Antenna Pattern Correction (A)APC method by moving the range feed in the focal plane of a Compact Antenna Test Range (CATR) instead of moving the Device Under Test (DUT) around in the Quiet Zone (QZ). The advantages are clear: it is easier (cost and accuracy wise) to implement a feed scanner than a DUT scanner; the method can be used for azimuth and elevation patterns and it can even be implemented using multiple feed horns to get to the same measurement time as with a single range feed. The capabilities of defocused measurements in the Compact Payload Test Range (CPTR) at ESA/ESTEC have been previously assessed  and they revealed a triply reflected ray  and a QZ ripple induced by periodic surface inaccuracies . This paper focuses on verifying the performance of the Focal Plane AAPC method for these effects. Use has been made of the well known DTU-ESA VAST-12 antenna .
Outdoor RCS Measurement Range for Spaceborne SAR Calibration Targets
The Microwaves and Radar Institute regularly performs calibration campaigns for spaceborne synthetic aperture radar (SAR) systems, among which have been X-SAR, SRTM, and ASAR. Tight performance specifications for future spaceborne SAR systems like TerraSAR-X and TanDEM-X demand an absolute radiometric accuracy of better than 1 dB. The relative and absolute radiometric calibration of SAR systems depends on reference point targets (i. e. passive corner reflectors and active transponders), which are deployed on ground, with precisely known radar cross section (RCS). An outdoor far-field RCS measurement facility has been designed and an experimental test range has been implemented in Oberpfaffenhofen to precisely measure the RCS of reference targets used in future X-band SAR calibration campaigns. Special attention has been given to the fact that the active calibration targets should be measured under the most realistic conditions, i. e. utilizing chirp impulses (bandwidth up to 500 MHz, pulse duration of 2 µs for a 300 m test range). Tests have been performed to characterize the test range parameters. They include transmit/receive decoupling, background estimation, and two different amplitude calibrations: both direct (calibration with accurately known reference target) and indirect (based on the radar range equation and individual characteristics). Based on an uncertainty analysis, a good agreement between both methods could be found. In this paper, the design details of the RCS measurement facility and the characterizing tests including amplitude calibration will be presented.
Application of the SWE-To-PWE Antenna Diagnostics Technique to an Offset Reflector Antenna
A new antenna diagnostics technique has been developed for the DTU-ESA Spherical Near-Field Antenna Test Facility at the Technical University of Denmark. The technique is based on the transformation of the Spherical Wave Expansion (SWE) of the radiated field, obtained from a spherical near-field measurement, to the Plane Wave Expansion (PWE), and it allows an accurate reconstruction of the field in the extreme near-field region of the antenna under test (AUT), including the aperture field. While the fundamental properties of the SWE-to-PWE transformation, as well as the influence of finite measurement accuracy, have been reported previously, we validate here the new antenna diagnostics technique through an experimental investigation of a commercially available offset reflector antenna, where a tilt of the feed and surface distortions are intentionally introduced. The effects of these errors will be detected in the antenna far-field pattern, and the accuracy and ability of the diagnostics technique to subsequently identify them will be investigated. Real measurement data will be employed for each test case.
Antenna Measurement at 650 GHZ With A Planar Near-Field Scanner
Accurate antenna measurements at sub-millimeter frequencies are very challenging. Especially the phase measurement accuracy is usually limited by the mechanical accuracy of the measurement equipment. The measurement techniques used, and the measurement results of a dual reflector feed system (DRFS) at 650 GHz are presented in this paper. Planarity error compensation technique was used that enabled accurate correction to the measured phase pattern without accurate pre-existing information of the planarity error of the planar near-field scanner. The measured DRFS beam agrees well with the simulated and the achieved measurement accuracy is good.
A Method to Correct Measurement Errors in Far-Field Antenna Ranges
Now-a-days, far-field ranges are being used to measure antenna radiation patterns. Two main types of ranges used are used for these measurements: direct and indirect illumination. In either case, the accuracy of the measurement is dependent upon the quality of the range quiet-zone fields. In direct illumination, phase and amplitude taper cause discrepancies in the fields. For indirect illumination, only amplitude taper must be accounted for. Additionally, stray signals and cross-polarization will further distort the quiet-zone fields and lead to measurement errors. This new methodology starts with the measured antenna data and a priori knowledge of the incident fields and estimates an Effective Aperture Distribution (EAD). The EAD compensates for these sources of error and can be used to predict the far-field radiation pattern of the antenna under test. Analytical results are presented for taper and stray signal analysis.
UCLA's Millimeter-Wave Bi-polar Planar Antenna Measurement System: A Novel Portable Design
As new antenna designs reach higher frequencies and smaller sizes, traditional large scale antenna chamber systems become ill-suited for measurement. External mixing, room-sized chambers, and expensive test equipment add large costs and burden to antenna measurement systems. A smaller, more cost effective system is proposed. Using the bipolar planar scanning technique developed at UCLA, a portable and movable millimeter-wave antenna chamber is currently under development. The chamber is being designed to fit on the end of a standard optical table and enjoys the space-saving and accuracy inherent to the bipolar planar configuration. Simple construction of the chamber will allow relatively easy assembly and disassembly and allow movement of the chamber from one table to another, if needed. Antenna of diameters up to 40cm can be accommodated and scan planes of up to ~160cm can be measured. Millimeter-wave frequencies from around 30GHz to 67GHz can be measured. Antennas measured will use planar near-field to far-field techniques. In particular, the post-process will follow the OSI/FFT method and will incorporate the phase retrieval techniques developed for the bipolar configuration. These phase-less measurements will allow the use of scalar millimeter-wave test equipment with much lower cost than comparable vector test equipment.
Design, Alignment and Calibration Requirements for a Sub-Millimeter Wave Frequency Tiltable Lightweight Scanner
This paper discusses design aspects related to a tiltable lightweight near-field scanning system for use at sub-millimeter frequencies. It addresses design issues as they relate to accuracy and scanner distortions from multiple causes. Calibration methods to measure and correct for anticipated and unanticipated errors are briefly addressed. Actual test results are presented. The tiltable scanner being discussed was designed for the Atacama Large Millimeter/submillimeter Array (ALMA)  and is being used by the National Radio Astronomy Observatory (NRAO) . It has many other applications by virtue of its light weight (approx. 120 lbs) and ability to be oriented at different angles. These include flight-line testing and other in-situ antenna test applications.
Polarimetric calibration of indoor and outdoor polarimetric radar cross section systems
We used a set of dihedrals to perform polarimetric calibrations on an indoor RCS measurement range. We obtain simultaneously hh, hv, vh, and vv polarimetric data as the calibration dihedrals rotate about the line-of-sight to the radar. We applied Fourier analysis to the data to determine the polarimetric system parameters, which are expected to be very small. We also obtained polarimetric measurements on two cylinders to verify the accuracy of the system parameters. We developed simple criteria to assess the data consistency over the very large dynamic range demanded by the dihedrals. We examined data contamination by system drift, dynamic range nonlinearities, and the presence of background and noise. We propose improved measurement procedures to enhance consistency between the dihedral and cylinder measurements and to minimize the uncertainty in the polarimetric system parameters. The final recommened procedure can be used to calibrate polarimetrically both indoor and outdoor ranges.
DATA REDUCTION IN THE NF-FF TRANSFORMATION TECHNIQUE WITH SPHERICAL SPIRAL SCANNING
An effective near-field – far-field transformation technique with spherical spiral scanning tailored for antennas having two dimensions very different from the third one is here proposed. To this end, an antenna with one or two predominant dimensions (as, e.g., an elongated or quasi-planar antenna) is no longer considered as enclosed in a sphere, but in a prolate or oblate ellipsoid, respectively, thus allowing one to remarkably reduce the number of required data. Moreover these source modellings remain quite general and contain the spherical one as particular case. 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.
Advanced Computational Tools for Antenna Placement Studies
Recent advances in computational electromagnetic tools have made antenna design possible along with integration of antennas on various ground, sea and air platforms. Numerical computations can be performed to evaluate the effects of antenna placement, radiation hazard, EMC/EMI, etc. The typical numerical approaches include full wave techniques such as Method of Moments (MoM), Multilevel Fast Multipole Method (MLFMM) and asymptotic techniques such as Physical Optics (PO) and Uniform Theory of Diffraction (UTD). For many practical applications, sometimes it is necessary to study the electromagnetic behavior on a specific structure over a broad frequency band, and therefore it is important to have some benchmark data on computational resources needed for some commonly used numerical techniques. In this study, representative full-size air, ground and sea platforms are considered and the frequency limit is pushed at different bands using several numerical techniques. The accuracy and computational resources are compared.
MODELING ANTENNA PATTERN DISTORTION CAUSED BY SUPPORTING STRUCTURES
This paper presents a technique for calculating the antenna pattern distortion caused by supporting structures such as buildings, towers, etc. The technique is based on ray tracing and the uniform theory of diffraction. The resulting distorted pattern can then be added to antenna databases and used as input to, for example, wireless network planning tools. The present method is fast and can considerably improve the accuracy of propagation calculations of radio frequency signals. A representative example from the application of this technique to an antenna mounted on the top of a building is presented.
Radiation Pattern Measurements and Predictions of the PLANCK RF Qualification Model
PLANCK is one of the scientific missions of the European Space Agency, devoted to observe the Cosmic Microwave Background radiation with unprecedented accuracy. One of the key factors for the performance is the radiation pattern of the telescope, especially the sidelobe performance in the direction of hot celestial bodies like Sun, Earth and Moon. The satellite will operate around the L2 Lagrangian point in deep space under cryogenic conditions. These conditions can not be realized in an antenna test range for a payload of this size. Therefore, the predictions for the performance under flight conditions depend highly on numerical simulations. The model to be used had never before been verified to this level of confidentiality. The challenge was to conduct a test campaign at frequencies up to 320 GHz (far beyond the normal range of the used CATR) with a very large object (the PLANCK RF Qualification Model with an aperture size of 1.5 m, i.e. more than 1500 wavelength at 320 GHz) to demonstrate Sidelobe Levels down to -90 dB. A selection of the measurement results and comparison with predictions will be presented.
FIELD ESTIMATION AND ASSESSING ACCURACY IN COMPLEX RF ENVIRONMENTS
A novel technique for estimating the spatial electromagnetic field distribution and its covariance error is presented based on variogram analysis and the statistical interpolation technique known as Kriging. The spatial structure of some field measurements are characterized by variogram analyses and their propagation properties identified. The physical implications of the Kriging interpolator functional fit to measured data is considered and illustrated. It is concluded that with specialist interpretation this new technique can be used as a valuable checking tool, or to reduce the number of field measurement, in a measurement programme, particularly when the costs of the latter are considered.
HIGH ACCURACY COMPACT RANGE ANTENNA MEASUREMENT FACILITY
Orbit/FR has installed a new compact range for antenna measurements at ACE Antenna Corp. The measurement facility covers a frequency range from 0.8 to 40GHz with a Quiet Zone size of 3 m diameter x 3 m length. The design of the compact range is similar to the one already installed by Orbit/FR at Ericsson (Sweden) with some improvements in the mechanical design and in the system parameters. An intensive simulation of the reflector serrations had allowed for finding its optimal profile, thus improving the quiet zone parameters at entire frequency range, especially at low frequencies, at which a number of base-station and mobile antennas are expected for testing by ACE Antenna Corp. A new design of a feed positioner and a baffle house added more convenience for the compact range alignment and operation. The system was installed and qualified in March 2008. The field probing has been performed within the entire operating frequency range, which then allows for evaluation of the antenna measurement accuracy. A system description as well as results of simulation and excerpt of the qualification data is presented in the paper.
PROCEDURE AND PROCESS OPTIMIZATION FOR REDUCTION OF MEASUREMENT UNCERTAINTIES IN RF TEST FACILITIES
The measurement accuracy of state-of-the-art RF test facilities like near-field or compact test ranges is influenced due to applied system hardware as well as operational facts which are influenced by human errors. The measurement errors of near-field test facilities were analyzed and published in the past times and are based on the 18-term error model of Newell . For compact test ranges and especially for the cross-polar free compensated compact range a similar error model was established at Astrium GmbH within a study for the satellite service provider INTELSAT  in order to define possible facility performance improvements and maximum achievable values for the measurement accuracy. It has to be remarked, that test programs for space applications require very stringent adherence to procedures and documentation of process steps during a test campaign. Within this paper, recommendations for process optimizations and procedures will be presented to guarantee the adherence to the valid error budgets and to minimize the Human Factor. A description of main error contributions in the Compensated Compact Range (CCR) of Astrium GmbH will be performed. Furthermore, the error budgets for pattern and gain measurements and achievable performance improvements will be given.
Characterization of measurement systems through extensive measurement campaigns
Within the European Union network "Antenna Center of Excellence" – ACE (2004-2007), a first intercomparison campaign among different European measurement systems, using the 12 GHz Validation Standard (VAST12) antenna, were carried out during 2004 and 2005. One of the challenges of that campaign was the definition of the accurate reference pattern. This was the reason why a dedicated measurement campaign for definition of the accurate reference pattern was hold during 2007 and beginning of 2008. This second campaign is described in the companion paper “Dedicated measurement campaign for definition of accurate reference pattern of the VAST12 antenna”. This dedicated measurement campaign was performed by Technical University of Denmark (DTU) in Denmark, SAAB Microwave Systems (SAAB) in Sweden and Technical University of Madrid (UPM) in Spain. This campaign consisted of a large number of measurements with slightly different configurations in each of the three institutions (2 spherical near field systems and one compact range). The purpose of this paper is to show the process to achieve the reference pattern from each institution and the evaluation of the accuracy. The acquisitions were performed systematically varying in applied scanning scheme, measurement distances, signal level and so on. The results are analyzed by each institution combining the measurement results in near or far field and extracting from these measurements: a “best” pattern, an evaluation of possible sources of errors (i.e. reflections, mechanical and electrical uncertainties) and an estimation of the items of the uncertainty budget.
Assessment of a Candidate Metallic Waveguide Standard, Based on S-parameter Uncertainty Due to Dimensional Manufacturing Errors.
An effort to ascertain the accuracy of the rectangular waveguide measurement technique for permittivity and permeability characterization of materials, has led to the development and application of a waveguide notch filter as a scattering parameter (S-parameter) reference standard. The S-parameters of this reference can be determined accurately using simulations that implement a full wave model of the waveguide measurement technique. The notch frequency response characteristic allows testing over the dynamic range of the measurement system. When fabricated in metal, the filter provides a predictable frequency response, has mechanical and temporal stability, and is reproducible using standard machining techniques. However, manufacturing errors introduce uncertainty in the measured S-parameters. Determining the sensitivity of S-parameter uncertainty as a function of manufacturing errors is important in assessing the appropriateness of the notch filter as a metallic standard for use throughout the material measurements community. This paper presents the characteristics of the filter, showing both calculated and measured S-parameter values, and provides an analysis that demonstrates the relationship between dimensional manufacturing tolerances and the resulting S-parameter uncertainty.
Total Radiated Power Measurement for Small Radio Terminals Using a Spheroidal Coupler
We propose a novel method for measuring the total radiated power (TRP) of small radio terminals, such as mobile phones, active RFID tags, and UWB devices, using a spheroidal cavity coupler in which an EUT and a receiving antenna are displaced symmetrically around the focal points of the spheroid. The proposed method provides a compact, low-cost TRP measurement system with high sensitivity and high speed, supporting TRP measurement for both in-band signals and higher-order spurious radiation. Although the behavior of electromagnetic waves in the coupler is complex due to multiple reflections, we can evaluate the maximum TRP from the EUT by using the displacement technique and comparison with a reference system using a reference transmitting antenna and signal generator. Computer simulation verifies that the method measures TRP with high accuracy.
Uncertainty evaluation through simulations of virtual acquisitions modified with mechanical and electrical errors in a cylindrical near-field antenna measurement system
An error simulator based on virtual cylindrical near-field acquisitions has been implemented in order to evaluate how mechanical or electrical inaccuracies may affect the antenna parameters. In outdoor ranges, where the uncertainty could be rather important due to the weather conditions, an uncertainty analysis a priori based on simulations is an effective way to characterize measurement accuracy. The tool implemented includes the modelling of the Antenna Under Test (AUT) and the probe and the cylindrical near-to-far-field transformation. Thus, by comparing the results achieved considering an infinite far-field and the ones obtained while adding mechanical and electrical errors, the deviations produced can be estimated. As a result, through virtual simulations, it is possible to determine if the measurement accuracy requirements can be satisfied or not and the effect of the errors on the measurement outcomes can be checked. Several types of results were evaluated for different antenna sizes, which allowed determining the effect of the errors and uncertainties in the measurement for the antennas under study.
A State of the Art Anechoic Chamber for Air vehicle Testing at Alenia Aeronautica
Electromagnetic Anechoic Chamber has recently been built by Alenia Aeronautica at Caselle South Plant: The Anechoic Chamber is a full anechoic chamber, and it has been designed to carry out electromagnetic vulnerability tests mainly on fighter and unmanned aircraft. In addition measurement can be carried out on many different vehicles that can be brought into the chamber through the main access door. A system to extract exhaust gas was installed in order to carry out tests on a wide variety of vehicles. The Anechoic Chamber has been designed to carry out both HIRF/EMC test and High Sensitivity RF measurement: in particular HIRF/EMC tests in the frequency range 30MHz ÷ 18GHz with the capability of radiating a very high intensity electromagnetic field and High Sensitivity RF measurement, including antenna pattern measurements on antennas installed on aircraft in the frequency range 500MHz ÷ 18GHz. During the design phase a 1/12th scale model of the chamber had been fabricated to assess the desired electromagnetic performance. In this phase of design the model was tested at the scale frequencies for Filed Uniformity, Site Attenuation and Free Space VSWR results. This study was published at the AMTA 2004 meeting. In addition to the physical model, during the construction phase, various computer simulations were performed to further define the detailed internal absorber layout and to define test acceptance methods for procedures not covered by the standards. The computer model analysis was conducted to identify areas of scattering that could be treated with higher performance absorbers to improve the chambers quiet zone performance. The identified “Fresnel Zones." have been treated with high performance absorbers optimized to provide improved performance at microwave frequencies. The absorber optimization was reported at the AMTA 2006 meeting. This optimization has allowed validation of the chamber according to the requirements of CIRSP 16-1-4 2007-02 in the range of frequency 30 MHz - 18GHz. The size (shield to shield) of chamber is 30m wide, 30m long and 20m high, and the 18m wide by 8.5m high main door allows the SUT access. The shielded structure is a welded structure of 3mm-thick steel panels which guarantees shielding effectiveness of more than 100 dB in the frequency range 100 kHz to 20GHz. The chamber includes a 10 meter diameter turntable to rotate a 30 ton SUT with an angular accuracy of ± 0.02° and a pathway to allow SUT access. Both the pathway and the turntable are permanently covered by ferrite tiles. A hoist system permits lifting of the SUT (max 25 tons) up to 10 meters from the turntable centre enabling EMC testing on aircraft with the landing gear retracted.
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