DTU-ESA Millimeter-Wave Validation Standard Antenna – Requirements and Design
Inter-comparisons and validations of antenna measurement ranges are useful tools allowing the detection of various problems in the measurement procedures, thus leading to improvements of the measurement accuracy and facilitating better understanding of the measurement techniques. The maximum value from a validation campaign is achieved when a dedicated Validation Standard (VAST) antenna specifically designed for this purpose is available. The widespread use of the known VAST-12 (12 GHz) antenna, developed by the Technical University of Denmark (DTU) in 1987 and operated by the DTU-ESA Facility since 1992, demonstrates the long-term value of the dedicated VAST antennas . The driving requirements of VAST antennas are their mechanical stability with respect to any orientation of the antenna in the gravity field and thermal stability over a given operational temperature range. The mechanical design shall ensure extremely stable electrical characteristics with variations typically an order of magnitude smaller than the measurement uncertainty. At the same time, it must withstand high g-loads under frequent transportations and it shall also support convenient handling of the VAST antenna (practical electrical and mechanical interfaces, low mass, attachment points for lifting, etc.). Today, there is a well identified need for increased operational frequencies to get access to large bandwidth for broadband communication. Upcoming satellite communication services utilize up/down link at K/Ka-bands, while the use of Q/V bands is contemplated for the feeder links in the coming years. In response to this need, a millimeter-wave VAST (mm-VAST) antenna is currently under development in a collaboration between the DTU and TICRA under contract from the European Space Agency. In this paper, the electrical and mechanical requirements of the DTU-ESA mm-VAST antenna are discussed and presented. Potential antenna types fulfilling the electrical requirements are briefly reviewed and the baseline design is described. The emphasis is given to definition of the requirements for the mechanical and thermal stability of the antenna, which satisfy the stringent stability requirement for the mm-VAST electrical characteristics.  S. Pivnenko et al., “Comparison of Antenna Measurement Facilities with the DTU-ESA 12 GHz Validation Standard Antenna within the EU Antenna Centre of Excellence”, IEEE Trans. Antennas Propagat., 2009, vol. 57, no. 7. pp. 1863-1878.
Advanced Positioner Control Techniques in Antenna Measurements
Antenna, Radome, and RCS testing systems rely on high-fidelity positioner systems to provide high-precision positioning of articles for RF testing. Historically, the industry has relied on linear PID control techniques in torque, velocity, and position control loops on individual axes to drive the positioners. Recently, advancements have been made in the use of advanced control hardware including multiple-DOF laser and optical feedback devices, brushless DC motors, VFD AC motors, and multi-drive torque-biased actuation. Advanced control techniques including single-axis error correction, multi-axis global error compensation, and multi-axis coordinated motion have been implemented to improve positioner accuracy. Here, a survey is conducted of control technologies in other industries such as machine tools and industrial robotics. An assessment is conducted on the viability of other advanced techniques to provide insight into the potential future control and capabilities of positioning systems in the RF testing industry. Candidate advanced techniques include gain scheduling and sliding-mode control which could provide improved accuracy over a wider range of conditions including varying loads and operating points caused by differing movement speeds or large variations in static loading. Dynamic input-shaping and feed-forward techniques could help suppress dynamic vibrations and improve dynamic tracking behavior for improved continuous-measurement scanning accuracy. Adaptive and non-linear control techniques might improve disturbance and error rejection for improved accuracy while managing dynamic-behavior drift allowing for adaptation to long-term positioner changes without re-tuning.
Dual-calibration Processing Based on Minimum Weighted Mean Squared Error (MWMSE) in RCS Measurement
Dual-calibration was first proposed by Chizever et al. in 1996 [AMTA'1996] and had get wide applications in evaluation of the uncertainty in radar cross section (RCS) measurement and calibration. In 2013, LaHaie proposed a new technique based on jointly minimizing the mean squared error (MMSE) [AMTA'2013] among the calibrated RCS of multiple calibration artifacts, which estimates both the calibration function and the calibration uncertainty for each artifact. MMSE greatly improves the estimation accuracy for the radar calibration function as well as results in lower residual and RCS calibration errors. This paper presents a modified version of LaHaie's MMSE by minimizing the weighted mean squared error (MWMSE) for RCS calibration processing from multiple calibrator measurements, which is related to the following functions and parameters: the calibration function; the theoretical and measured RCS; the number of calibration artifacts the number of frequency samples and the weight for ith calibration artifacts which may be defined in terms of the theoretical RCS of all the calibration artifacts. For example, if the weight is defined as the inverse of the total theoretical RCS of the ith calibration artifacts for all frequency samples, the error then represents the total relative calibration error instead of an absolute error as in MMSE. MWMSE then means that an optimal calibration function is found in terms of minimum total relative calibration error, which is expected for most applications. Numerical simulation results are presented to demonstrate the usefulness of the proposed technique.
Phase Interferometry in a Planar Near-Field Scanner
This paper explores the accuracy capabilities of a two element phase interferometer measurement in a planar near-field scanner. Traditional phase interferometer applications utilize wide field of view antennas such as spirals making the utilization of planar near-field measurements less than ideal. In this application, high directivity antennas were utilized which allowed us to consider a planar near-field measurement solution. Leaving the AUT stationary and the stability of the planar near-field coordinate system were primary considerations in deciding to utilize a planar near-field measurement system. Typical interferometer performance metrics include comparing measured phase differences to ideal element phase differences at the same locations. Often the nominal drawing locations are used to generate the ideal element phase difference curves. The sensitivity of actual element vector displacement values versus ideal displacements can be reduced by deriving the best-fit displacement vector from the measured data and is utilized in the processing and reporting of results. This paper reviews the measurements, analysis techniques and results from this investigation and illustrates the capabilities of a planar near-field scanner to perform these types of measurements with a high degree of measurement fidelity.
Spherical Geometry Selection Used for Error Evaluation
ABSTRACT Spherical near-field error analysis is extremely useful in allowing engineers to attain high confidence in antenna measurement results. NSI has authored numerous papers on automated error analysis and spherical geometry choice related to near field measurement results. Prior work primarily relied on comparison of processed results from two different spherical geometries: Theta-Phi (0 =?= 180, -180 = f = 180) and Azimuth-Phi (-180 =?= 180, 0 = f = 180). Both datasets place the probe at appropriate points about the antenna to measure two different full spheres of data; however probe-to-antenna orientation differs in the two cases. In particular, geometry relative to chamber walls is different and can be used to provide insight into scattering and its reduction. When a single measurement is made which allows both axes to rotate by 360 degrees both spheres are acquired in the same measurement (redundant). They can then be extracted separately in post-processing. In actual fact, once a redundant measurement is made, there are not just two different full spheres that can be extracted, but a continuum of different (though overlapping) spherical datasets that can be derived from the single measurement. For example, if the spherical sample density in Phi is 5 degrees, one can select 72 different full sphere datasets by shifting the start of the dataset in increments of 5 degrees and extracting the corresponding single-sphere subset. These spherical subsets can then be processed and compared to help evaluate system errors by observing the variation in gain, sidelobe, cross pol, etc. with the different subset selections. This paper will show the usefulness of this technique along with a number of real world examples in spherical near field chambers. Inspection of the results can be instructive in some cases to allow selection of the appropriate spherical subset that gives the best antenna pattern accuracy while avoiding the corrupting influence of certain chamber artifacts like lights, doors, positioner supports, etc. Keywords: Spherical Near-Field, Reflection Suppression, Scattering, MARS. REFERENCES Newell, A.C., "The effect of measurement geometry on alignment errors in spherical near-field measurements", AMTA 21st Annual Meeting & Symposium, Monterey, California, Oct. 1999. G. Hindman, A. Newell, “Spherical Near-Field Self-Comparison Measurements”, Proc. Antenna Measurement Techniques Association (AMTA) Annual Symp., 2004. G. Hindman, A. Newell, “Simplified Spherical Near-Field Accuracy Assessment”, Proc. Antenna Measurement Techniques Association (AMTA) Annual Symp., 2006. G. Hindman & A. Newell, “Mathematical Absorber Reflection Suppression (MARS) for Anechoic Chamber Evaluation and Improvement”, Proc. Antenna Measurement Techniques Association (AMTA) Annual Symp., 2008. Pelland, Ethier, Janse van Rensburg, McNamara, Shafai, Mishra, “Towards Routine Automated Error Assessment in Antenna Spherical Near-Field Measurements”, The Fourth European Conference on Antennas and Propagation (EuCAP 2010) Pelland, Hindman, “Advances in Automated Error Assessment of Spherical Near-Field Antenna Measurements”, The 7th European Conference on Antennas and Propagation (EuCAP 2013)
Design and Validation of Compact Antenna Test Ranges using Computational EM
The design of modern Compact Antenna Test Ranges (CATRs) is a challenging task, and to achieve strong performance, simulation using computational electromagnetics is a vital part of the design process. However, the large electrical size, geometrical complexity and high accuracy requirements often mean that the available computer resources are not sufficient for running the simulation. In the present paper, we highlight some recent developments that allow for much larger, faster and more accurate simulations than was possible just a few years ago, and apply them to realistic ranges. The conclusion is clear: Modern software tools allow designers of CATRs to achieve better performance in shorter time than was previously possible.
A New Method for VHF/UHF Characterization of Anisotropic Dielectric Materials
Recent interest in anisotropic metamaterials and devices made from these materials has increased the need for advanced RF material characterization. Moreover, the quest for measurement of inhomogeneous and anisotropic materials at VHF and UHF frequencies has long been one of the primary stretch goals of the RF materials measurement community. To date, the only viable method for these types of materials has been either fully filled or partially filled VHF waveguides, which are large, expensive, and slow. This paper introduces a new fixture design that greatly simplifies the process of obtaining intrinsic properties for inhomogeneous and anisotropic dielectric materials. The fixture combines low frequency capacitance and high frequency coaxial airline concepts to measure cube shaped specimens, and is termed an “RF Capacitor”. Furthermore, a significant limitation of past measurement methods is their reliance on approximate analytical models to invert material properties. These analytical models restrict the available geometries and frequency ranges that a measurement fixture can have. The present method avoids this limitation by implementing a new inversion technique based on a full-wave, finite difference time domain (FDTD) solver to exactly model the measurement geometry. In addition, this FDTD solver is applied in a novel way to enable inversion of frequency-dependent dielectric properties within seconds. This paper presents the fixture design and calibration for this new measurement method, along with example measurements of isotropic and anisotropic dielectric materials. In particular, 3” cube specimens are measured and the bulk dielectric properties in the three principal planes are determined by measuring the same specimen in three different orientations within the measurement fixture. Finally, calculations are presented to show the relative accuracy of this method against a number of probable uncertainty sources, for some characteristic materials.
A Reduced Uncertainty Method for Gain over Temperature Measurements in an Anechoic Chamber
Gain over Temperature (G/T) is an antenna parameter of importance in both satellite communications and radio-astronomy. Methods to measure G/T are discussed in the literature [1-3]. These methodologies usually call for measurements outdoors where the antenna under test (AUT) is pointed to the “empty” sky to get a “cold” noise temperature measurement; as required by the Y-factor measurement approach . In reference , Kolesnikoff et al. present a method for measuring G/T in an anechoic chamber. In this approach the chamber has to be maintained at 290 kelvin to achieve the “cold” reference temperature. In this paper, a new method is presented intended for the characterization of lower gain antennas, such as active elements of arrays. The new method does not require a cold temperature reference thus alleviating the need for testing outside or maintaining a cold reference temperature in a chamber. The new method uses two separate “hot” sources. The two hot sources are created by using two separate noise diode sources of known excess noise ratios (ENR) or by one source and a known attenuation. The key is that the sources differ by a known amount. In a conventional Y-factor measurement , when the noise source is turned off, the noise power is simply the output attenuator acting as a 50 ohm termination for the rest of the receive system. But by using two known noise sources, the lower noise temperature source takes the place of T-cold in the Y-factor equations. The added noise becomes the difference in ENR values. An advantage of this approach is that it allows all the ambient absorber thermal noise temperature change effects to be small factors thus reducing one of the sources of uncertainty in the measurement. This paper provides simulation data to get an approximation of the signal loss from the probe to the antenna under test (AUT). Another critical part of the method is to correctly define the reference plane for the measurement. Preliminary measurements are presented to validate the approach for a known amplifier attached to an open ended waveguide (OEWG) probe which is used as the AUT.  Kraus J. Antennas 2nd ed.1988 McGraw-Hill: Boston, Massachusetts.  Kraus J. Radio Astronomy Cygnus-Quasar Books 1986.  Dybdal R. B. “G/T Comparative Measurements” 30th Annual Antenna Measurement Techniques Association Annual Symposium (AMTA 2008), Boston, Massachusetts, November 2008.  “Noise Figure Measurement Accuracy – The Y-Factor Method”, Agilent Technologies, Application Note AN57-2.  Kolesnikoff, P. Pauley, R. and Albers, L “G/T Measurements in an Anechoic Chamber” 34th Annual Antenna Measurement Techniques Association Annual Symposium (AMTA 2012), Bellevue, Washington Oct 2012. Keywords: Gain over Temperature (G/T), Satellite Communication, Radio Astronomy, Noise Figure Measurement
Estimating Measurement Uncertainties in Compact Range Antenna Measurements
Methods for determining the uncertainty in antenna measurements have been previously developed and presented. The IEEE recently published a document  that formalizes a methodology for uncertainty analysis of near-field antenna measurements. In contrast, approaches to uncertainty analysis for antenna measurements on a compact range are not covered as well in the literature. Unique features of the compact range measurement technique require a comprehensive approach for uncertainty estimation for the compact range environment. The primary difference between the uncertainty analyses developed for near-field antenna measurements and an uncertainty analysis for a compact range antenna measurement lies in the quality of the incident plane wave illuminating the antenna under test from the compact range reflector. The incident plane wave is non-ideal in amplitude, phase and polarization. The impact of compact range error sources on measurement accuracy has been studied [2,3] and error models have been developed [4,5] to investigate the correlation between incident plane wave quality and the resulting measurement uncertainty. We review and discuss the terms that affect gain and sidelobe uncertainty and present a framework for assessing the uncertainty in compact range antenna measurements including effects of the non-ideal properties of the incident plane wave. An example uncertainty analysis is presented. Keywords: Compact Range, Antenna Measurement Uncertainty, Error Analysis References: 1. IEEE Standard 1720-2012 Recommended Practices for Near-Field Antenna Measurements. 2. Bingh,S.B., et al, “Error Sources in Compact Test Range”, Proceedings of the International Conference on Antenna Technologies ICAT 2005. 3. Bennett, J.C., Farhat, K.S., “Wavefront Quality in Antenna Pattern Measurement: the use of residuals.”, IEEE Proceedings Vol. 134, Pt. H, No. 1, February 1987. 4. Boumans, M., “Compact Range Antenna Measurement Error Model”, Antenna Measurement Techniques Association 1996 5. Wayne, D., Fordham, J.A, Mckenna, J., “Effects of a Non-Ideal Plane Wave on Compact Range Measurements”, Antenna Measurement Techniques Association 2014
Predicting the Performance of a Very Large, Wideband Rolled-Edge Reflector
Achieving a very large quiet zone across a wide frequency band, in a compact range system, requires a physically large reflector with a suitable surface accuracy. The size of the required reflector dictates attention to several important processes, such as how to manufacture the desired surface across a large area and the practicality of transportation and installation. This inevitably leads to the segmentation of the reflector into multiple panels; which must be fabricated, installed, and aligned to each other to conform to the required geometry. Performance predictions must take into account not only the surface accuracy of the individual panels but also their alignment errors. This paper presents the design approach taken on a recent project for a compact range system utilizing a blended rolled-edge reflector that produces a 5 meter quiet zone across a frequency range of 350 MHz to 40 GHz. It discusses the physical segmentation strategy, the fabrication methodology, the intermediate qualification of panels, the panel alignment technique, and the laser-based metrology methodology employed. Performance analysis approach and results will be presented for the geometry as conceived and then for the realized panelized reflector as machined and aligned.
A New Method for Millimeter-Wave Characterization of Thin Resistive Fabrics
As millimeter-wave applications become more widely available technologies, there is a demand to know material properties for design and application purposes. However, many mass produced materials are either not specified at these frequencies or the price materials can be costly. Therefore the easiest method for characterization is by measurement. Traditional methods of this measurement type involve the reflectivity of a fabric sample placed on a flat metallic reference plate. However, this method has some major difficulties at these high frequencies. For example, the surface of the reference plate must be very flat and smooth and must be carefully oriented such that their surface is precisely facing the transmitting and receive and antennas. Furthermore the electrically large size of the reference plate of this setup makes it difficult to measure in far-field and anechoic range time is expensive. Resistive and conductive fabrics have applications such as shielding, anti-static, and radio wave absorption. Radio wave absorption and radar cross section engineering is currently of high interest to the automotive industry for testing newly emerging automotive radar systems. Such fabric measurement has already been utilized to accurately characterize artificial skin for radar mannequins to recreate the backscattering of human targets at 77 GHz. This paper presents a new and convenient method for measuring the reflective properties of conductive and resistive materials at millimeter wave frequencies by wrapping fabrics around a metallic reference cylinder. This new approach to fabric characterization method is able to obtain higher accuracy and repeatability despite the difficulties of measuring at high frequency.
Achieving Impressive Global Positioning and Stability in a High Fidelity Antenna Measurement System
Highly accurate antenna measurements can require precise alignment and positioning of the probe antenna to the antenna under test. The positioning of the antenna during acquisition can involve the movement of several simultaneous axes of motion. This places a global positioning accuracy requirement on the positioning system. To achieve precision in global positioning and alignment, an understanding of dominant error factors such as load induced deflection/resonance, thermal deflection, positioning error sources and mechanical alignment tolerances is essential. This paper focuses on how global accuracy and stability were achieved, addressing these factors, on a recently delivered large far field antenna measurement system. The system involved eight axes of positioning with the ability to position 950 lbs antenna under test 19.5 ft above the chamber floor achieving 0.007 inch and 0.005 degrees positioning accuracy relative to the global range coordinate system. Stability of the probe antenna after motion was within 0.001 inch. Key Words: Global Position Accuracy, Far Field, Position Stability, Simultaneous Motion, Position Error Correction, High Accuracy, Precise Motion
Speed and Accuracy Considerations in Modern Phase Center Measurements
This paper presents a method for determining the phase center of an antenna based on pattern measurements made over multiple frequencies with a two axis spherical positioning system. Mathematical calculations are used to determine the best-fit sphere for the measured phase data. The origin of the sphere is the phase center of the antenna at the frequency of interest. This method provides fast, flexible multi-frequency measurement of an antenna’s phase center. Results validating the proposed method are presented using both simulated data and measured data. In order to determine the accuracy of this method, a dipole is translated a precise distance within the measurement coordinate system. The difference between phase center measurements made before and after the translation gives an indication of the potential accuracy of the measurements. Also, major contributors to phase center measurement uncertainty are discussed with consideration to reducing the overall phase center measurement uncertainty.
An Innovative Close-Range Antenna Scanner System for Obtaining Far-Field Radiation Pattern of Installed Antenna at Short Distances
We have successfully designed and developed an innovative “CLose-range Antenna Scanner System” (or CLASS) suitable for measuring the far-field radiation pattern of installed antennae at short distances. The system consists of three key components: (1) a uniquely designed lens horn antenna that generates plane waves in close proximity, (2) a mechanical x-y scanner to scan the antenna-under-test, and (3) a customized stitching software to compute the far-field antenna pattern from the measured field information. The developed system has a scan area of 4.6 x 4.6 m, with resolutions of ±0.1mm in both the x and y traverse directions. The scanner structure is designed in a scalable fashion to cater for measurement of antenna installed at various locations (e.g. front and sides) on a platform. The system is capable of measurement from 1 to 18 GHz and generates far-field radiation pattern with a gain accuracy of ±1 dB.
Development of a FMCW Radar Sensor For Soil Humidity Estimation
To determine the proper moisture content in the soil is critical to get maximum grow in plants and crops and its estimation it is used to regulate the amount of irrigation that it is needed. For this reason, many sensors that measure water content have been developed to give the grower some feedback of the water content. Some methods such as the ones based in gravimetric properties are accurate but labor consuming, other such as the tension meters require periodic service, the neutron probe is also accurate but expensive. The more popular sensor is based in electrical resistance measurement that gives acceptable accuracy and it is not expensive. However, this sensor has the disadvantage that needs to be buried in the soil. Here, we are exploring the characteristics of electromagnetic propagation and its scattering properties as a tool to identify the physical soil composition. The presence of water changes drastically the dielectric properties of the soil affecting the reflected signal. In this research, we are assessing the viability of a sensor based in FMCW radar technology for water detection with the advantage of being portable and low cost. The research involves the fabrication of a directive antenna operating in a broadband regimen, transmitter/ receiver circuit and the signal processing of the return signal adjusted to the detection of moisture in soil. We present the calibration methods and graphic results of the intensity of the reflected signal of dry bare soil, wet soil, and soil covered by plants.
A Study on the Effects of Influence Factors for Antenna Radiation Efficiency Measurements in Anechoic Chamber
?Radiation efficiency is an important attribute of an antenna that can be calculated from its gain and directivity. This paper focuses on investigating the effects of influence factors for antenna radiation efficiency measurement in an anechoic chamber (AC). The gain transfer method (GTM) is used widely during the gain measurement, but the results can be influenced by many factors. A comparison of gain measurement performed by GTM and the three-antenna method (TAM) is presented. All measurements were carried out between 1 GHz and 8 GHz in an anechoic chamber with a double-ridged waveguide horn antenna as the antenna under test (AUT), which has a relatively broad half-power beamwidth. The results show that the maximum difference between the two methods is about 1.5 dB and the GTM may bring greater measurement uncertainty. To evaluate the influence of directivity and its repeatability, two sets of directivity measurements were performed using four different antenna mounting brackets, namely: Rohacell foam, Tufnol, metal, and metal covered by radio frequency absorber. Amongst the antenna mounting brackets, the Tufnol bracket gives the best repeatability performance. The antenna axial symmetrical properties were also assessed for each antenna mounting bracket except for Rohacell foam. The results shows that the gain measurement has more influence over characterization of antenna radiation efficiency as compared with the directivity measurement. To improve accuracy for radiation efficiency measurement, one suggests to use TAM for the antenna gain measurement.
The DTU-ESA Millimeter-Wave Validation Standard Antenna - Manufacturing and Testing
Inter-comparisons of antenna test ranges serve the purpose of validating the measurement accuracy of a given range before it can be qualified to perform certain measurements, which is particularly important for space applications, where antenna specifications are very stringent. Moreover, by verifying the measurement procedures and identifying sources of errors and uncertainties, inter-comparison campaigns improve our understanding of strengths and limitations of different measurement techniques, which, in turn, leads to further improved measurement accuracies. The lesson learned from early comparison campaigns executed by the Technical University of Denmark (DTU) in early 80s on some readily available antennas says that proper inter-comparisons can only be done on dedicated antennas, whose design is driven by stringent requirements on their rigidity and mechanical stability. Furthermore, well-defined reference coordinate systems are essential. These principles have convincingly been proven valid by the VAST-12 antenna designed by DTU in late 80s, which in more than 20 years has demonstrated its usefulness and a long-term value. Currently, the satellite communication industry is actively commercializing the mm-wave frequency bands (K/Ka-bands) in its strive for wide frequency bandwidth and higher bit-rates. The next step is the exploration and exploitation of the Q/V-band. In this scenario, the European Space Agency (ESA) is expanding its portfolio of VAlidation STandard antennas (VAST) into mm-waves to ensure accurate measurements of the next generation communication antennas. This time, ESA demands all four bands (K/Ka/Q/V-bands) to be covered by a single VAST antenna. In this contribution, we report our efforts in designing, fabricating, and testing a new precision tool for antenna test range qualification and inter-comparisons at mm-waves -- the mm-VAST antenna. In particular, we present the details of the antenna mechanical design, fabrication and assembling procedures. The performance verification test plan as well as first measurement results will also be discussed.
CATR Quiet Zone Modelling and the Prediction of "Measured" Radiation Pattern Errors: Comparison using a Variety of Electromagnetic Simulation Methods
The single-offset compact antenna test range (CATR) is a widely deployed technique for broadband characterization of electrically large antennas at reduced range lengths . The nature of the curvature and position of the offset parabolic reflector as well as the edge geometry ensures that the resulting collimated field is comprised of a pseudo transverse electric and magnetic (TEM) wave. Thus, by projecting an image of the feed at infinity, the CATR synthesizes the type of wave-front that would be incident on the antenna under test (AUT) if it were located very much further away from the feed than is actually the case with the coupling of the plane-wave into the aperture of the AUT creating the classical measured “far-field” radiation pattern. The accuracy of a pattern measured using a CATR is primarily determined by the phase and amplitude quality of the pseudo plane-wave with this being restricted by two main factors: amplitude taper (which is imposed by the pattern of the feed), and reflector edge diffraction, which usually manifests as a high spatial frequency ripple in the pseudo plane wave . It has therefore become customary to specify CATR performance in terms of amplitude taper, and amplitude & phase ripple of this wave over a volume of space, termed the quiet-zone (QZ). Unfortunately, in most cases it is not directly apparent how a given QZ performance specification will manifest itself on the resulting antenna pattern measurement. However, with the advent of powerful digital computers and highly-accurate computational electromagnetic (CEM) models, it has now become possible to extend the CATR electromagnetic (EM) simulation to encompass the complete CATR AUT pattern measurement process thereby permitting quantifiable accuracies to be easily determined prior to actual measurement. As the accuracy of these models is paramount to both the design of the CATR and the subsequent determination of the uncertainty budget, this paper presents a quantitative accuracy evaluation of five different CEM simulations. We report results using methods of CATR modelling including: geometrical-optics with geometrical theory of diffraction , plane-wave spectrum , Kirchhoff-Huygens  and current element , before presenting results of their use in the antenna pattern measurement prediction for given CATR-AUT combinations. REFERENCES C.G. Parini, S.F. Gregson, J. McCormick, D. Janse van Rensburg “Theory and Practice of Modern Antenna Range Measurements”, IET Press, 2014, ISBN 978-1-84919-560-7. M. Philippakis, C.G. Parini, “Compact Antenna Range Performance Evaluation Uging Simulated Pattern Measurements”, IEE Proc. Microw. Antennas Propag., Vol. 143, No. 3, June 1996, pp. 200-206. G.L. James, “Geometrical Theory of Diffraction for Electromagnetic Waves”, 3rd Edition, IET Press, 2007, ISBN 978-0-86341-062-8. S.F. Gregson, J. McCormick, C.G. Parini, “Principles of Planar Near-Field Antenna Measurements”, IET Press, 2007.
Computation of the Far Field Radiated by Aperiodic Sampled Planar Fields by Means of NUFFT
It is a common practice when computing radiation patterns from non-uniformly sampled planar fields to interpolate the samples into a regular grid , although it might cause inaccuracies due to the interpolation process. The non-uniform fast Fourier transform (NUFFT) has been applied to process near field measurements in non-uniform planar grids with arbitrary precision , and also to analyze aperiodic arrays . However, samples are usually treated as punctual sources. In this contribution, an efficient and accurate method to calculate the far field radiated by non-uniformly sampled planar fields which comply the Nyquist theorem using the non-uniform fast Fourier transform (NUFFT) is shown. The method takes into account the amplitude of the unit cell radiation pattern, which allows to compute more accurately the copolar and crosspolar components of the far field with regard to the array factor , which models the samples as punctual sources. For measured fields it is assumed that post-processing has been done, for instance, taking into account probe corrections. Because the NUFFT is precision-dependent, a discussion of how its accuracy can affect the computed radiated fields will be carried out. Numerical examples will be provided to show the accuracy and performance of the NUFFT with regard to the FFT and direct evaluation of the far fields. Finally, a study of computing times comparing the FFT, NUFFT and direct evaluation will be presented. References  Y. Rahmat-Samii, L. I. Williams, and R. G. Yaccarino, “The UCLA bi-polar planar-near-field antenna-measurement and diagnostics range,” IEEE Antennas Propag. Mag., vol. 37, no. 6, pp. 16–35, Dec. 1995.  R. C. Wittmann, B. K. Alpert, and M. H. Francis, “Near-field antenna measurements using nonideal measurement locations,” IEEE Trans. Antennas Propag., vol. 46, no. 5, pp. 716–722, May 1998.  A. Capozzoli, C. Curcio, G. D'Elia, and A. Liseno, “Fast phase-only synthesis of conformal reflectarrays,” IET Microw. Antennas Propag., vol. 4, no. 12, Dec. 2010.
Comparison of Reflector Antenna Measurements and Simulations
In antenna measurement, well-established procedures are consolidated to determine the associated measurement uncertainty for a given antenna and measurements scenario. Similar criteria for establishing uncertainties in numerical modeling of the same antenna are still to be established. In this paper, we investigate the achievable agreement between antenna measurement and simulation when external error sources are minimized. The test object, is a reflector fed by a wideband dual ridge horn (SR40-A and SH4000) manufactured by MVG. This highly stable reference antenna has been selected to minimize uncertainty related to finite manufacturing and material parameter accuracy. Two frequencies, 10.7GHz and 18GHz have been selected for detailed investigation. The antenna has been measured by several measurement facilities (spherical, cylindrical and planar near field ranges) across Europe in the frame of the EurAAP/WG5 “Facility Comparison Campaign” activity. The purpose of this intercomparison campaign is the comparison of the different antenna measurement facilities, throughout Europe, considering measurement procedures and uncertainty estimates. The antenna has been simulated using a full CAD model, in step compatible format and using different numerical methods from different software vendors.