AMTA Paper Archive
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Extrapolation Range for D-band Standard Gain Horn Antenna Measurement
This paper describes an mm-wave extrapolation range installed at KRISS, which may be used for testing standard gain antennas by using the three-antenna extrapolation technique in the frequency range from 110 GHz to 325 GHz. It consists of a precision linear slide and an mm-wave S-parameters measurement system. The precision linear slide for changing the separation distance between transmitting and receiving antennas is realized with a linear motor with 1.6 meter long on a precision stone surface plate. The mm-wave measurement system for measuring S-parameters at extrapolation antenna measurements consists of a 67 GHz vector network analyzer used as a main frame and three frequency extenders which are operating at three frequency bands (D-band (110 -170 GHz), G-band (140-220 GHz) and J-band (220-325 GHz)). The S-parameters measurement system is calibrated with TRL/LRL method. The general procedure of the extrapolation technique is as follows; 1) The effect of multiple reflections between transmitting and receiving antennas is removed from data measured at a reduced distance. 2) A polynomial is determined for curve-fitting the data removed the effect of multiple reflections. 3) Finally, far-field antenna properties are calculated from the polynomial. In this paper, a method using measured S-parameters for reducing multiple reflections between transmitting and receiving antennas is used. Power gain of D-band standard gain horn antennas is measured with the mm-wave extrapolation range. Description of detailed measurement system and measurement result will be presented at the symposium.
Utilization Of An Octocopter As A Two-Way Field Probe For Electro-Magnetic Field Measurements At An Outdoor Radar Cross Section Range
RCS and Antenna measurement accuracy critically depends on the quality of the incident field. Both compact and far field ranges can suffer from a variety of contaminating factors including phenomena such as atmospheric perturbation, clutter, multi-path, as well as Radio Frequency Interference (RFI). Each of these can play a role in distorting the incident field from the ideal plane wave necessary for an accurate measurement. Methods exist to mitigate or at least estimate the measurement uncertainty caused by these effects. However, many of these methods rely on knowledge of the incident field amplitude and phase over the test region. Traditionally the incident field quality is measured directly using an electromagnetic probe antenna which is scanned through the test region. Alternately, a scattering object such as a sphere or corner reflector is used and the scattered field measured as the object is moved through the field. In both cases the probe/scatterer must be mounted on a structure to move and report the position in the field. This support structure itself acts as a moving clutter source that perturbs the incident field being measured. Researchers at the Air Force Institute of Technology (AFIT) have recently investigated a concept that aims to eliminate this clutter source entirely. The idea is to leverage the advances in drone technology to create a free flying field probe that doesn’t require any support structure. We explore this concept in our paper, detailing the design, hardware, and software developments required to perform a concept demonstration measurement in AFIT’s RCS measurement facility. Measured data from several characterization tests will be presented to validate the method. The analysis will include an estimate of the applicability of the technique to a large outdoor RCS measurement facility.
In-situ Diagnosis of Direction Finding Antenna using Optically-fed Transmitting Miniature Probes
Direction Finding (DF) Antennas are usually designed and tested in controlled environments. However, antenna far field response may change significantly in its operational environment. In such perturbing or not -controlled close context, the antennas calibration validity becomes a major issue which can lead to DF performance degradation and to a costly re-calibration process. Even if in-situ re-calibration is still complicated; the DF antenna response can be monitored, during the mission, in order to ensure the DOA accuracy. This paper presents an innovative design and the performance of a low-disturbing solution to detect the near field antenna response deviations from a nominal case. The proposed system is based on an array of transmitting miniature dipoles deployed all around the DF antennas. These probes are optically fed through a non-biased photodiode that carries the direct conversion into a RF signal at the desired frequency. The detection re-used the DF receiving RF chains to analyze any deviation (complex values) of the antennas array manifold. Compared to the Optically Modulated Scatterer (OMS) technique, the benefits of the proposed approach are demonstrated experimentally over a frequency decade (UHF band). First a better sensitivity is shown (higher than 80 dB on the monitored link), and secondly the phase detection is made really simple compared to the OMS technique. Finally, a relation between this in-situ diagnosis mode and the DF angular direction accuracy is established. Thus the capacity to detect, on the near field response, the presence of various types of closed obstacles (open trap on the carrier, additional antenna…) which perturb significantly the far field antenna response, is evaluated.
Detailed Uncertainty Analysis of the Electrically Small Antenna Efficiency Measurement
The radiated efficiency is a key performance indicator for multi-standards frequency agile electrically small antennas (ESA) that are mounted on wireless IoT sensors. One of the techniques to estimate it, consists to integrate, over all the angular directions, the gain measured in the far field condition. The gain-comparison method is usually implemented in the CEA LETI testbench ; which requires an accurate knowledge of the standard horn gain. The introduction of a new RF-optical link to remove coaxial cable perturbation on ESA radiation, in our test bench has raised the opportunity to proceed to an error budget analysis. This paper delivers the main results of this study where the impact of several parameters such as the optical fiber movement, the horn position, the received power level, chamber imperfection… have been evaluated. We have carried on the three antennas method (one Vivaldi and two TEM standard horns) to estimate the complex transfer function of the three antennas. The overall goal is to estimate the detailed uncertainty analysis of the ESA efficiency measurement over a large band of frequencies. This work aims to identify the most impacting effects on uncertainty and to initiate the discussion with the AMTA community how to decrease them.
Correction of Non-ideal Probe Orientations for Spherical Near-Field Antenna Measurements
Positioning in near-field antenna measurements is crucial and often an absolute position accuracy of ?\50 is required. This can be difficult to achieve in practice, e.g. for robotic arm measurement systems and/or high frequencies. Therefore, optical measurement devices are used to precisely measure the position and orientation. The information can be used to correct the position and orientation during the measurement or in the near-field to far-field transformation. The latter has the benefit that the measurement acquisition is typically faster because no additional correction movements are needed. Different methods for correction of non-ideal measurement positions in r, ? and f have been presented in the past. However, often not only the relative position but also the orientation between the antenna under test (AUT) and the probe coordinate system is not perfect. So far, correction and investigation of the related non-ideal probe orientations has been neglected due to the assumption that the probe receiving pattern is broad. In this paper, non-ideal probe orientations will be investigated and a spherical wave expansion procedure which corrects non-ideal probe orientations and positions will be presented. This is achieved by including an arbitrary probe pointing in the probe response calculation by additional Euler rotations of the probe receiving coefficients. The introduced pointwise higher-order probe correction scheme allows an exact spherical wave expansion of the radiated AUT field. The transformation is based on solving a system of linear equations and, thus, has a higher complexity compared to Fourier-based methods. However, it will be shown that most of the calculations can be precomputed during the acquisition and that solving the linear equation system can be accelerated by using iterative techniques such as the conjugate gradient method. The applicability of the proposed method is demonstrated by measurements where an intentional misalignment is introduced. Furthermore, the method can be used to include full probe correction in the translated spherical wave expansion algorithm. In conclusion, the proposed procedure is a beneficial extension of spherical wave expansion methods and can be applied in different measurement scenarios.
Determination of the Far-Field Radiation Pattern of a Vehicle Mounted VHF Antenna From a Set of Sparse Near-Field Measurements
The paper summarizes the performance of a new near-field to far-field (NF/FF) transform approach for a VHF vehicle mounted AUT test case, and compares the approach with the spherical measurement approach. The NF/FF transformation is based on the solution of an inverse problem in which the measured NF and predicted FF values are attributed to a set of equivalent electric and magnetic surface currents which lie on a convex arbitrary surface that is conformal to the antenna under test (AUT). The NF points are conformal to the AUT, reducing the number of samples and relaxing positioning requirements used in conventional spherical, NF/FF geometries. A pseudo inversion of the matrix representing the mapping of the equivalent sources into the near-field samples is obtained by using the singular value decomposition (SVD), which is used to form an approximation of the inverse of the matrix. This inverse, when multiplied by the NF measurement vector, solves for the efficiently radiating components of the current, which are used to compute the FF in a straightforward manner. Keywords—Antenna Near-Field to Far-Field Transformation, Electromagnetic Inverse Problems.
Radiation Center Estimation from Near-Field Data Using a Direct and an Iterative Approach
Spherical Near-Field (SNF) measurements are an established technique for the characterization of an Antenna Under Test (AUT). The normal sampling criterion follows the Nyquist theorem, taking equiangular samples. The sampling step size depend on the smallest sphere that, centered in the measurement’s coordinate system, encloses the AUT, i.e. the global minimum sphere. In addition, a local minimum sphere can be defined as the sphere with minimum radius which, centered in the AUT, encloses it alone. The local minimum sphere is always equal or smaller than the global minimum sphere, being equal when the AUT is centered in the measurement’s coordinate system. It is assumed that the local minimum sphere’s center coincides with the radiation center. Furthermore, it is possible to compute a Translated Spherical Wave Expansion (TSWE) centered in the local minimum sphere, thus needing less measurement points, as long as the relative position of its center is known. Due to practical reasons, it is not always possible to easily locate the radiation center. In this paper, the relative position of the radiation center of an AUT with respect to the measurement's coordinate system’s center is estimated from SNF data using two approaches. The first approach takes the phase center as an estimation of the radiation center and is based on the method of moving reference point, strictly valid for the far-field case, analyzing its error at different near-field distances. The second approach is based on a spherical modes' spectrum analysis: the closer the AUT’s radiation center is to the coordinate system's center, the larger the power fraction in the lower modes will be. The proposed algorithm iteratively displaces the SWE and checks the power in a predefined number of modes until the convergence criterion is fulfilled. It is important to note that no near-field to far-field transformation is used, for the less measurement points taken do not allow it. A thorough analysis of the estimation error is done by simulation for different cases and antennas. The estimation error of both methods is compared and discussed, highlighting the convenience of each method depending on the requirements.
Nearfield Antenna Measurements over Seawater – Some Preliminary Thoughts
The principles of near-field antenna measurements and scanning in Cartesian and spherical coordinates are well established and documented in the literature, and in standards used on antenna ranges throughout government, industry, and academic applications. However the measurement methods used and the mathematics that are applied to compute the gain and radiation of the pattern of the test antenna from the near-field data assume typically that the antenna is operating in free space. This leaves several questions open when dealing with antennas operating over a lossy ground plane, such as the ocean damp soil, etc. In this paper, we shall discuss some of the motivation behind an examination of the physics and mathematics involved in performing a near-field antenna measurement over a seawater ground plane. Examples of past work in this are shall be discussed along with some of the challenges of performing far field antenna measurements in the presence of the air-sea interface. These discussions lead to some fundamental questions about how one defines gain in this environment and whether or not a near field approach could be beneficial. This will lead to some discussion of when and how the existing modal field expansions used in near-field measurements may need to be adjusted to account for the presence of the ground plane created by the ocean surface. An example of the limiting case of an antenna operating over a metallic ground plane will be discussed as a stepping stone to the more general problem of an antenna operating over a lossy ground plane.
Assessment of a 3D-Printed Aluminum Corrugated Feed Horn at 118.7503 GHz
We investigate all-metal 3D printing as a viable option for millimeter wave applications. 3D printing is finding applications across many areas and may be a useful technology for antenna fabrication. The ability to rapidly fabricate custom antenna geometries may also help improve cub satellite prototyping and development time. However, the quality of an antenna produced using 3D printing must be considered if this technology can be relied upon. Here we investigate a corrugated feed horn that is fabricated using the powder bead fusion process for use in the PolarCube cube satellite radiometer. AlSi10Mg alloy is laser fused to build up the feed horn, including the corrugated structure on the inner surface of the horn. The intricate corrugations, and tilted waveguide feed transition of this horn made 3D printing a compelling and interesting process to explore. We will discuss the fabrication process and present measurement data at 118.7503 GHz. Gain extrapolation and far-field pattern results obtained with the NIST robotic antenna range CROMMA are presented. Far-field pattern data were obtained from a spherical near-field scan over the front hemisphere of the feed horn. The quasi-Gaussian HE11 hybrid mode supported by this antenna results in very low side lobe levels which poses challenges for obtaining good SNR at large zenith angle during spherical near field measurements. This was addressed through using a single alignment and electrical calibration while autonomously changing between extrapolation and near-field measurements using the robotic arm in CROMMA. The consistency in parameters between extrapolation and near-field measurements allowed the extrapolation data to be used in-situ as a diagnostic. Optimal near-field scan radius was determined by observing the reflection coefficient S11 during the extrapolation measurement. The feed horn-to-probe antenna separation for which |S11| was reduced to 0.1 dB peak-to-peak was taken as the optimal near-field scan radius for the highest measurement SNR. A comparison of these measurements to theoretical predictions is presented which provides an assessment of the performance of the feed horn.
Development of A New Atom-Based SI Traceable Electric-Field Metrology Technique
One of the keys to developing new science and technologies is to have sound metrology tools and techniques. Whenever possible, we would like these metrology techniques to make absolute measurements of the physical quantity. Furthermore, we would like to make measurements directly traceable to the International System of Units (SI). Measurements based on atoms provide such a direct SI traceability path and enable absolute measurements of physical quantities. Atom-based measurements have been used for several years; most notable are time (s), frequency (Hz), and length (m). There is a need to extend these atom-based techniques to other physical quantities, such as electric (E) fields. We are developing a fundamentally new atom-based approach for that will lead to a self-calibrated, SI traceable E-field measurement and has the capability to perform measurements on a fine spatial resolution in both the far-field and near-field. This new approach is significantly different from currently used field measurement techniques in that it is based on the interaction of radio-frequency (RF) E-fields with Rydberg atoms (alkali atoms placed in a glass vapor-cell that are excited optically to Rydberg states). The Rydberg atoms act like an RF-to-optical transducer, converting an RF E-field strength to an optical-frequency response. In this new approach, we employ the phenomena of electromagnetically induced transparency (EIT) and Autler-Townes splitting. This splitting is easily measured and is directly proportional to the applied RF E-field amplitude and results in an absolute SI traceable measurement. The technique is very broadband allowing self-calibrated measurements over a large frequency band including 500 MHz to 500 GHz (and possibly up to 1 THz and down to 10's of megahertz). We will report on the development of this new metrology approach, including the first fiber-coupled vapor-cell for E-field measurements. We also discuss key applications, including self-calibrated measurements, millimeter-wave and sub-THz measurements, field mapping, and sub-wavelength and near-field imaging. We show results for mapping the fields inside vapor cells, for measuring the E-field distribution along the surface of a circuit board, and for measuring the near-field at the aperture in a cavity.
Nonredundant Near-Field-Far-Field Transformation from Probe Positioning Errors Affected Bi-Polar Data
Among the near-field – far-field (NF-FF) transformation techniques, the one employing the bi-polar scanning is particularly interesting, since it retains all the advantages of that using the plane-polar one, while requiring a mechanically simple, compact, and cheaper measurement facility . In fact, in this scan, the antenna under test (AUT) rotates axially, while the probe is mounted at the end of an arm that rotates around an axis parallel to the AUT one. An effective probe voltage representation on the scanning plane requiring a minimum number of bi-polar NF data has been developed in , by properly exploiting the nonredundant sampling representations of electromagnetic (EM) fields  and considering the AUT as enclosed in an oblate ellipsoid. A 2-D optimal sampling interpolation (OSI) formula is then employed to efficiently recover the NF data required by the traditional plane-rectangular NF-FF transformation  from the acquired nonredundant bi-polar samples. It is so possible to considerably reduce the number of the needed NF data and corresponding measurement time with respect to the previous approach , which did not exploit the nonredundant sampling representations. However, due to an imprecise control of the positioning systems and their finite resolution, it may be impossible to exactly locate the probe at the points fixed by the sampling representation, even though their position can be accurately read by optical devices. Therefore, it is very important to develop an effective algorithm for an accurate and stable reconstruction of the NF data needed by the NF-FF transformation from the acquired irregularly spaced ones. A viable and convenient strategy  is to retrieve the uniform samples from the nonuniform ones and then reconstruct the required NF data via an accurate and stable OSI expansion. In this framework, two different approaches have been proposed. The former is based on an iterative technique, which converges only if there is a biunique correspondence associating at each uniform sampling point the nearest nonuniform one, and has been applied in  to the uniform samples retrieval in the case of cylindrical and spherical surfaces. The latter, based on the singular value decomposition (SVD) method, does not exhibit this constraint and has been applied to the nonredundant bi-polar  scanning technique based on the oblate ellipsoidal modeling. However, it can be conveniently used only when the uniform samples recovery can be split in two independent one-dimensional problems. The goal of this work is not only to provide the experimental validation of the SVD based technique , but also to develop the approach using the iterative technique and experimentally assess its effectiveness.  L.I. Williams, Y. Rahmat-Samii, R.G. Yaccarino, “The bi-polar planar near-field measurement technique, Part I: implementation and measurement comparisons,” IEEE Trans. Antennas Prop., vol. 42, pp. 184-195, Feb. 1994.  F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “Data reduction in the NF-FF transformation with bi-polar scanning,” Microw. Optic. Technol. Lett., vol. 36, pp. 32-36, 2003.  O.M. Bucci, C. Gennarelli, C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop., vol. 46, pp. 351-359, March 1998.  E.B. Joy, W.M. Leach, Jr., G.P. Rodrigue, D.T. Paris, “Application of probe-compensated near-field measurements,” IEEE Trans. Antennas Prop., vol. AP-26, pp. 379-389, May 1978.  O.M. Bucci, C. Gennarelli, G. Riccio, C. Savarese, “Electromagnetic fields interpolation from nonuniform samples over spherical and cylindrical surfaces,” IEE Proc. Microw. Antennas Prop., vol. 141, pp. 77-84, April 1994. F. Ferrara, C. Gennarelli, M. Iacone, G. Riccio, C. Savarese, “NF–FF transformation with bi-polar scanning from nonuniformly spaced data,” Appl. Comp. Electr. Soc. Jour., vol. 20, pp. 35-42, March 2005.
Analysis of Near-Field RCS Behavior for mm-Wave Automotive Radar Testing Procedures
Millimeter wave vehicular radar operating in the 77 GHz band for automatic emergency breaking (AEB) applications in detecting vehicles, pedestrians, and bicyclists, test data has shown that the radar cross section (RCS) of a target decreases significantly with distance at short range distances typically measured by automotive radar systems, where the reliable detection is most critical. Some attribute this reduction to a reducing illumination spot size from the antenna beam pattern. Another theory points to the spherical phase front due to measurement in the Fresnel region of the target, when the distance for the far-field zone is not met. The illumination of the target depends on the antenna patterns of the radar, whereas the Fresnel region effects depend on the target geometry and size. Due to fluctuations in measured data for RCS as a function of range in the near-field, upper and lower bounds for the target RCS versus range have been determined empirically as a method for describing the expected RCS of target. So far, the range-dependent RCS bounds used in AEB test protocols have been determined empirically. The study discussed in this paper aims to study the underlying physics that produces range-dependent RCS in near field and provide analytical model of such behavior. The resultant analytical model can then be used to objectively determine the RCS upper and lower bounds according to the radar system parameters such as antenna patterns and height. A comparison of the analytically predicted model and empirical near-field RCS as a function of range data will be presented for pedestrian, bicyclist, and vehicle targets.
Nonredundant NF-FF Transformation with Spherical Spiral Scan for a Non-Centered Quasi-Planar Antenna Under Test
Among the near-field - far-field (NF-FF) transformations, that with spherical scan  is the most appealing due to its feature to allow the whole radiation pattern reconstruction of the antenna under test (AUT). To get a considerable measurement time saving, spherical NF-FF transformations for AUTs with one or two predominant dimensions, requiring a minimum number of NF data, have been developed in , by using the nonredundant sampling representations of the electromagnetic (EM) fields  and adopting a prolate or oblate ellipsoid to shape the AUT. Another effective possibility to save the measurement time is to make faster the scan by collecting the NF data through continuous and synchronized movements of the probe and AUT. To this end, NF-FF transformations with spherical spiral scan have been recently proposed. They rely on the nonredundant representations and use optimal sampling interpolation (OSI) formulae  to effectively recover the NF data needed by the traditional spherical NF-FF transformation  from the acquired ones. The nonredundant sampling representation on the sphere from spiral samples and the related OSI expansion have been developed in [4-6] by adopting a spherical AUT model and choosing the spiral pitch equal to the sample spacing needed to interpolate along a meridian. Then, NF-FF transformations with spherical spiral scan for long or quasi-planar AUTs  have been obtained by applying the unified theory of spiral scans for non-volumetric AUTs . Unfortunately, due to practical constraints, it is not always possible to mount the AUT in such a way that it is centered on the scanning sphere centre. In this case, the number of NF data required by the NF-FF transformation  and the related measurement time can remarkably increase, due to the corresponding grow of the minimum sphere radius. Aim of this work is the development of a fast and accurate nonredundant NF-FF transformation with spherical spiral scan suitable for quasi-planar antennas, which requires practically the same number of NF data both in the centered and offset mountings of the AUT. To this end, an offset mounted quasi-planar AUT is modeled as contained in a oblate ellipsoid, and an effective representation of the probe voltage over the scanning sphere, using a minimum number of samples collected on a proper spiral wrapping it, is developed by applying the unified theory of spiral scans for non-volumetric AUTs  in the spherical coordinate system having the origin coincident with the AUT centre at distance from the scanning sphere one. The related OSI expansion allows to accurately reconstruct the NF data required for the NF-FF transformation.  J. Hald, J.E. Hansen, F. Jensen, F.H. Larsen, Spherical near-field antenna measurements, J.E. Hansen, (ed.), London, Peter Peregrinus, 1998.  O.M. Bucci, C. Gennarelli, G. Riccio, C. Savarese, “Data reduction in the NF–FF transformation technique with spherical scanning,” Jour. Electr. Waves Appl., vol. 15, pp. 755-775, June 2001.  O.M. Bucci, C. Gennarelli, C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop., vol. 46, pp. 351-359, March 1998.  O.M. Bucci, F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “NF–FF transformation with spherical spiral scanning,” IEEE Antennas Wireless Prop. Lett., vol. 2, pp. 263-266, 2003.  J F. D’Agostino, F. Ferrara, J.A. Fordham, C. Gennarelli, R. Guerriero, M. Migliozzi, “An experimental validation of the near-field - far-field transformation with spherical spiral scan,” IEEE Antennas Prop. Magaz., vol. 55, pp. 228-235, Aug. 2013.  F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “Theoretical foundations of near-field–far-field transformations with spiral scannings,” Prog. in Electr. Res., vol. 61, pp. 193-214, 2006.  R. Cicchetti, F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Near-field to far-field transformation techniques with spiral scannings: a comprehensive review,” Int. Jour. Antennas Prop., vol. 2014, ID 143084, 11 pages, 2014.  F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “The unified theory of near–field–far–field transformations with spiral scannings for nonspherical antennas,” Prog. in Electr. Res. B, vol. 14, pp. 449-477, 2009.
Application of the Translated-SWE Algorithm for the Characterization of Antennas Installed on Cars Using a Minimum Number of Samples
The Translated Spherical Wave Expansion (Translated-SWE) has been recently proposed as a powerful Near Field to Far Field (NF/FF) transformation tool which allows to reduce the number of samples in offset spherical NF measurements. The algorithm is based on the definition of a new reference system located on the Device Under Test (DUT) rather than on the center of the measurement sphere. The translation of the measurement system on the DUT allows to represent it with a minimum number of spherical modes (smaller minimum sphere) thus the reduction of the NF sampling points (down-sampling). The validation of the Translated-SWE have been presented in previous publications in case of DUT offset displaced along the Z-axis. This may occur in case of mechanical constraints of the measurement system such as mast or stand-offs of fixed length, used to handle the DUT. Similarly, in other measurement situations, the DUT is intentionally displaced offset wrt the center of rotation to enhance the echo reduction capabilities of the modal filtering performed on the SWE spectrum. It has been also shown that in such measurement scenarios the Translated-SWE can be effectively used allowing a significant reduction of the sampling points and thus of the testing time. Antennas installed on complex structure, like cars, is another example of offset radiating devices. In many practical case, the currents induced by the fed antenna on the structure have only a localized effect (e.g. higher directive antennas and/or antennas working at higher frequencies). In such situations a down-sampled acquisition can be performed taking advantage of the Translated-SWE which is run moving the reference system on the fed antenna so that only the portion of structure surrounding that antenna is taken into account. The size of the measured portion of the structure will of course depend on the density of the applied sampling while the remaining part will be neglected. In this paper the Translated-SWE algorithm will be applied to antenna installed on cars in generic offset position. To this purpose the algorithm has been updated in order to be able to deal with generic XYZ-offsets.
Ka-Band Measurement Results of the Irregular Near-Field Scanning System PAMS
The portable antenna measurement system PAMS was developed for arbitrary and irregular near-field scanning. The system utilizes a crane for positioning of the near-field probe. Inherent positioning inaccuracies of the crane mechanics are handled with precise knowledge of the probe location and a new transformation algorithm. The probe position and orientation is tracked by a laser while the near-field is being sampled. Far-field patterns are obtained by applying modern multi-level fast multipole techniques. The measurement process includes full probe pattern correction of both polarizations and takes into account channel imbalances. Because the system is designed for measuring large antennas the RF setup utilizes fiber optic links for all signals from the ground instrumentation up to the gondola, at which the probe is mounted. This paper presents results of the Ka-band test campaign in the scope of an ESA/ESTEC project. First, the new versatile approach of characterizing antennas in the near-field without precise positioning mechanics is briefly summarized. The setup inside the anechoic chamber at Airbus Ottobrunn, Germany is shown. Test object was a linearly polarized parabolic antenna with 33dBi gain at 33GHz. The near-fields were scanned on a plane with irregular variations of over a wavelength in wave propagation. Allowing these phase variations in combination with a non-equidistant grid gives more degree of freedom in scanning with less demanding mechanics at the cost of more complex data processing. The setup and the way of on-the-fly scanning are explained with respect to the crane speed and the receiver measurement time. Far-fields contours are compared to compact range measurements for both polarizations to verify the test results. The methodology of gain determination is also described under the uncommon near-field constraint of coarse positioning accuracy. Finally, the error level assessment is outlined on the basis of the classic 18-term near-field budgets. The assessment differs in the way the impact of the field transformation on the far-field pattern is evaluated. Evaluation is done by testing the sensitivity of the transformation with a combination of measured and synthetic data.
Free-Space Antenna Far-Field Extraction from Near-Field Measurements Above Metallic Ground
Antenna measurements above a material half-space are becoming an interesting aspect of near-field measurements especially for automotive antenna tests. Upcoming measurement facilities will be equipped with a dielectric or metallic ground. The near-field is sampled on a measurement surface in the vicinity of the device under test (DUT) above the ground, e.g. on a hemisphere. Thus, the effect of the ground has to be considered in the subsequent near-field to far-field transformation in order to obtain the far-field of the DUT above the ground plane. Assuming the metallic ground of the facility to be perfectly conducting, the ground effects are considered by introducing image sources below the ground plane in addition to the primary sources of the DUT above the ground plane. If coupling effects between the DUT and the ground plane are negligible, the primary sources correspond to the sources of the DUT in free-space. As a consequence, by separating the primary sources from the image sources, the free-space far-field of the DUT can be obtained from near-field measurements above ground. This means that measurement ranges with a ground plane can also be used to obtain free-space far-fields. In electromagnetic simulations, the primary sources can be placed in arbitrary environments, e.g. for communication channel evaluations. The quality of the primary sources extraction process mainly depends on the distance of the DUT sources from the ground plane as well as on the localization property of the employed equivalent sources which e.g. can be electric and/or magnetic surface currents or spherical modes. In this contribution, the numerical properties of the forward operator describing the relation between the DUT sources and the signal of the probe antenna above ground are analyzed in detail. The requirements for the unique determination of the primary sources from the near-field observations by inverting the operator are identified. Based on numerical investigations and real measurements obtained in a hemispherical near-field measurement facility, it will be shown that dependent on the ratio of the geometrical extensions of the DUT and its height above the ground as wells as on the strength of the coupling between the DUT and the ground, the free-space DUT far-field can be extracted with high quality.
Filtering Antenna-to-Antenna Reflections in Antenna Extrapolation Measurements
At NIST, we have developed a precision, wide-band, mmWave modulated-signal source with traceability to primary standards. We are now extending the traceability path for this modulated-signal source into free space to be used for verifying over-the-air measurements in 5G, wireless receivers. However, to obtain a traceable modulated signal in free space, the full scattering matrix of the radiating antenna must be measured. We have extended the extrapolation methods used at NIST, based on the work of Newell, et al. . The extrapolation measurement provides a very accurate, far-field, on-axis, scattering matrix between two antennas. When combined with scattering-matrix measurements made with permutations of pairs of three antennas, far-field scattering, and, thus, gain, is obtained for each antenna. This allows an accurate extrapolation of the antenna’s near-field pattern. We have incorporated the extrapolation fitting algorithms into a Monte Carlo uncertainty engine called the NIST Microwave Uncertainty Framework (MUF) . The MUF provides a framework to cascade scattering matrices from various elements, while propagating uncertainties and maintaining any associated correlations. By incorporating the extrapolation measurements, and the three-antenna method into the MUF, we may provide traceability of all measurement associated with the gain, including the scattering parameters. In this process, we studied several aspects of the gain determination. In this work, we show simulations determining the efficacy of filtering to reduce the effect of multiple reflection on the extrapolation fits. We also show comparisons of using only amplitude (as is traditionally done) to using the full complex data to determine gain. Finally, we compare uncertainties associated with choices in the number of expansion terms, systematic alignment errors, uncertainties in vector network analyzer calibrations and measurements, and phase error introduced by cable movement. With these error mechanisms and their respective correlations, we illustrate the NIST MUF analysis of the antenna scattering-matrix with data at 118 GHz.  A. C. Newell, R. C. Baird, and P. Wacker “Accurate Measurement of Antenna Gain and Polarization at reduced distances by an extrapolation technique” IEEE Transactions on Antennas and Propagation. Vol. 21, No 4, July 1973 pp. 418-431.  D. F. Williams, NIST Microwave Uncertainty Framework, Beta Version. NIST, Boulder, CO, USA, Jun. 2014. [Online]. Available: http://www.nist.gov/pml/electromagnetics/related-software.cfm
Group Delay Measurement For Satellite Payload Testing
Equivalent Isotropically Radiated Power (EIRP), Saturating Flux Density (SFD) and Group Delay (GD) are three system level parameters often measured during the characterization of spacecraft systems. EIRP is of interest for transmitters, SFD for receivers and GD for the entire up/down link. A test methodology for EIRP and SFD was first presented in  and  and a detailed procedure presented in . To date GD has only been measured under far-field (or simulated far-field) conditions. In , a concept for measuring GD in a planar near-field (PNF) range is described, but no methodology is presented. In this paper, we present a method for measuring GD in a planar near-field range. The technique is based on a set of three antenna pairs, measured sequentially, from which the insertion phase of the measurement system and the near-field probe  can be resolved. Once these parameters are known, insertion phase for the device under test (i.e. a Tx or Rx antenna) can be measured and GD calculated as the negative frequency derivative of the insertion phase. An added complexity in the case of a near-field measurement is the near-field probe is in close proximity to the device under test (not far-field condition) for which compensation is needed. We show through simulation and measurement, that the plane wave expansion allows us to compute a correction factor for the proximity of the probe to the device under test; thus allowing correction of the measured insertion phase. The final step in measuring payload GD through both uplink and downlink channels is to set up a fixed Tx probe in close proximity to the Rx antenna and an equivalent Rx probe in close proximity to the Tx antenna and performing a through measurement as one would do on a far-field range. Correction factors for compensating for the proximity of both probes are then applied, based on independent a-priori Rx and Tx case measurements performed on the antennas. Simulated and measured data will be presented to demonstrate the process and to illuminate some of the finer nuances of the correction being applied. Index Terms— Group Delay, Planar Near-Field, Antenna Measurements, Three Antenna Method.  A. C. Newell, R. D. Ward and E. J. McFarlane, “Gain and power parameter measurements using planar near-field techniques”, IEEE Trans. Antennas &Propagat, Vol 36, No. 6, June 1988  A. C. Newell, “Planar near-field antenna measurements”, NIST EM Fields Division Report, Boulder, CO, March 1994.  D. Janse van Rensburg and K. Haner, “EIRP & SFD Measurement methodology for planar near-field antenna ranges”, Antenna Measurement Techniques Association Conference, October 2014.  C. H. Schmidt, J. Migl, A. Geise and H. Steiner, “Comparison of payload applications in near field and compact range facilities”, Antenna Measurement Techniques Association Conference, October 2015.  A. Frandsen, D. W. Hess, S. Pivnenko and O. Breinbjerg, “An augmented three-antenna probe calibration technique for measuring probe insertion phase”, Antenna Measurement Techniques Association Conference, October 2003.
Measurements of Low Gain VHF Antennas in Spherical Multi-Probe NF Systems
Measurement of the radiation properties of low gain antenna operating at VHF frequencies is well known to be a challenging task. Such antennas are sometimes tested in outdoor Far Field (FF) ranges which are unfortunately subject to errors caused by the electromagnetic pollution and scattering from the environment. Near Field (NF) measurements performed in shielded anechoic chambers are thus preferable to outdoor ranges. However, also in such cases, the accuracy of the results may be compromised by the poor reflectivity of the absorbing material which might be not large enough wrt the VHF wavelength. Other source of errors may be caused by the truncation of the scanning area which generates ripple on the FF pattern after NF/FF transformation. Spherical multi-probe systems developed by MVG are optimal measurement solution for low directive Device Under Test (DUT). Such systems allow to perform a quasi-full spherical acquisition combining a rotation of the DUT along azimuth, with a fast electronically scanned multi-probe vertical arch. The DUT can be accommodated on masts made of polyester material which allows to minimize the interaction with the DUT. Measurements of low directive device above 400 MHz performed with such type of systems have been demonstrated to be accurate and extremely fast in previous publications. In this paper, measurements of a low directivity antenna, performed at VHF frequencies in a MVG spherical multi-probe system, will be presented. The antenna in this study is an array element, part of a larger array, which has been developed for space-born AIS applications. Gain and pattern accuracy of the measurement will be demonstrated by comparison with full wave simulation of the tested antenna.
Measurement of Antenna System Noise Temperature Using Planar Near-Field Data
This paper presents the results of a new measurement technique to determine antenna system noise temperature using data acquired from a planar near-field measurement. The ratio of antenna gain to system noise temperature (G/T) is usually determined in a single measurement when the antenna is alternately pointed towards the “cold sky” and a hot radio source such as the sun or a star with a known flux density. The antenna gain is routinely determined from near-field measurements and with the development of this new technique, the system noise temperature can also be determined. The ratio of G/T can therefore be determined from planar near-field data without moving the antenna to an outdoor range. The noise temperature is obtained by using the plane-wave spectrum of the planar near-field data and focusing on the portion of the spectrum in the evanescent or “imaginary space” portion of the spectrum. Near-field data is obtained using a data point spacing of l/4 or smaller and the plane-wave spectrum is calculated without applying any probe correction or Cos(q) factor. The spectrum is calculated over real space corresponding to propagating modes of the far-field pattern and also the evanescent or imaginary space region where . Actual evanescent modes are highly attenuated in the latter region and therefore the spectrum in this region must be produced by “errors” in the measured data. Some error sources such as multiple reflections will produce distinct localized lobes in the evanescent region and these are recognized and correctly identified by using a data point spacing of less than l/2 to avoid aliasing errors in the far-field pattern. It has been observed that the plane wave spectrum beyond these localized lobes becomes random with a uniform average power. This region of the spectrum must be produced by random noise in the near-field data that is produced by all sources of thermal noise in the electronics and radiated noise sources received by the antenna. By analysing and calibrating this portion of the spectrum in the evanescent region the near-field noise power can be deduced and the corresponding noise temperature determined. Simulated and measured data will be presented to illustrate and validate the measurement and analysis techniques. Keywords — Planar Near-Field, G/T, Figure-of-Merit Measurements, Simulation, Plane Wave Spectrum.
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