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The spherical multipole based near-field far-field transformation is extended to near-field antenna measurements above a perfectly electrically conducting (PEC) ground plane. As the effect of the ground plane is considered in the transformation by applying the image principle to the spherical modes radiated by the device under test (DUT), the near-field measurement points above the ground plane are sufficient to fully characterize the radiation behavior of the DUT above PEC ground. The nonequispaced fast Fourier transform (NFFT) is employed in the forward operator of the inverse problem in order to apply the transformation to e.g. spiral scans which are favorable to large and heavy scanner systems. If the elevation axis is located above or below the ground plane, an additional translation operator is integrated into the transformation to consider such an offset in the mechanical system. The proposed method is applied to synthetic and simulated automotive antenna near-field data in order to show its effectiveness.
The uncertainty of the far field, obtained from antenna planar near field measurements, against the dynamic range is investigated by means of statistical analysis. The dynamic range is usually limited by the noise floor for low level signals and by the receiver saturation for high level signals. The noise level could be important for high measurement rate, which requires the usage of a high signal level to ensure a sufficient signal to noise ratio. As a result the nonlinearities are increasing, thus a compromise must be accomplished. To evaluate the effects of the limited near field dynamic range on the far field, numerical simulations are performed for dipoles array. Initially, the synthetic near field data corresponding to a given antenna under test were generated and directly processed to yield the corresponding far field patterns. Many far field parameters such as gain, beam width, maximum sidelobe level, etc. are determined and recorded as the error-free values of these parameters. Afterwards, the synthetic near field data are deliberately corrupted by noise and receiver nonlinearities while varying the amplitude through small, medium and large values. The error-corrupted near field data are processed to yield the far field patterns, and the error-corrupted values of the far field parameters are calculated. Finally, a statistical analysis was conducted by means of comparison between the error-corrupted parameters and the error-free parameters to provide a quantitative evaluation of the effects of near field errors on the different far field parameters.
There are some antennas where rapid validation is required, maintaining a reduced measurement space and sufficient accuracy in the calculation of some antenna parameters as gain. In particular, for cellular base station antennas in production phase the measurement time is a limitation, and a rapid check of the radiation performance becomes very useful. Also, active phased arrays require a high measurement time for characterizing all the possible measurement conditions, and special antenna measurement systems are required for their characterization. This paper presents a single or dual cut near field antenna test procedure for the measurement of the gain of antennas, especially for separable array antennas. The test set-up is based on an azimuth positioner and a near to far field transformation software based on the expansion of the measurements in cylindrical modes. The paper shows results for gain measurements: first near to far field transformation is performed using the cylindrical modes expansion assuming a zero-height cylinder. This allows the use of a FFT in the calculation of the far field pattern including probe correction. In the case of gain, a near to far field transformation factor is calculated for theta = 0 degrees, using the properties of separable arrays. This factor is used in the gain calculation by comparison technique. Depending on the antenna shape one or two main cuts are required for the calculation of the antenna gain: for linear arrays it is enough to use the vertical cut (larger dimension of the antenna), for planar array antenna 2 cuts are necessary, unless the array was squared assuming equal performance in both planes. Also, this method can be extrapolated to other kind of antennas: the paper will check the capabilities and limitations of the proposed method. The paper is structured in this way: section 1 presents the measurement system. Section 2 presents the algorithms for near to far field transformation and gain calculation. Section 3 presents the validation of the algorithm. Section 4 presents the results of the measurement of different antennas (horns, base station arrays, reflectors) to analyze the limitations of the algorithm. Section 5 includes the conclusions.
Among the near-field–far-field (NF–FF) transformations, that adopting the spherical scanning is particularly interesting, since it allows the complete antenna pattern reconstruction and avoids the error due to the scanning zone truncation. The classical spherical NF–FF transformation [1] has been modified in [2] by exploiting the spatial quasi-bandlimitation properties of the electromagnetic (EM) fields [3]. In particular, the choice of the highest spherical wave has been rigorously determined by these properties instead to be fixed by a rule-of-thumb related to the minimum sphere enclosing the antenna under test (AUT). The nonredundant sampling representations of the EM fields [4] have been properly applied to develop effective NF–FF transformations, requiring a number of NF data remarkably lower than that needed by the classical transformation [1] when considering nonvolumetric antennas. In particular, a quasi-planar AUT has been modelled by an oblate ellipsoid [2] or by a double bowl [5], whereas a long AUT has been shaped by a prolate ellipsoid [2] or by a cylinder with two hemispherical caps (rounded cylinder) [5]. Unfortunately, for practical constraints, it is not always possible to mount the AUT in such a way that it is centred on the scanning sphere centre. In such a case, the number of NF data needed by the classical NF–FF transformation [1] and the related measurement time can considerably grow, due to the corresponding increase of the minimum sphere radius. To overcome this drawback, a new spherical NF–FF transformation has been recently proposed in [6], by developing a properly modified version of the spherical wave expansion, wherein the spherical wave functions are defined with respect to the AUT centre instead of the scanning sphere one. Although the number of needed NF data is drastically reduced with respect to that fixed by the rule of the minimum sphere radius, it results to be slightly greater than the one corresponding to a centred mounting. Aim of this work is to properly exploit the nonredundant representations of EM fields to develop a nonredundant spherical NF–FF transformation for long antennas, based on rounded cylinder modelling, which requires the same number of NF data in both cases of centred and offset mounting of the AUT. It will be so possible to remarkably reduce the number of NF data and the related measurement time with respect to that required by the approach [6]. [1] J. Hald, J.E. Hansen, F. Jensen, and F.H. Larsen, Spherical near-field antenna measurements, J.E. Hansen, (ed.), London, Peter Peregrinus, 1998. [2] O.M. Bucci, C. Gennarelli, G. Riccio, and C. Savarese, “Data reduction in the NF–FF transformation technique with spherical scanning,” Jour. Electromagn. Waves Appl., vol. 15, pp. 755-775, June 2001. [3] O.M. Bucci and G. Franceschetti, “On the spatial bandwidth of scattered fields,” IEEE Trans. Antennas Prop., vol. AP-35, pp. 1445-1455, Dec. 1987. [4] O.M. Bucci, C. Gennarelli, and 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, 1998. [5] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, and M. Migliozzi, “Effective antenna modellings for NF–FF transformations with spherical scanning using the minimum number of data,” Int. Jour. Antennas Prop., vol. 2011, ID 936781, 11 pages. [6] L.J. Foged, P.O. Iversen, F. Mioc, and F. Saccardi, “Spherical near field offset measurements using downsampled acquisition and advanced NF/FF transformation algorithm,” Proc. of EUCAP 2016, paper 1570229473, Davos, Apr. 2016.
An innovative method for the diagnosis of large reflector antennas from far field data in radio astronomical application is presented, which is based on the optimization of the number and the location of the far field sampling points required to retrieve the antenna status in terms of feed misalignments. In these applications a continuous monitoring of the Antenna Under Test (AUT), and its subsequent reassessment, is necessary to guarantee the optimal performances of the radiotelescope. The goal of the method is to reduce the measurement time length to minimize the effects of the time variations of both the measurement setup and of the environmental conditions, as well as the issues raised by the complex tracking of the source determined by a prolonged acquisition process. Furthermore, a short measurement process helps to shorten the idle time forced by the maintenance activity. The field radiated by the AUT is described by the aperture field method. The effects of the feed misalignments are modeled in terms of an aberration function, and the relationship between this function and the Far Field Pattern is recast in the linear map by expanding on a proper set of basis functions the perturbation function of the Aperture Field. These basis functions are determined using the Principal Component Analysis. Accordingly, from the Far Field Pattern, assumed measured in amplitude and phase, the unknown parameters defining the antenna status can be retrieved. The number and the position of the samples is then found by a Singular Values Optimization (SVO).
Several recent articles [1-9] have focused on assessing spherical near field (SNF) errors induced by using a non-ideal probe, i.e. a probe that has modal content. This paper explores this issue from the perspective of the probe response constants, defined by [10], which are the mathematical representation of the effect of the antenna under test (AUT) subtending a finite angular portion of the probe pattern at measurement distance . The probe response constants are a function of the probe modal coefficients, the size of the AUT (i.e. the AUT minimum sphere radius ), and the measurement distance , and thus can be used to evaluate the relative contribution of probe content as both measurement distance and AUT size varies. After a brief introduction, the first section of this paper reviews the theory describing the probe response constants; the second section provides some examples of the probe response constants for a simulated broadband quad-ridge horn, and the final section examines measured AUT pattern errors induced by using the corresponding probe response constants in a conventional SNF-to-FF transform. References: [1] A. C. Newell and S. F. Gregson, “Effect of Higher Order Modes in Standard Spherical Near-Field Probe Correction,” in AMTA 2015 Proceedings, Long Beach, CA, 2015. [2] Y. Weitsch, T. F. Eibert, and L. G. T. van de Coevering, “Investigation of Higher Order Probe Corrected Near-Field Far-Field Transformation Algorithms for Preceise Measurement Results in Small Anechoic Chambers, in AMTA 2015 Proceedings, Long Beach, CA, 2015. [3] A. C. Newell and S. F. Gregson, “Estimating the Effect of Higher Order Azimuthal Modes in Spherical Near-Field Probe Correction,” in EuCAP 2014 Proceedings, The Hague, 2014. [4] A. C Newell and S. F. Gregson, “Higher Order Mode Probes in Spherical Near-Field Measurements, in EuCAP 2013 Proceedings, Gothenburg, 2013. [5] A. C. Newell and S. F. Gregson, “Estimating the Effect of Higher-Order Modes in Spherical Near-Field Probe Correction,” in AMTA 2012 Proceedings, Seattle, WA, 2012. [6] T. A. Laitinen and S. Pivnenko, “On the Truncation of the Azimuthal Mode Spectrum of High-Order Probes in Probe-Corrected Spherical Near-Field Antenna Measurements,” in AMTA 2011 Proceedings, Denver, CO, 2011. [7] T. A. Laitinen, S. Pivnenko, and O. Breinbjerg, “Theory and Practice of the FFT/Matrix Inversion Technique for Probe-Corrected Spherical Near-field Antenna Measurements with High-Order Probes,” IEEE Trans. Antennas and Prop., Vol. 58, No. 8, August 2010. [8] T. A. Laitinen, J. M. Nielsen, S. Pivnenko, and O. Breinbjerg, On the Application Range of General High-Order Probe Correction Technique in Spherical Near-Field Antenna Measurements,” in EuCAP 2007 Proceedings, Edinburgh, 2007. [9] T. A Laitinen, S. Pivnenko, and O. Breinbjerg, “Odd-Order Probe Correction Technique for Spherical Near-Field Antenna Measurements,” Radio Sci., Vol. 40, No. 5, 2005. [10] J. E. Hansen ed., Spherical Near-Field Antenna Measurements, London: Peregrinus, 1988.
Compact Antenna Test Range (CATR) provide convenient testing, directly in far-field conditions of antenna systems placed in the Quiet Zone (QZ). Polarization performance is often the reason that a more expensive, complex, compensated dual reflector CATR is chosen rather than a single reflector CATR. For this reason, minimizing the QZ cross-polarization of a single reflector CATR has been a challenge for the industry for many years. A new, dual polarised feed, based on conjugate matching of the undesired cross polar field in the QZ on a full wave-guide band, has recently been developed, manufactured and tested. The CXR feed (cross polar reduction feed) has shown to significantly improve the QZ cross-polar discrimination of standard single reflector CATR systems. In previous papers, the CXR feed concept has been discussed and proved using a limited scope demonstrator and numerical analysis. In this paper, for the first time, the exhaustive testing of the dual polarised feed operating in the extended WR-75 waveguide band (10-16 GHz) is presented. Accuracy improvements, achieved in antenna cross-polar testing, using this feed is also illustrated by measured examples.
An inverse source reconstruction method with great potential in radome diagnostics is presented. Radomes are designed to enclose antennas to protect them, from e.g. weather conditions. Frequency selective surface (FSS) radomes are designed to conceal the antennas and provide stealth properties, by transmitting specific frequencies and be reflective for other frequencies. Ideally, the radome is expected to be electrically transparent. However, tradeoffs are necessary to fulfill properties such as aerodynamics, robustness, lightweight, weather persistency, stealth properties, etc. One tradeoff is the existence of inevitable defects. Specifically, for examples, seams in large radomes, lightning strike protection, Pitot tubes, rain caps, or lattice dislocations in frequency selective radomes. In all these examples of defects, it is essential to diagnose their influences, since they degrade the electromagnetic performance of the radomes if not carefully attended and analyzed. In this contribution, we investigate if source reconstruction can be employed to localize and image the disturbances from the defects on the surface of the radome. Employing far-field measurements remove the need for probe compensation. An artificial puck plate (APP) radome with dislocations in the lattice is investigated. An APP radome is a frequency selective surface (FSS) and it consists of a thick perforated conducting frame, where the apertures in the periodic lattice are filled with dielectric pucks. Due to the double curvature of an FSS surface, gaps and disturbances in the lattice may cause deterioration of the radome performance. Source reconstruction methods determine the equivalent surface currents close to the object of interest. The reconstructions are established by employing an integral representation in combination with an integral equation. The geometry of the object on which the fields are reconstructed is arbitrary. However, the problem is ill-posed and needs regularization. The equivalent surface currents are reconstructed on a body of revolution with the method of moment (MoM), and the problem is regularized with a singular value decomposition (SVD). The aim is to back-propagate a measured far field to determine the field components on the radome surface. The purpose is to investigate if defects on a frequency selective surface (FSS) lattice can be localized.
An inverse source reconstruction method with great potential in radome diagnostics is presented. Radomes are designed to enclose antennas to protect them, from e.g. weather conditions. Frequency selective surface (FSS) radomes are designed to conceal the antennas and provide stealth properties, by transmitting specific frequencies and be reflective for other frequencies. Ideally, the radome is expected to be electrically transparent. However, tradeoffs are necessary to fulfill properties such as aerodynamics, robustness, lightweight, weather persistency, stealth properties, etc. One tradeoff is the existence of inevitable defects. Specifically, for examples, seams in large radomes, lightning strike protection, Pitot tubes, rain caps, or lattice dislocations in frequency selective radomes. In all these examples of defects, it is essential to diagnose their influences, since they degrade the electromagnetic performance of the radomes if not carefully attended and analyzed. In this contribution, we investigate if source reconstruction can be employed to localize and image the disturbances from the defects on the surface of the radome. Employing far-field measurements remove the need for probe compensation. An artificial puck plate (APP) radome with dislocations in the lattice is investigated. An APP radome is a frequency selective surface (FSS) and it consists of a thick perforated conducting frame, where the apertures in the periodic lattice are filled with dielectric pucks. Due to the double curvature of an FSS surface, gaps and disturbances in the lattice may cause deterioration of the radome performance. Source reconstruction methods determine the equivalent surface currents close to the object of interest. The reconstructions are established by employing an integral representation in combination with an integral equation. The geometry of the object on which the fields are reconstructed is arbitrary. However, the problem is ill-posed and needs regularization. The equivalent surface currents are reconstructed on a body of revolution with the method of moment (MoM), and the problem is regularized with a singular value decomposition (SVD). The aim is to back-propagate a measured far field to determine the field components on the radome surface. The purpose is to investigate if defects on a frequency selective surface (FSS) lattice can be localized.
For near-field antenna measurement it is sometimes desirable or necessary to measure only the magnitude of the near-field - to perform so-called phaseless (or amplitude-only or magnitude-only) near-field antenna measurements [1]. It is desirable when the phase measurements are unreliable due to probe positioning inaccuracy or measurement equipment inaccuracy, and it is necessary when the phase reference of the source is not available or the measurement equipment cannot provide phase. In particular, as the frequency increases near-field phase measurements become increasingly inaccurate or even impossible. However, for the near-field to far-field transformation it is necessary to obtain the missing phase information in some other way than through direct measurement; this process is generally referred to as the phase retrieval. The combined process of first measuring the magnitudes of the field and subsequently retrieving the phase is referred to as a phaseless near-field antenna measurement technique. Phaseless near-field antenna measurements have been the subject of significant research interest for many years and numerous reports are found in the literature. Today, there is still no single generally accepted and valid phaseless measurement technique, but several different techniques have been suggested and tested to different extents. These can be divided into three categories: Category 1 – Four magnitudes techniques, Category 2 – Indirect holography techniques, and Category 3 -Two scans techniques. This paper provides an overview of the different phaseless near-field antenna measurement techniques and their respective advantages and disadvantages for different near-field measurement setups. In particular, it will address new aspects such as probe correction and determination of cross-polarization in phaseless near-field antenna measurements. [1] OM. Bucci et al. “Far-field pattern determination by amplitude only near-field measurements”, Proceedings of the 11’th ESTEC Workshop on Antenna Measurements, Gothenburg, Sweden, June 1988.
Near field Inverse Synthetic Aperture Radar (ISAR) Radar Cross Section (RCS) measurements are used in this study to obtain geometrically correct images of full scale objects placed on a turntable. The images of the targets are processed using a method common in the compressive sensing field, Basis Pursuit Denoise (BPDN). A near field model based on isotropic point scatterers is set up. This target model is naturally sparse and the L1-minimization method BPDN works well to solve the inverse problem. The point scatterer solution is then used to obtain far field RCS data. The methods and the developed algorithms required for the imaging and the RCS extraction are described and evaluated in terms of performance in this paper. A comparison to image based near to far field methods utilizing conventional back projection is also made. The main advantage of the method presented in this paper is the absence of noise and side lobes in the solution of the inverse problem. Most of the RCS measurements on full scale objects that are performed at our measurement ranges are set up at distances shorter than those given by the far field criterion. The reasons for this are, to mention some examples, constraints in terms of available equipment and considerations such as maximizing the signal to noise in the measurements. The calibrated near-field data can often be used as recorded for diagnostic measurements but in many cases the far field RCS is also required. Data processing is then needed to transform the near field data to far field RCS in those cases. Separate features in the images containing the point scatterers can be selected using the method presented here and a processing step can be performed to obtain the far field RCS of the full target or selected parts of the target, as a function of angle and frequency. Examples of images and far field RCS extracted from measurements on full scale targets using the method described in this paper will be given.
At AMTA 2006, we introduced the world to a system and method for over-the-air (OTA) testing of MIMO wireless devices with the concept of the boundary array technique, whereby the far-field over the air RF propagation environment is emulated to produce the realistic near field multi-path propagation conditions necessary for MIMO communication. Last year, the CTIA released Version 1.0 of their "Test Plan for 2x2 Downlink MIMO and Transmit Diversity Over-the-Air Performance," which standardizes on the boundary array technique (commonly referred to as the Multi-Probe Anechoic Chamber technique to differentiate it from the use of a reverberation chamber) for MIMO OTA testing. As the wireless industry just now prepares to perform certification testing for MIMO OTA performance for existing 4G LTE devices, the rest of the community is looking forward to the development of 5G. The corresponding future releases of the 3GPP wireless standard are expected to standardize the use of Massive MIMO in existing cellular communication bands. Massive MIMO is similar to the concept of mulit-user MIMO in IEEE 802.11ac Wi-Fi radios, but is taken to the extreme, with potentially hundreds of antennas and radios per cellular base station. This high level of radio to antenna integration at the base station will for the first time drive the industry beyond just antenna pattern measurements of base stations and OTA performance testing of handsets to full OTA performance testing of these integrated systems. At the same time, handset design is evolving to use adaptive antenna systems that will pose additional testing challenges. Likewise, manufacturers are looking to evaluate real-world usage scenarios that aren't necessarily represented by the test cases used for mobile device certification testing. This paper will discuss a number of these advances and illustrate ways that the MIMO OTA test systems must evolve to address them.
Spherical near-field (SNF) antenna test systems offer unique advantages over other types of measurement configurations and have become increasingly popular over the years as a result. To yield high accuracy far-field radiation patterns, it is critical that the rotators of the SNF scanner are properly aligned. Many techniques using optical instruments, laser trackers, low cost devices or even electrical measurements [1 - 3] have been developed to align these systems. While these alignment procedures have been used in practice with great success, some residual alignment errors always remain. These errors can sometimes be quantified with high accuracy and low uncertainty (known error) or with large uncertainties (unknown error). In both cases, it is important to understand the impact that these SNF alignment errors will have on the far-field pattern calculated using near-field data acquired on an SNF scanner. The sensitivity to various alignment errors has been studied in the past [4 - 6]. These investigations concluded that altering the spherical acquisition sampling grid can drastically change the sensitivity to certain alignment errors. However, these investigations were limited in scope to a single type of measurement system. This paper will expand upon this work by analyzing the effects of spherical alignment errors for a variety of different measurement grids and for different SNF implementations (phi-over-theta, theta-over-phi) [7]. Results will be presented using a combination of physical alignment perturbations and errors induced via simulation in an attempt to better understand the sensitivity to SNF alignment errors for a variety of antenna types and orientations within the measurement sphere. Keywords: Spherical Near-Field, Alignment, Uncertainty, Errors. References [1] J. Demas, “Low cost and high accuracy alignment methods for cylindrical and spherical near-field measurement systems”, in the proceedings of the 27th annual Meeting and Symposium, Newport, RI, USA, 2005. [2] S. W. Zieg, “A precision optical range alignment tecnique”, in the proceedings of the 4th annual AMTA meeting and symposium, 1982. [3] A.C. Newell and G. Hindman, “The alignment of a spherical near-field rotator using electrical measurements”, in the proceedings of the 19th annual AMTA meeting and symposium, Boston, MA, USA, 1997. [4] A.C. Newell and G. Hindman, “Quantifying the effect of position errors in spherical near-field measurements”, in the proceedings of the 20th annual AMTA meeting and symposium, Montreal, Canada, 1998. [5] A.C. Newell, G. Hindman and C. Stubenrauch, “The effect of measurement geometry on alignment errors in spherical near-field measurements”, in the proceedings of the 21st annual AMTA meeting and symposium, Monterey, CA, USA, 1999. [6] G. Hindman, P. Pelland and G. Masters, “Spherical geometry selection used for error evaluation”, in the proceedings of the 37th annual AMTA meeting and symposium, Long Beach, CA, USA, 2015. [7] C. Parini, S. Gregson, J. McCormick and D. Janse van Rensburg, Theory and Practice of Modern Antenna Range Measurements. London, UK: The Institute of Engineering and Technology, 2015
At AMTA 2006, we introduced the world to a system and method for over-the-air (OTA) testing of MIMO wireless devices with the concept of the boundary array technique, whereby the far-field over the air RF propagation environment is emulated to produce the realistic near field multi-path propagation conditions necessary for MIMO communication. Last year, the CTIA released Version 1.0 of their "Test Plan for 2x2 Downlink MIMO and Transmit Diversity Over-the-Air Performance," which standardizes on the boundary array technique (commonly referred to as the Multi-Probe Anechoic Chamber technique to differentiate it from the use of a reverberation chamber) for MIMO OTA testing. As the wireless industry just now prepares to perform certification testing for MIMO OTA performance for existing 4G LTE devices, the rest of the community is looking forward to the development of 5G. In the search for ever more communication bandwidth, the wireless industry has set its sights on broad swaths of unused spectrum in the millimeter wave (mmWave) region above 20 GHz. The first steps into this area have already been standardized as 802.11ad by the members of the WiGig Alliance for short range communication applications in the unlicensed 60 GHz band, with four 2.16 GHz wide channels defined from 58.32-65.88 GHz. With the potential for phenomenal bandwidths like this, the entire telecommunications industry is looking at the potential of using portions of this spectrum for both cellular backhaul (mmWave links from tower to tower) as well as with the hopes of developing the necessary technology for mobile communication with handsets. The complexity of these new radio systems and differences in the OTA channel model at these frequencies, not to mention limitations in both the frequency capabilities and resolution requirements involved, imply the need for a considerably different environment simulation and testing scenarios to those used for current OTA testing below 6 GHz. The traditional antenna pattern measurement techniques used for existing cellular radios are already deemed insufficient for evaluating modern device performance, and will be even less suitable for the adaptive beamforming arrays envisioned for mmWave wireless devices. Likewise, the array resolution and path loss limitations required for a boundary array system to function at these frequencies make the idea of traditional OTA spatial channel emulation impractical. However, as we move to technologies that will have the radio so heavily integrated with the antenna system that the two cannot be tested separately, the importance of OTA testing cannot be understated. This paper will discuss the potential pitfalls we face and introduce some concepts to attempt to address some of the concerns noted here.
Although In the 1970’s and 1980’s the near-field technology was proven to work properly for antenna characterization. It was until the late 1990’s that antenna communities begun to trust this technology and depend heavily on it. This same scenario could happen to the phase-less near-field technologies. It is true that there is still much to be done in the sense of reliability of these techniques. Nevertheless there are still situations where these techniques must be applied. This paper will be dealing with the phase-less near-field antenna measurement technique. The well-known iterative Fourier transformation (IFT) technique is used. The amplitude of the field distribution on concentric spheres surrounding the antenna under test (AUT) is used to reconstruct the phase information necessary for the spherical near-field to far-field transformation (SNFFF). It will be shown that despite its geometrical and computational complexity this technique can be applied on the spherical case achieving very good accuracy. In addition this paper makes use of global optimization techniques especially genetic algorithm (GA) to establish an initial estimate of the phase distribution necessary for the algorithm which is later on fine-tuned using the local optimization i.e. IFT to retrieve a closer estimate of the solution. It will be shown that except for the null positions the far-field accuracy can be enhanced. The implementation of the GA will be shortly given and the concept of masks, which simplifies the implementation, will be discussed.
Electro-optic (EO) probes provide an ultra-wideband, high-resolution, non-invasive technique for polarimetric near-field scanning of antennas and phased arrays. Unlike conventional near field scanning systems which typically involve metallic components, the small footprint all-dielectric EO probes can get extremely close to an RF device under test (DUT) without perturbing its fields. In this paper, we discuss and present measurement results for EO field mapping of a dual-band circularly polarized active phased array that operates at two different S and C bands: 2.1GHz and 4.8GHz. The array uses probe-fed, cross-shaped, patch antenna elements at the S-band and dual-slot-fed rectangular patch elements at the C-band. At each frequency band, the array works both as transmitting and receiving antennas. The antenna elements have been configured as scalable array tiles that are arranged together to create larger apertures. Near-field scan maps and far-field radiation patterns of the dual-band active phased array will be presented at the bore sight and at different scan angles and the results will be validated with simulation data and measurement results from an anechoic chamber.
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.
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.
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.
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.