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Accuracy

Speed and Accuracy Considerations in Modern Phase Center Measurements
James Huff, November 2015

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
Serguei Matitsine,Tse-Tong Chia, Alexander Zubov, Rengasamy Kumaran, Hock-Kwee Oh, November 2015

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
Maria C. Gonzalez,Christian Hurd, Jose Enrique Almanza Medina, Xiaoguang Liu, November 2015

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
Qi Wanquan ,Tian Hong Loh, November 2015

?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
Oleksiy Kim,Sergey Pivnenko, Olav Breinbjerg, Rolf Jørgensen, Niels Vesterdal, Kim Branner, Peter Berring, Christen Malte Markussen, Maurice Paquay, November 2015

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.

Computation of the Far Field Radiated by Aperiodic Sampled Planar Fields by Means of NUFFT
Daniel Rodríguez Prado,Manuel Arrebola, Marcos Rodríguez Pino, Fernando Las-Heras, November 2015

It is a common practice when computing radiation patterns from non-uniformly sampled planar fields to interpolate the samples into a regular grid [1], 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 [2], and also to analyze aperiodic arrays [3]. 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 [3], 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 [1] 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. [2] 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. [3] 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.

CATR Quiet Zone Modelling and the Prediction of "Measured" Radiation Pattern Errors: Comparison using a Variety of Electromagnetic Simulation Methods
Clive Parini,Rostyslav Dubrovka, Stuart Gregson, November 2015

The single-offset compact antenna test range (CATR) is a widely deployed technique for broadband characterization of electrically large antennas at reduced range lengths [1]. 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 [2]. 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 [3], plane-wave spectrum [4], Kirchhoff-Huygens [4] and current element [3], before presenting results of their use in the antenna pattern measurement prediction for given CATR-AUT combinations. REFERENCES [1]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. [2]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. [3]G.L. James, “Geometrical Theory of Diffraction for Electromagnetic Waves”, 3rd Edition, IET Press, 2007, ISBN 978-0-86341-062-8. [4]S.F. Gregson, J. McCormick, C.G. Parini, “Principles of Planar Near-Field Antenna Measurements”, IET Press, 2007.

Comparison of Reflector Antenna Measurements and Simulations
M. Sierra Castañer,L. Foged, M. Saporetti, E. Jørgensen, T. Voigt, D. Tallini, M. Orefice, G. Giordanengo, G. Dassano, M. Böttcher, A. Wien, J. Serna, D. Pérez de Diego, F. Calvano, November 2015

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.

Comparison of the Near-field Measurements between a Commercial Open-Ended Rectangular Waveguide Probe and its Equivalent Probe in SIW Technology
Nuria Esparza,Pablo Alcón, Luis F. Herrán, Fernando Las-Heras, November 2015

Rectangular open-ended waveguide probes are commonly used in near-field antenna measurements because their frequency behaviour is widely well-known and modeled. Nevertheless, in the last years, the substrate integrated waveguide technology has been developed as a harder competitor. These new circuits are a compromise between the advantages of classical rectangular waveguides, such as high quality factor and low losses, and the advantages of planar circuits, such as low cost and easy compact integration. Also, SIWs present lower weight and dimensions than their equivalent circuits based on metallic waveguides. In this paper we study, under the same measurement conditions, the behaviour of a commercial open-ended rectangular waveguide probe and its equivalent probe in SIW technology. We will compare the near-field measurements obtained with both probes and will show that SIW probes present higher spatial resolution than their equivalent commercial probes. So, SIW probes can detect possible abrupt electric field circuit changes with more accuracy than commercial rectangular waveguides, under the same measurement conditions. The ability of the presented probes has been investigated measuring the simulated amplitude and phase of the electric field of a pyramidal horn placed a few centimetres of the probes. And the study has been validated with the measurements of a microstrip antenna in X-band that presents non-uniform electric field.

Computational Electromagnetic Modeling of Near-Field Antenna Test Systems Using Plane Wave Spectrum Scattering Matrix Approach
Allen Newell,Stuart Gregson, November 2014

In recent years a number of analyses and simulations have been published that estimate the effect of using a probe with higher order azimuthal modes with standard probe corrected spherical transformation software.  In the event the probe has higher order modes, errors will be present within the calculated antenna under test (AUT) spherical mode coefficients and the resulting asymptotic far-field parameters [1, 2, 3, 4] where the simulations were harnessed to examine these errors in detail.  Within those studies, a computational electromagnetic simulation (CEM) was developed to calculate the output response for an arbitrary AUT/probe combination where the probe is placed at arbitrary locations on the measurement sphere ultimately allowing complete near-field acquisitions to be simulated.  The planar transmission equation was used to calculate the probe response using the plane wave spectra for actual AUTs and probes derived from either planar or spherical measurements.  The planar transmission formula was utilized as, unlike the spherical analogue, there is no limitation on the characteristics of the AUT or probe thereby enabling a powerful, entirely general, model to be constructed.  This paper further extends this model to enable other measurement configurations and errors to be considered including probe positioning errors which can result in ideal first order probes exhibiting higher order azimuthal mode structures.  The model will also be used to determine the accuracy of the Chu and Semplak near-zone gain correction [5] that is used in the calibration of pyramidal horns.  The results of these additional simulations are presented and discussed. Keywords: near-field, antenna measurements, near-field probe, spherical alignment, spherical mode analysis. REFERENCES A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction”, Antenna Measurement Techniques Association (AMTA) 34th Annual Meeting & Symposium, Bellevue, Washington October 21-26, 2012. A.C. Newell, S.F. Gregson, “Higher Order Mode Probes in Spherical Near-Field Measurements”, 7th European Conference on Antennas and Propagation (EuCAP 2013) 8-12 April 2013. A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction”, Antenna Measurement Techniques Association (AMTA) 35th Annual Meeting & Symposium, Columbus, Ohio, October 6-11, 2013. A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Azimuthal Modes in Spherical Near-Field Probe Correction”, The 8th European Conference on Antennas and Propagation (EuCAP 2014) 6-11 April 2014. T.S. Chu, R.A. Semplak, “Gain of Electromagnetic Horns,’’ Bell Syst. Tech. Journal, pp. 527-537, March 1965

On the Use of Basis Pursuit and a Forward Operator Dictionary to Separate Specific Background Types from Target RCS Data
Brian Fischer,Ivan Lahaie, Mark Hawks, Timothy Conn, November 2014

RCS measurements are often comprised of a combination of the coherent summation of many things in addition to the desired target.  Those other things contribute to error in RCS measurements and include noise, clutter and background, which can be further characterized according to specific types.  An approach has been developed that is capable of capturing and separating certain types of noise, clutter and background based on specific forward models to include RFI, target support (e.g., pylon), and many others, such that engineers can clearly see the separated components and selectively choose to include, exclude, or edit as the case may be.  This approach affords far more flexibility than classic image edit reconstruct (IER), and offers more editing accuracy than Fourier-based approaches including entire phase history based approaches.  This paper describes the basic approach and shows examples with measured data.

Antenna Alignment and Positional Validation of a mmWave Antenna System Using 6D Coordinate Metrology
David Novotny,Joshua Gordon, Jeff Guerrieri, November 2014

Antenna alignment for near-field scanning was typically done at NIST with multiple instruments (theodolites, electronic levels, motor encoders) to align multiple stacked motion stages (linear, rotation).  Many labs and systems are now using laser trackers to measure ranges and perform periodic compensation across the scan geometry.  We are now seeing the use of laser trackers with 3D coordinate metrology to align ranges and take positional data.  We present the alignment techniques and positional accuracy and uncertainty results of a mmWave antenna scanning system at 183 GHz. We are using six degree-of-freedom (6DOF) AUT and Probe measurements (x, y, z, yaw, pitch, roll) to align the AUT and then to align the scan geometry to the AUT.  We are using a combination of 3DOF laser tracker measurements with a combined 6DOF laser tracker/photogrammetry sensor. We combine these measurements using coordinated spatial metrology to assess the quality of each motion stage in the system, tie the measurements of each individual alignment together, and to assess scan geometry errors for position and pointing.  Finally we take in-situ 6DOF position measurements to assess the positional accuracy to allow for positional error correction in the final pattern analysis. The knowledge of the position and errors allow for the correction of position and alignment of the probe at every point in the scan geometry to within the repeatability of the motion components (~30 µm). The in-situ position knowledge will eventually allow us to correct to the uncertainty of the measurement (~15 µm). Our final results show positioning errors on the spherical scan surface have an average error of ~30 µm with peak excursions of ~100 µm. This robust positioning allows for accurate analysis of the RF system stability. Our results show that at 183 GHz, our RF repeatability with movement over 180° orientation change with a 600 mm offset to be less than ±0.05 dB and ±5°.

Investigations on Gain Measurement Accuracies at Limited Far-Field Conditions
Engin Gülten,Andreas Drexler, Josef Migl, Jürgen Habersack, November 2014

Driven by the mobile data communications needs of market broadband antennas at the upper frequency bands are already state-of-the-art, e.g. at the Ka-Band. For the characterization of an antenna the antenna gain is one of the major test parameters. This measurement task is already challenging for standard applications at the Ka-Band. However, for the calibration of remote station antennas utilized in high precision test facilities, e.g. the compact range, even higher measurement accuracies are typically required in order to fulfil the overall system performance within the later test facility. Therefore the requirement for this investigation is to improve the measurement set-up and also the steps to get a failure budget which is better than ± 0.15 dB. Every antenna gain measurement technique is affected by required changes in the measurement setup, e.g. the Device under Test (DUT) or the remote station, respectively. This results for example in a variation of mismatch with resulting measurement errors. To determine and compensate the occurred mismatches, the scattering parameters of the involved components have to be measured and be evaluated with a corresponding correction formula. To quantify the effect for the gain measurement accuracy the remaining uncertainty of the mismatch correction values is examined. Another distortion is caused by multiple reflections between the antenna apertures. To reduce this error source, four additional measurements each with a decreased free space distance should be performed. In addition to the common methods, this paper explains in detail an advanced error correction method by using the singular value decomposition (SVD) and compares this to the standard mean value approach. Finally the restricted distance between both antennas within the applied anechoic far-field test chamber has to be analysed very critically and optionally corrected for the far-field gain at an infinite distance in case the measurement distance is fulfilling the minimum distance requirement, only. The paper will discuss all major error contributions addressed above, show correction approaches and verify these algorithms with exemplary gain measurements in comparison to the expected figures.

Advantages and Disadvantages of Various Hemispherical Scanning Techniques
Eric Kim,Anil Tellakula, November 2014

When performing far field or near field antenna measurements on large antennas, it is often necessary to have various types of mechanical positioning systems to achieve the required hemispheric scans.  Measurement systems employing a single-arm gantry, a dual-arm gantry, a fixed arch moving probe, or a fixed arch multi-probe have been paired with either an azimuth positioner or a vehicle turntable to provide hemispheric scanning of the object being tested. This paper will highlight the key characteristics of various scanning methods making comparisons between the different techniques.  Positioning and system accuracy, speed, stowing ability, calibration, frequency range, upgradability, relative cost and other key aspects of the various techniques will be discussed in detail to help the end user during the system design and selection process.  In addition, the paper will highlight novel hemispheric and truncated spherical scanning approaches. In many applications, the success of the entire project often centers on the judicious selection of the positioning subsystem.  This paper will provide guidance toward making the proper selection of the scanning concept as well as of the positioning system.

Equiangular Phase Shifting Holography for THz Near-field/Far-field Prediction
Gary Junkin,Josep Parrón Granados, Pedro de Paco Sánchez, Yi Lu, November 2014

A three-step equiangular (120º) phase shifting holography (EPSH) technique is proposed for THz antenna near-field/far-field prediction. The method is attractive from the viewpoint of receiver sensitivity, phase accuracy over the entire complex plane, simplified detector array architecture, as well as reducing planarity requirements of the near-field scanner. Numerical modeling is presented for the holographic receiver performance, using expected phase shift calibrations errors and phase shift noise. The receiver model incorporates responsivity and thermal noise specifications of a commercial Schottky diode detector. Additionally, simulated near-field patterns at 372GHz demonstrate the convenience of the method for accurate and high dynamic range THz near-field/far-field predictions, using a phase-shifter calibrated to ±0.1°.

Achieving High Accuracy from a Near-field Scanner without Perfect Positioning
George Cheng,Yong Zhu, Jan Grzesik, November 2014

We propose a technique which achieves highly accurate near-field data as well as far-field patterns despite the positioning inaccuracy of the scanner in the antenna near-field measurements. The method involves position sensing hardware in conjunction with data processing software. The underlying theory is provided by the Field Mapping Algorithm (FMA), which transforms exactly the measured field data on a conventional planar, spherical, or cylindrical surface, indeed on any enclosing surface, to any other surface of interest.  In our modified near-field scanning system, a position recording laser device is attached to the probe. The positions of data grid points are thus found and recorded along with the raw RF data.  The raw data acquired over an irregular, imperfect surface is subsequently converted exactly to a designated, regular surface of canonical type based on the FMA and its associated position information.  Once the near-field data is determined at all required grid points, the far-field pattern per se is obtained via a conventional near-field-to-far-field transformation.  Moreover, and perhaps just as importantly, the interplay between our FMA and the free-form position/RF recording methodology just described allows us to bypass entirely the arduous task of strict antenna alignment.  The free-form position/RF data are simply propagated by the FMA software to some perfectly aligned reference surface ideally adapted as a springboard for any intended far-field buildup. Our proposed marriage of a standard scanning system and a position recorder, with otherwise imperfect RF/location data restored to ideal status under the guidance of the FMA, clearly offers the advantage of high precision at minimal equipment cost.  It is, simply stated, a win-win budget/accuracy RF measurement solution. Two analytic examples and one measurement case are given for demonstration.  The first example is a circular aperture within an infinite conducting plane, the second is a 10 lambda x 10 lambda dipole array antenna.  The measurement case involves a waveguide slot array antenna.  In all three cases, the near-field data were deliberately acquired over imperfectly located grid points. The FMA was then applied to obtain near-field data at the preferred, regularly arranged grid points from these position compromised values.  Excellent grid-to-grid near-field comparison and calculated far-field results were obtained.

Testing of Panels And Radomes Using De-embedding To Reduce Probe Interaction Errors
Henry Burger, November 2014

Legacy methods for testing the performance of radome panels and finished radomes have always been in isolation from the system antenna, for many reasons. The legacy method of testing employed horn antennas at relatively close distances, a fixed-frequency signal source, and primitive receivers. More modern systems used much better receivers capable of measuring both phase and amplitude, and these gave way to automatic network analyzers. The network analyzer system also replaces the fixed-frequency source, because it has its own step-frequency source. The rest of the setup remains the same. A network analyzer can itself be calibrated, but that calibration cannot include the probe antennas, nor can it account for interactions, particularly at normal incidence. With increasing demands on performance, it is essential that the interaction effects of the probe antennas with the radome be removed.    The micorwave integrated circuit industry has the identical problem. The circuit probes that are used to reach into the circuit assemblies have very small tips, and the internal elements to accomplish this size reduction make probe mataching difficult. Thus the probe parameters become embedded into the overall measured response. The circuit testing community has developed a process to de-embed these probes, yielding the S-parameters of the circuit under test in isolation from surroundings.    This paper investigates a method for applying this closed-system technique to open-system testing, such as panel-measuremsnt tables, by using a secondary calibration technique that is adapted to open systems. This effectively extends the calibration of the analyzer system to encompass the probes, thus improving accuracy.

Dual Compact Range Electrical Versus Mechanical Bore Sight Alignment
Hulean Tyler,Frank Soliman, David Kim, November 2014

There are many methods of aligning feeds on a dual cylindrical parabolic sub-reflector compact range.  Presented in this paper is a laser tracker and Field probe method that was used to align the RF feed to the sub-reflectors.   The laser tracker provides real time positional error measurements that are mapped and these results are used to fine tune the alignment of RF feed to the phase centers of the dual cylindrical parabolic sub-reflectors.  Field probe test scans are performed to verify QZ performance of various alignment positions measured comparing scans of amplitude, phase and taper.  The laser tracker alignment method provides an efficient and a highly accurate method to achieving precision alignment of the RF feed to the sub-reflector system installed into the dual reflector compact range.  High accuracy antenna measurements in a compact range require precision alignment of the RF feed to the sub-reflectors phase center.  The quality and size of the RF plane wave field of the quiet zone (QZ) performance is affected by the alignment of the RF feed and sub-reflector system combination.   This alignment is accomplished through mechanical adjustments of the x-y-z axis RF feed positioning system.   Measurements of both mechanical and electrical bore site is performed and compared across the full measurement spectrum to verify the compact antenna test range (CATR) system positioning accuracy.

A Comparison of Material Measurement Accuracy of RF Spot Probes to a Lens-Based Focused Beam System
John Schultz,James Maloney, Kathleen Maloney, Rebecca Schultz, November 2014

A popular method for microwave characterization of materials is the free-space focused beam technique, which uses lenses or shaped reflectors to focus energy onto a confined region of a material specimen. In the 2-18 GHz band, 60 cm diameter lenses are typically spaced 30 to 90 cm from the specimen under test to form a Gaussian focused beam with plane-wave like characteristics at the focal point. This method has proved popular because of its accuracy and flexibility. Another free-space measurement technique that has been employed by some is the use of dielectrically loaded antennas that are placed in close proximity to a specimen. In this alternate technique, the dielectrically loaded antennas are smaller than lenses, making the hardware more compact and lower cost, however this is done at the expense of potentially reduced accuracy. This paper directly compares a standard laboratory focused beam system to a measurement system based on some recently developed RF spot probes. The spot probes are specially designed antennas that are encapsulated in a dielectric and optimized to provide a small illumination spot 3 to 8 cm in front of the probe. Experimental measurements of several dielectric, magnetic, and resistive specimens were measured by both systems for direct comparison. With these data, uncertainty analysis comparisons were made for both fixtures to establish measurement limits and capability differences between the two methods. Understanding these uncertainties and measurement limits are key to implementing compact spot probes in a manufacturing setting for quality assurance purposes.

The CROMMA Facility at NIST Boulder: A Unified Coordinated Metrology Space for Millimeter-Wave Antenna Characterization
Joshua A. Gordon,David Novotny, Mike Francis, Ron Wittmann, Miranda Butler, Jeffrey Guerrieri, November 2014

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.







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