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We describe two theoretical bases for an algorithm for back-projection. The first is (1) Fourier inversion of the mathematical expression for the far electric field components in terms of the aperture electric field. The second is (2) Fourier inversion of the complete vectorial transmitting characteristic of Kerns' scattering matrix. It is this characteristic that results from the standard process of planar near-field (PNF) scanning and the ensuing reduction of the PNF transmission equation. We demonstrate that the theoretical approaches (1) and (2) yield identical back-projection algorithms. We report on back-projection measurements of an 18 inch X-band flat plate phased array using the far-field obtained from both planar and spherical near-field scanning. The spherical measurements were made on a large arch range.
A new simple approach is presented to calibrate the gain of standard gain horn antennas operating in the millimeter-wave frequency band. In terms of calibration, it is difficult to accurately measure the gain of standard gain horn antennas in the far-field region due to the space limitation. Therefore, near-field measurement methods are generally used to calibrate the gain of standard horn antennas. The extrapolation range method is one of the most accurate measurement methods in the near-field region. In the conventional extrapolation range method, a moving average process is applied to remove multiple reflections between antennas. Moving average can only remove multiple reflections between antennas. Therefore, electromagnetic absorbers are required to remove other reflections increasing measurement uncertainties. The time-domain gating method in extrapolation range allows us to remove all reflection waves, and achieve accurate antenna gain calibration without absorbers. The time-domain gating also reduces the number of measurement positions in the extrapolation ranges and obtains the gain of antennas in wide frequency ranges. In this paper, we compare the theoretical value with the time-domain gating method without absorbers by measuring three W-band standard gain horn antennas.
Thermal and other random noise sources give rise to an error contribution in spherical near field measurement systems [12]. With modern receivers and sufficient amplification in the system this term often give an insignificant contribution in the overall measurement uncertainty. However, in special cases the uncertainty linked to random noise may be more significant and the proper treatment of this term is needed to evaluate the impact on overall measurement uncertainty. The motivation for this paper comes from observations on spherical near field (SNF) measurement of relatively small antennas using a high degree of oversampling. In a multiprobe system this is generally the case particularly in measurements of small antennas like dipoles as shown in Figure 1. In these cases the near field to far field (NF/FF) processing is performed with data collected from all probes and some truncation of the spherical mode spectrum depending on the antenna size. The term modal filtering is often used to describe the deliberate truncation of the mode spectrum. What can be observed is that the effective signal to noise ratio (S/N) in small antenna measurements in which modal filtering is applied during NF/FF processing are often much better than the apparent S/N in the “raw” near field (NF) data. Parseval’s theorem, which states that power computed in either domains equals the power in the other explains this difference. The “noise power” is spread out on all available spherical modes and therefore reduced when the mode spectrum is truncated by modal filtering at the appropriate order/distance depending on the size of the antenna. In this paper we present a formal discussion on how the residual noise power after NF/FF processing is affected by the processing parameters. It will be shown that the “effective S/N” can be calculated directly from simple formulas from the applied sampling and filtering. The formulas will be validated by an experimental setup.
The paper presents a new system, to be used in near-field antenna characterization, which (in its simplest implementation) automatically assists the alignment of the Antenna Under Test (AUT) and the definition of the uniform/non-uniform sampling and filtering procedures. The system, based on a very cheap hardware (3D, coded structured-light, digitalization device), is able to provide a complete description of the geometry of the measurement set-up and of the movements of its parts, probe included. In this paper the simplest case, the alignment of the AUT, is considered. In this case, the system determines the coordinates of the surface points of the AUT in the Laboratory Reference System (LRS), providing the position and the orientation of the AUT in the LRS. The acquisition of the geometrical data on the AUT requires only few seconds, with a negligible human intervention. The acquired data are then processed to provide the desired setting of the AUT either manually or by means of computer controlled actuators, ensuring the accuracy suited to millimetre-wave band. The geometrical information can be exploited also to make possible the correction via software, when movements of the AUT should be avoided. The accurate information on the AUT geometry can be fruitfully exploited in advanced, high-performance filtering and sampling approaches, again drastically reducing any human interaction with the system. The performance of the system is discussed by referring to a prototype working in the millimetre-wave band.
The Mathematical Absorber Reflection Suppression (MARS) technique is a method to reduce scattering errors in near-field and far-field antenna measurement systems. Previous tests by the authors had indicated that NSI's MARS technique was not as effective for directive antennas. A recent development of a scattering reduction technique for cylindrical near-field measurements has demonstrated that it can also work well for directive antennas. These measurements showed that the AUT shouldbeoffsetfromtheorigin byadistanceatleastequal to the largest dimension of the AUT rather than only 1-3 wavelengthswhich hadbeenusedfor smallerantennasin the earlier MARS measurements. Spherical near-field measurementshaverecently beenconcludedwhich confirm that with the larger offsets, the MARS technique can be applied to directive antennaswith excellent results. The MARS processing has recently been modified to produce significantly improved results. This improvement isespeciallyusefulfor antennaswherethephasecenterof the horns is located inside the horn and varies with frequency like pyramidal Standard Gain Horns (SGH). Fewermodesarerequired for thetranslatedpatternandthe filtering is more effective at reducing the effect of scattering. The improvement is very apparent for pyramidal horns.
Modern vehicle telematics subsystems often employ wireless interfaces. The design and evaluation of these subsystems involves measurement of antenna characteristics or Over-The-Air (OTA) performance of the subsystem as installed in a vehicle. Several subsystems servicing multiple user applications may be installed in a single vehicle, with antenna structures located anywhere on or within the vehicle. In general, the radiation characteristics of each subsystem must be measured over a partial spherical surface surrounding the vehicle and of sufficient radius to be outside the reactive near-field of the Device Under Test (DUT). This paper describes a distributed axis spherical scanning system designed for vehicle applications. The elevation axis which supports the probe antenna has a measurement radius of 25 ft (7.62m). The elevation positioner is supported on a hydraulic vertical lift axis to permit the adjustment of the measurement coordinate origin to be in the same horizontal plane as the DUT phase center. The measurement instrumentation system supports VNA based antenna pattern measurements or active OTA testing of telematics subsystems. The system is suitable for outdoor or indoor measurement facilities. An outdoor installation is described.
We present an innovative Near-Field test range, named Compact Near-Field (CNF) test range, using photonic probes and advanced Near-Field Far-Field transformations (NFFF). The photonic probe allows distances of one wavelength or less between AUT and probe, drastically reducing test range and scanner dimensions, improving the Signal to Clutter Ratio and the Signal to Noise Ratio, and reducing the scanning area and time. The NFFF, properly formulated as a linear inverse problem, further improves the rejection to clutter, noise and truncation error. The advantages of CNF test ranges are numerically foreseen and experimental results are presented under both, planar and cylindrical scanning geometries.
The Phaseless techniques have gained considerable attention during the past two decades in the antenna measurements community. The removal of the phase measurements has some immediate advantages over the common vectorial measurements. They are cost effective, well-adapted for higher frequencies and insensitive to phase instabilities. Recent advances in the near-field phaseless antenna measurement techniques have provided cost-effectiveness and robustness. A potential drawback of these techniques is the necessity of two-plane amplitude measurements in comparison to the standard one-plane near-field amplitude and phase measurements. This additional second-plane phaseless measurement typically requires longer overall measurement time. Consequently it is desirable to speed up the phaseless measurements by some means. The linear spiral measurement technique is utilized with the planar phaseless measurements to characterize a high directive circularly polarized reflector antenna. This spiral measurement scheme provides noticeable reduction of the measurements time while retaining all of the advantageous of the phaseless technique. A circularly polarized offset reflector antenna is measured in the UCLA bi-polar facility in the linear spiral mode to assess the applicability of the proposed scheme.
This note highlights the connection of antenna gain to the measurement of insertion loss based on established SNF formulations, relating directly among antenna transmission coefficients, antenna gain, acquired SNF raw scan data and the parameters acquired during a range insertion loss measurement. It shows how the measured insertion loss parameters are applied in normalizing raw SNF scan data in determining antenna gain.
The electromagnetic field within a test volume can be determined by use of spherical scanning techniques. Characterization of the field within the sphere requires compensation for probe-pattern effects. We provide a simple analysis to estimate uncertainties associated with this deconvolution.
The probe correction of near-field measured data can be considered as being composed of two parts. The first part is a pattern correction that corrects for the effects of the aperture size and shape of the probe and can be analyzed in terms of the far-field main component pattern of the probe. The second part is due to the non-ideal polarization properties of the probe. If the probe responded to only one vector component of the incident field in all directions, this correction would be unnecessary. But since all probes have some response to each of two orthogonal components, the polarization correction must be included. The polarization correction will be the focus of the following discussion. Previous studies have derived and tested general equations to analyze polarization uncertainty12. This paper simplifies these equations for easier application. The results of analysis and measurements for Planar, Cylindrical and Spherical near-field measurements will be summarized in a form that is general, easily applied and useful. Equations and graphs will be presented that can be used to estimate the uncertainty in the polarization correction for different AUT/Probe polarization combinations and measurement geometries. The planar case will be considered first where the concepts are derived from the probe correction theory and computer simulation and then extended to the other measurement geometries.
An effective near-field – far-field transformation technique with spherical spiral scanning tailored for antennas having two dimensions very different from the third one is here proposed. To this end, an antenna with one or two predominant dimensions (as, e.g., an elongated or quasi-planar antenna) is no longer considered as enclosed in a sphere, but in a prolate or oblate ellipsoid, respectively, thus allowing one to remarkably reduce the number of required data. Moreover these source modellings remain quite general and contain the spherical one as particular case. Numerical tests are reported for demonstrating the accuracy of the far-field reconstruction process and its stability with respect to random errors affecting the data.
Dipoles are a typical reference antenna in measurements. Because its performance is calculable even in the near field it is commonly used as a reference. But while the ideal dipole is a calculable device, the actual reference dipole used in the lab can be far from the ideal. In this paper end fed sleeve dipoles commonly used as references in wireless measurements and traditional quarter wavelength dipoles used in a wide variety of applications including RFID testing are study. Misalignment, manufacturing tolerances, variations on dielectric, and messy solder points will be analyzed numerically and in some cases compared with measured data to see the effects of these problems on the final performance of a reference dipole unit.
We introduce a new way to visualize electromagnetic waves. The sum total of wave information, essentially everywhere save for the source current distribution, can be determined from just a single data bank acquired across a near-field surface. The waves can be recovered and displayed in terms of their intensity or phase differential over the entire region of interest. A field mapping algorithm is proposed which obtains the field everywhere, both interior and exterior to the measurement surface, based on a single near-field data acquisition. The field mapping algorithm is a direct, closedform solution which is numerically straightforward and efficient. Verification is demonstrated by analytic examples, numerical simulation results, and hardware measurement. Excellent agreement is evident in all cases.
This paper describes an application of well known microwave holography to the practical case of the space antenna for the European Navigation System GALILEO. The antenna consists in an array of 45 patch elements, divided into six sectors, fed by a two level beam forming network. In fact, the procedure described in this paper has been used in the frame of the development of the GALILEO Navigation antenna to identify element feeding errors. A planar hologram on the aperture plane of the array has been obtained by a set of spherical near field measurements. Sampling the resulting aperture field distribution (in amplitude and phase) allowed reconstructing the excitation law and identifying errors. The developed procedure was validated with a number of test cases assessing numerical errors introduced by the process. Applying the back-projection to the measured far-field led to discover that some sectors of the array were overfed and that errors were present in the central power divider responsible of the first power distribution in the antenna. A new power divider was then manufactured and integrated into the array leading to a well performing antenna.
Practical ISAR measurements must often be made in the near-field. Scatterers are illuminated by a spherical wavefront, generating a continuum of incident angles due to parallax. Ignoring this, radar image processing produces geometrically distorted images whose utility diminishes the more deeply into the near-field the measurements are made. The underlying assumption that a target may be accurately modeled as a collection of isotropic point scatterers can enormously widen in angle. Yet, by considering parallax (with attention to phase), near-field measurements can produce quasi-far-field images, whose Fourier transform bears a greater likeness to a far-field RCS signature. A technique is presented and explored whereby each image pixel is focused at angles normal to the incident spherical wavefront by compensating for parallax. The focused coordinates are spatially variant, but for a pixel exactly containing a point scatterer, the resulting focused IQ pairs are identical with those in the far-field.
The measurement accuracy of state-of-the-art RF test facilities like near-field or compact test ranges is influenced due to applied system hardware as well as operational facts which are influenced by human errors. The measurement errors of near-field test facilities were analyzed and published in the past times and are based on the 18-term error model of Newell [1]. For compact test ranges and especially for the cross-polar free compensated compact range a similar error model was established at Astrium GmbH within a study for the satellite service provider INTELSAT [2] in order to define possible facility performance improvements and maximum achievable values for the measurement accuracy. It has to be remarked, that test programs for space applications require very stringent adherence to procedures and documentation of process steps during a test campaign. Within this paper, recommendations for process optimizations and procedures will be presented to guarantee the adherence to the valid error budgets and to minimize the Human Factor. A description of main error contributions in the Compensated Compact Range (CCR) of Astrium GmbH will be performed. Furthermore, the error budgets for pattern and gain measurements and achievable performance improvements will be given.
Within the European Union network "Antenna Center of Excellence" – ACE (2004-2007), a first intercomparison campaign among different European measurement systems, using the 12 GHz Validation Standard (VAST12) antenna, were carried out during 2004 and 2005. One of the challenges of that campaign was the definition of the accurate reference pattern. This was the reason why a dedicated measurement campaign for definition of the accurate reference pattern was hold during 2007 and beginning of 2008. This second campaign is described in the companion paper “Dedicated measurement campaign for definition of accurate reference pattern of the VAST12 antenna”. This dedicated measurement campaign was performed by Technical University of Denmark (DTU) in Denmark, SAAB Microwave Systems (SAAB) in Sweden and Technical University of Madrid (UPM) in Spain. This campaign consisted of a large number of measurements with slightly different configurations in each of the three institutions (2 spherical near field systems and one compact range). The purpose of this paper is to show the process to achieve the reference pattern from each institution and the evaluation of the accuracy. The acquisitions were performed systematically varying in applied scanning scheme, measurement distances, signal level and so on. The results are analyzed by each institution combining the measurement results in near or far field and extracting from these measurements: a “best” pattern, an evaluation of possible sources of errors (i.e. reflections, mechanical and electrical uncertainties) and an estimation of the items of the uncertainty budget.
A near-field measurement technique for the prediction of asymptotic far-field antenna patterns from data obtained from a modified cylindrical, or plane-polar, near-field measurement system is presented. This technique utilises a simple change in facility alignment to enable near-field data to be taken over the surface of a conceptual right cone [1, 2], or right conic frustum [3, 4] thereby allowing existing facilities to characterise wide-angle antenna performance in situations where hitherto they would perhaps have been limited by truncation. This paper aims to introduce the measurement technique, describe the novel probe-corrected near-field to far-field transform algorithm which is based upon a cylindrical mode expansion of the measured fields before presenting preliminary results of both computational electromagnetic simulations and actual range measurements. As this paper recounts the progress of ongoing research, it concludes with a discussion of the remaining outstanding issues and presents an overview of the planned future work.
In this work an experimental validation of the nearfield – far-field transformation technique with helicoidal scanning tailored for elongated antennas is provided. Such a transformation relies on the theoretical results relevant to the nonredundant sampling representations of the electromagnetic fields and makes use of an optimal sampling interpolation algorithm, which allows the reconstruction of the near-field data needed by the near-field – far-field transformation with cylindrical scan. In such a case, a prolate ellipsoid is employed to model an elongated antenna, instead of the sphere adopted in the previous approach. It is so possible to consider measurement cylinders with a diameter smaller than the source height, thus reducing the error related to the truncation of the scanning surface. The comparison of the reconstructions obtained from the data directly measured on the classical cylindrical grid with those recovered from the nonredundant measurements on the helix assesses the validity of this innovative scanning technique.