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Among the near-field – far-field (NF–FF) transformation techniques, the one employing the plane-polar scanning has attracted a considerable attention [1]. In this framework, efficient sampling representations over a plane from a nonredundant number of plane-polar samples, which stays finite also for an unbounded scanning plane, have been developed, by applying the nonredundant sampling representations of the EM fields [2] and assuming the antenna under test (AUT) as enclosed in an oblate ellipsoid [3] or in a double bowl [4], namely, a surface formed by two circular bowls with the same aperture diameter but eventually different lateral bends. These effective representations make possible to accurately recover the NF data required by the plane-rectangular NF–FF transformation [5] from a nonredundant number of NF data acquired through the plane-polar scanning. A remarkable reduction of the number of the needed NF data and, as a consequence, of the measurement time is so obtainable. 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 [6] is to retrieve the uniform samples from the nonuniform ones and then reconstruct the required NF data via an accurate and stable optimal sampling interpolation (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 [6] to the uniform samples reconstruction in the case of cylindrical and spherical surfaces. The latter, based on the singular value decomposition method, does not exhibit this constraint and has been applied to the nonredundant plane-polar [7] scanning technique based on the oblate ellipsoidal modelling. 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 to develop these two techniques for compensating known probe positioning errors in the case of the nonredundant plane-polar scanning technique using the double bowl modelling [4]. Experimental tests will be performed at the UNISA Antenna Characterization Lab in order to assess their effectiveness. [1] Y. Rahmat-Samii, V. Galindo Israel, and R. Mittra, “A plane-polar approach for far-field construction from near-field measurements,” IEEE Trans. Antennas Prop., vol. AP-28, pp. 216-230, 1980. [2] 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, 1998. [3] O.M. Bucci, F. D’Agostino, C. Gennarelli, G. Riccio, and C. Savarese, “NF–FF transformation with plane-polar scanning: ellipsoidal modelling of the antenna,” Automatika, vol. 41, pp. 159-164, 2000. [4] O.M. Bucci, C. Gennarelli, G. Riccio, and C. Savarese, “Near-field–far-field transformation from nonredundant plane-polar data: effective modellings of the source,” IEE Proc. Microw. Antennas Prop., vol. 145, pp. 33-38, 1998. [5] E.B. Joy, W.M. Leach, Jr., G. P. Rodrigue and D.T. Paris, “Application of probe-compensated near-field measurements,” IEEE Trans. Antennas Prop., vol. AP-26, pp. 379-389, May 1978. [6] 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, 1994. [7] F. Ferrara, C. Gennarelli, G. Riccio, C. Savarese, “Far field reconstruction from nonuniform plane-polar data: a SVD based approach,” Electromagnetics, vol. 23, pp. 417-429, July 2003
The single-offset compact antenna test range (CATR) is a widely deployed technique for broadband characterization of electrically large antennas at reduced range lengths [1]. The nature of the curvature and position of the offset parabolic reflector as well as the edge geometry ensures that the resulting collimated field is comprised of a pseudo transverse electric and magnetic (TEM) wave. Thus, by projecting an image of the feed at infinity, the CATR synthesizes the type of wave-front that would be incident on the antenna under test (AUT) if it were located very much further away from the feed than is actually the case with the coupling of the plane-wave into the aperture of the AUT creating the classical measured “far-field” radiation pattern. The accuracy of a pattern measured using a CATR is primarily determined by the phase and amplitude quality of the pseudo plane-wave with this being restricted by two main factors: amplitude taper (which is imposed by the pattern of the feed), and reflector edge diffraction, which usually manifests as a high spatial frequency ripple in the pseudo plane wave [2]. It has therefore become customary to specify CATR performance in terms of amplitude taper, and amplitude & phase ripple of this wave over a volume of space, termed the quiet-zone (QZ). Unfortunately, in most cases it is not directly apparent how a given QZ performance specification will manifest itself on the resulting antenna pattern measurement. However, with the advent of powerful digital computers and highly-accurate computational electromagnetic (CEM) models, it has now become possible to extend the CATR electromagnetic (EM) simulation to encompass the complete CATR AUT pattern measurement process thereby permitting quantifiable accuracies to be easily determined prior to actual measurement. As the accuracy of these models is paramount to both the design of the CATR and the subsequent determination of the uncertainty budget, this paper presents a quantitative accuracy evaluation of five different CEM simulations. We report results using methods of CATR modelling including: geometrical-optics with geometrical theory of diffraction [3], plane-wave spectrum [4], Kirchhoff-Huygens [4] and current element [3], before presenting results of their use in the antenna pattern measurement prediction for given CATR-AUT combinations. REFERENCES [1]C.G. Parini, S.F. Gregson, J. McCormick, D. Janse van Rensburg “Theory and Practice of Modern Antenna Range Measurements”, IET Press, 2014, ISBN 978-1-84919-560-7. [2]M. Philippakis, C.G. Parini, “Compact Antenna Range Performance Evaluation Uging Simulated Pattern Measurements”, IEE Proc. Microw. Antennas Propag., Vol. 143, No. 3, June 1996, pp. 200-206. [3]G.L. James, “Geometrical Theory of Diffraction for Electromagnetic Waves”, 3rd Edition, IET Press, 2007, ISBN 978-0-86341-062-8. [4]S.F. Gregson, J. McCormick, C.G. Parini, “Principles of Planar Near-Field Antenna Measurements”, IET Press, 2007.
Abstract Antenna far field phase pattern is important for some applications. It can be directly obtained in pattern measurement by far field test range (FFTR) or compact range (CR). However, it is found that the antenna far field phase pattern measured by current near field test range (NFTR) is not correct. For a uniform phase feeding plane array, its far field phase pattern should be near constant in 3dB beam width. However, the antenna phase pattern measured by current NFTR looks square curve vs angle. This paper found out that the root cause of the error is due to different reference planes. Both the amplitude pattern and the phase pattern obtained by current NFTR, in fact, refer to the probe scanner plane, not the antenna plane. This shifting of the reference plane has no effect on amplitude pattern, but has effect on phase pattern. After that, a correction method is proposed. One example is used for the root cause finding and correction technique explanation. According to this paper, if one wants to get phase pattern using NFTR, it is necessary to measure the distance between AUT and probe aperture accurately so as to correct it accurately after measurement and obtain accurate phase pattern.
In 2015, the Space Communications and Navigation (SCaN) Testbed project completed an S-Band ground station located at the NASA Glenn Research Center in Cleveland, Ohio. This S-Band ground station was developed to create a fully characterized and controllable dynamic link environment when testing novel communication techniques for Software Defined Radios and Cognitive Communication Systems. In order to provide a useful environment for potential experimenters, it was necessary to characterize various RF devices at both the component level in the lab and at the system level after integration. This paper will discuss some of the lab testing of the ground station components, with a particular focus / emphasis on the near-field measurements of the antenna. It will then describe the methodology for characterizing the installed ground station at the system level via a TDRS satellite, with specific focus given to the characterization of the ground station antenna pattern, where the max TDRS transmit power limited the validity of the non-noise floor received power data to the antenna main lobe region. Finally, the paper compares the results of each test as well as provides lessons learned from this type of testing methodology.
Within the standard scheme for probe-corrected spherical data-processing, it has been found that for an efficient computational implementation it is necessary to restrict the characteristics of the probe pattern such that it contains only azimuthal modes for which µ = ±1 [1, 2, 3]. This first-order pattern restriction does not however extend to placing a limit on the polar index mode content and therefore leaves the directivity of the probe unconstrained. Clearly, when using this widely utilized approach, errors will be present within the calculated probe-corrected test antenna spherical mode coefficients for cases where the probe is considered to have purely modes for which µ = ±1 and where the probe actually exhibits higher order mode structure. A number of analysis [4, 5, 6, 7, 8] and simulations [9, 10, 11, 12] can be found documented within the open literature that estimate the effect of using a probe with higher order modes. The following study is a further attempt to develop guidelines for the azimuthal and polar properties of the probe pattern and the measurement configuration that can be utilized to reduce the effect of higher order spherical modes to acceptable levels. ? [1] P.F. Wacker, ”Near-field antenna measurements using a spherical scan: Efficient data reduction with probe correction”, Conf. on Precision Electromagnetic Measurements, IEE Conf. Publ. No. 113, pp. 286-288, London, UK, 1974. [2] F. Jensen, ”On the probe compensation for near-field measurements on a sphere”, Archiv für Elektronik und Übertragung-stechnik, Vol. 29, No. 7/8, pp. 305-308, 1975. [3] J.E. Hansen, (Ed.) “Spherical near-field antenna measurements”, Peter Peregrinus, Ltd., on behalf of IEE, London, 1988. [4] T.A. Laitinen, S. Pivnenko, O. Breinbjerg, “Odd-order probe correction technique for spherical near-field antenna measurements,” Radio Sci., vol. 40, no. 5, 2005. [5] T.A. Laitinen, O. Breinbjerg, “A first/third-order probe correction technique for spherical near-field antenna measurements using three probe orientations,” IEEE Trans. Antennas Propag., vol. 56, pp. 1259–1268, May 2008. [6] T.A. Laitinen, J. M. Nielsen, S. Pivnenko, O. Breinbjerg, “On the application range of general high-order probe correction technique in spherical near-field antenna measurements,” presented at the 2nd Eur. Conf. on Antennas and Propagation (EuCAP’07), Edinburgh, U.K. Nov. 2007. [7] T.A. Laitinen, S. Pivnenko, 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 Propag., vol. 58,, No. 8, pp. 2623–2631, August 2010. [8] T.A. Laitinen, S. Pivnenko, “On the truncation of the azimuthal mode spectrum of high-order probes in probe-corrected spherical near-field antenna measurements” AMTA, Denver, November 2012. [9] A.C. Newell, S.F. Gregson, “Estimating the effect of higher order modes in spherical near-field probe correction”, AMTA 34th Annual Meeting & Symposium, Seattle, WA, October. 2012. [10] A.C. Newell, S.F. Gregson, “Higher Order Mode probes in Spherical Near-Field Measurements”, EuCAP, Gothenburg, April, 2013. [11] A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction”, AMTA 35th Annual Meeting & Symposium, Seattle, WA, October. 2013. [12] A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Azimuthal Modes in Spherical Near-Field Probe Correction”, EuCAP, The Hague, April, 2014.
Open-area test site (OATS) is a basic range for measuring omni antennas at VHF/HUF band. Reflections from the trees nearby, from the edge of the metal ground plane of an OATS are researched with the aid of ultra-broadband calculable dipole antennas (CDAs). Usually, these reflections are detrimental to precise antenna measurements from 20 MHz to 1 GHz; however they are very difficult to analyze accurately, since no rigorous theory exists on the relationship between the reflections and the configurations of an OATS. For this difficulty, a pair of very accurate and broadband CDAs are manufactured and verified with a slightly modified near-field method, whose site-insertion-loss deviation (?SIL) between measurements and simulation is less than 0.3 dB over 10 MHz to 340 MHz for a single pair of dipole elements resonated at 90 MHz. Based on the optimized CDAs, the effects of ground plane sizes, wire mesh shapes around the edge of the metal ground plane, trees nearby and especially masts are researched. The research shows that the reflections from the edge of an optimized ground plane is less than 0.1 dB at 10 m range. Finally, the performance of an OATS with these optimizations is verified: for 10 m separation, ?SIL is within 0.26 dB at horizontal polarization (HP) and within 0.34 dB at vertical polarization (VP) for the typical 24 frequencies from 30 MHz to 1 GHz; at 20 m separation, ?SIL is within 0.59 dB (HP) and 0.85 dB (VP) from 20 MHz to 500 MHz. An example for the uncertainty of calibration the free-space antenna factor of tuned dipole antennas are provided, too.
End-of-the-line over-the-air (OTA) testing of fully assembled wireless devices is one of the most important tests done in factories. It is designed to detect defective devices to avoid them being shipped out to the end customers. There are many requirements in designing over-the-air test systems for factory testing, including small factory real estate, measurement repeatability, and fast test time. These requirements prompt to challenges in OTA test system designs. Few existing widely-used test systems exist: near-field coupling systems where the test antenna is located very near the device’s antenna under test, small TEM cells, and shielded enclosures with one or several test antennas. Each technology has advantages and disadvantages, such as system size, defect detection capabilities/limitations, and performance measurement correlation to that from a far-field method. However, they all lack in dealing with improving test time with devices having technologies working with multiple simultaneous antennas/streams. For example, the current test time for a 2-antenna device (MIMO or received diversity capable devices) is doubled because each antenna chain is tested sequentially. Furthermore, possible coupling effect between antennas is not typically tested. The newly proposed OTA test system is an adaptive system with an array of test antenna elements inside a shielded enclosure. It takes advantage of the multi-path environment inside the enclosure to adapt itself and create a static channel environment with the specified requirement needs. For example, to improve test time for a 2-antenna device, the system groups the antenna elements of the system into two arrays to create two signal streams creating a 2x2-matrix channel with the cross-coupled matrix values minimized (e.g. minimization of the matrix condition number). This created static channel environment with optimized isolation between the two direct signal paths enables testing of the two antenna streams concurrently with minimized perturbation between the streams, hence reducing test time by almost half. The system will reconfigure the antenna elements for each test channel. This proposed new method of an adaptive over-the-air test system opens up to new ways of testing fully-assembled wireless devices in factories and also enables testing of certain performance qualities that current existing OTA test systems cannot perform.
Measurement uncertainty is a vital parameter when assessing the performance of an antenna. Common measurement procedures such as field-probing give performance parameters of the quiet zone, such as amplitude-taper and ripple. However, relating these measurements to the actual measurement uncertainty is difficult at best. Furthermore the gain of the used probe has large influence on the outcome of the performance parameters, making measurement chamber intercomparison based on these parameters difficult. Quiet zone spherical near-field scanning fully describes the field distribution inside the quiet zone. Probe correction can be applied to compensate for the probe influence on the spherical modes. The mode spectrum consists of all electric waves propagating into the quiet zone. From the mode spectrum several performance parameters of the quiet zone can be derived. As an example the main beam power is concentrated in the m=±1 spectrum when aligned with the z-axis. Since other sources, having a different angle, have their mode spectrum spread over the m-spectrum, the power in m=±1 can be divided by the power in m?±1. This provides a signal-to-noise ratio which can be directly related to measurement uncertainty. Using the signal-to-noise ratio a new determination of the measured pattern uncertainty is found. In contrast to parameter derived from field-probing the new parameter is more general and comprehensive. In the paper we will derive new performance parameter and apply them to measured data in a CATR.
The National Institute of Standards and Technology (NIST) has developed computationally efficient algorithms for probe location and polarization compensation in near- to far-field transformations for use when measurements are not made on the standard canonical grids. A major application of such methods is at higher frequencies, where it is difficult or impractical to locate a probe to required tolerances for the standard transforms. Our algorithms require knowledge of the actual position of the probe at the measurement points. This information can be furnished by state-of-the-art optical tracking devices. Probe position information is routinely obtained by the NIST CROMMA (Configurable Robotic MilliMeter-wave Antenna) Facility. Even at lower frequencies, probe-location compensation techniques allow in principle, the use of less precise and therefore, less expensive scanning hardware. Our approach also provides the flexibility to process data intentionally collected on nonstandard grids (plane-polar, spiral, etc.) or with mixed geometries (such as a cylinder with a hemispherical or planar end cap). We present simulations and actual probe position compensation results at 183 GHz. The possibility of compensating for known variations in the probe pointing is considered.
Mars rover Direct-to-Earth (DTE) communication is an exciting new development that can maintain transfer of high volumes of scientific data from Mars to Earth. Currently, large orbiting assets are used as a relay to return scientific data, often containing higher data rates than current DTE systems. Therefore, the goal of this paper is to investigate antenna array topologies to augment DTE systems to support high data rates. The antenna design is complex, having to simultaneously support dual-band, high gain, high power handling, and circular polarization capabilities. An exhaustive study of patch elements in literature shows that current geometries are infeasible for a Mars rover DTE system. A CP Half E-shaped patch element is developed, containing important dual band S11/AR performance in the required RX and TX bands while featuring a single-feed single-layer design. Moreover, various subarray architectures are evaluated to determine if the gain requirements can be achieved. To meet this gain requirement, a 4x4 subarray topology is designed which allows a modular, scalable, and high gain design. To feed each of the 4x4 element subarrays, a stripline feed network is developed, consisting of a binomial impedance transformer and a four stage 1:2 power divider. This feed network supported a broadside radiation pattern for the subarray topology. These components are then integrated, first through a full wave simulation in HFSS. This rigorous study showed support for Mars rover DTE communications systems. The integrated subarray design is then fabricated and measured using a spherical near-field chamber in the UCLA Center for High Frequency Electronics (CHFE) facilities where measurements showed a very good comparison to the simulation results. Overall this integrated subarray design was successful, showing dual-band, high gain, high power handling, and CP performance.
It is a common practice when computing radiation patterns from non-uniformly sampled planar fields to interpolate the samples into a regular grid [1], although it might cause inaccuracies due to the interpolation process. The non-uniform fast Fourier transform (NUFFT) has been applied to process near field measurements in non-uniform planar grids with arbitrary precision [2], and also to analyze aperiodic arrays [3]. However, samples are usually treated as punctual sources. In this contribution, an efficient and accurate method to calculate the far field radiated by non-uniformly sampled planar fields which comply the Nyquist theorem using the non-uniform fast Fourier transform (NUFFT) is shown. The method takes into account the amplitude of the unit cell radiation pattern, which allows to compute more accurately the copolar and crosspolar components of the far field with regard to the array factor [3], which models the samples as punctual sources. For measured fields it is assumed that post-processing has been done, for instance, taking into account probe corrections. Because the NUFFT is precision-dependent, a discussion of how its accuracy can affect the computed radiated fields will be carried out. Numerical examples will be provided to show the accuracy and performance of the NUFFT with regard to the FFT and direct evaluation of the far fields. Finally, a study of computing times comparing the FFT, NUFFT and direct evaluation will be presented. References [1] Y. Rahmat-Samii, L. I. Williams, and R. G. Yaccarino, “The UCLA bi-polar planar-near-field antenna-measurement and diagnostics range,” IEEE Antennas Propag. Mag., vol. 37, no. 6, pp. 16–35, Dec. 1995. [2] R. C. Wittmann, B. K. Alpert, and M. H. Francis, “Near-field antenna measurements using nonideal measurement locations,” IEEE Trans. Antennas Propag., vol. 46, no. 5, pp. 716–722, May 1998. [3] A. Capozzoli, C. Curcio, G. D'Elia, and A. Liseno, “Fast phase-only synthesis of conformal reflectarrays,” IET Microw. Antennas Propag., vol. 4, no. 12, Dec. 2010.
Reflectarrays have been widely studied in the past 3 decades and several techniques have been developed for the synthesis of shaped-beam far-field radiation patterns [1]. Also, some near-field applications have been studied, such as imaging [2] or RFID [3]. In this contribution, a near-field synthesis technique is proposed for the reflectarray quiet zone optimization, which can be of interest in the design of probes for compact antenna test ranges (CATR) at high frequencies. The near-field of the reflectarray is characterized by a simple radiation model which computes the near field of the whole antenna as far-field contributions of each element. The reflectarray unit cell is considered the unit radiation element and its far field is computed employing the second principle of equivalence. Then, at each point in space, all contributions from the elements of the reflectarray are added in order to obtain the near field [4]. This simple model has been validated through simulations with GRASP [5] and also through near-field measurements. Then it has been used to optimize the near field of the reflectarray. The Intersection Approach algorithm is used to optimize both amplitude and phase of the near field radiated by the antenna, and uses the Levenberg-Marquardt algorithm [6] as backward projector. This optimization increases the size of the quiet zone generated by the reflectarray. References [1] J. Huang and J. A. Encinar, Reflectarray Antennas Wiley-IEEE Press, 2008. [2] H. Kamoda et al., "60-GHz electronically reconfigurable large reflectarray using single-bit phase shifters," IEEE Trans. Antennas Propag., vol. 59, no. 7, pp. 2524–2531, July 2011. [3] Hsi-Tseng Chou et al., "Design of a near-field focused reflectarray antenna for 2.4 GHz RFID reader applications," IEEE Trans. Antennas and Propag., vol. 59, no. 3, pp. 1013–1018, March 2011. [4] D. R. Prado, M. Arrebola, M. R. Pino, F. Las-Heras, "Evaluation of the quiet zone generated by a reflectarray antenna," International Conference on Electromagnetics in Advanced Applications (ICEAA), pp. 702–705, 2-7 Sept. 2012. [5] "GRASP Software", TICRA, Denmark, http://www.ticra.com. [6] J. Álvarez et al., “Near field multifocusing on antenna arrays via non-convex optimisation,” IET Microw. Antennas Propag., vol. 8, no. 10, pp. 754–764, Jul. 2014.
In antenna measurement, well-established procedures are consolidated to determine the associated measurement uncertainty for a given antenna and measurements scenario. Similar criteria for establishing uncertainties in numerical modeling of the same antenna are still to be established. In this paper, we investigate the achievable agreement between antenna measurement and simulation when external error sources are minimized. The test object, is a reflector fed by a wideband dual ridge horn (SR40-A and SH4000) manufactured by MVG. This highly stable reference antenna has been selected to minimize uncertainty related to finite manufacturing and material parameter accuracy. Two frequencies, 10.7GHz and 18GHz have been selected for detailed investigation. The antenna has been measured by several measurement facilities (spherical, cylindrical and planar near field ranges) across Europe in the frame of the EurAAP/WG5 “Facility Comparison Campaign” activity. The purpose of this intercomparison campaign is the comparison of the different antenna measurement facilities, throughout Europe, considering measurement procedures and uncertainty estimates. The antenna has been simulated using a full CAD model, in step compatible format and using different numerical methods from different software vendors.
Rectangular open-ended waveguide probes are commonly used in near-field antenna measurements because their frequency behaviour is widely well-known and modeled. Nevertheless, in the last years, the substrate integrated waveguide technology has been developed as a harder competitor. These new circuits are a compromise between the advantages of classical rectangular waveguides, such as high quality factor and low losses, and the advantages of planar circuits, such as low cost and easy compact integration. Also, SIWs present lower weight and dimensions than their equivalent circuits based on metallic waveguides. In this paper we study, under the same measurement conditions, the behaviour of a commercial open-ended rectangular waveguide probe and its equivalent probe in SIW technology. We will compare the near-field measurements obtained with both probes and will show that SIW probes present higher spatial resolution than their equivalent commercial probes. So, SIW probes can detect possible abrupt electric field circuit changes with more accuracy than commercial rectangular waveguides, under the same measurement conditions. The ability of the presented probes has been investigated measuring the simulated amplitude and phase of the electric field of a pyramidal horn placed a few centimetres of the probes. And the study has been validated with the measurements of a microstrip antenna in X-band that presents non-uniform electric field.
Comparisons are made between far-field patterns of an X-band polarization reference horn obtained using the equally-spaced, latitude, and uniform near-field measurement grids. All of the far-fields were obtained by transforming the measured near-field data. Measurement and data processing times are also presented, such that the reader can understand the benefits and drawbacks of the equally-spaced, latitude, and uniform grids. In addition to these comparisons, the sampling requirements of the latitude grid are investigated. In the past, it has been recommended to thin the uniform grid near the poles of the measurement sphere, which is referred to as latitude sampling. The typical method is to multiply the number of sample points required on the equator by a sin(theta) weighting function to obtain the number of sample points required near the poles. However, it will be shown that the sin(theta) weighting function may lead to aliasing in certain cases, and a new method is proposed which is guaranteed to minimize aliasing for any antenna-under-test. We refer to this new grid as the Maximum Fourier Content (MFC) latitude grid.
We describe millimeter-wave near-field measurements made with the new National Institute of Standards and Technology (NIST) robotic scanning system. This cost-effective system is designed for high-frequency performance, is capable of scanning in multiple configurations, and is able to track measurement geometry at every point in a scan. We have measured a WR-5 standard gain horn at 183 GHz using the spherical near-field method. We compare these results to a theoretical model and to a direct far-field measurement.
In military applications, where low sidelobes and high precision in beam pointing are vital, a phased array antenna beamformer requires to be calibrated regarding the cabling that connects the beamformer to the antenna and mutual coupling between antenna elements. To avoid problems associated with mismatched phase transmission lines between the beamformer and the antenna and include the coupling effects, beamforming network characterization must be done with the antenna integrated to the beamformer. In this paper, a method to characterize the beamformer of a slotted waveguide array antenna in the antenna measurement range is introduced. The antenna is a travelling wave slotted waveguide array scanning in the elevation plane. The elevation pattern of the antenna is a shaped beam realized by a phase-only beamformer. The calibration channel includes serial cross-guide couplers fed by a single waveguide line. The channel is integrated to the waveguide lines of the antenna. In the first phase of the characterization, the far field pattern of each antenna element is obtained from the near field measurements at the “zero” states of the phase shifters. In the second stage, all states of the phase shifters are measured automatically using the calibration channel described above. The results of calibration channel measurements are used to determine the changes in phase and magnitude for different states of phase shifters. The phase and magnitude of the peak point of the far field pattern is referenced to the zero state measurement of the calibration channel. Phase only pattern synthesis is carried out using the results of both zero-state near field and calibration channel measurements and the required phase shifter states are determined accordingly. Measured patterns show good agreement with the theoretical patterns obtained in the synthesis phase.
Antenna measurement facilities face their physical limits with the growing size of today’s large and narrow packed antenna farms of telecom satellites but also of large unfurlable reflector antennas for low frequency telecom applications. The special operational constraints that come along when measuring such large future antennas demand for new measurement approaches, especially if the availability or realization of present measurement systems with large anechoic chambers is not an option. This paper presents a new system called PAMS (Portable Antenna Measurement System). The most characteristic part of PAMS is that the RF instrumentation is installed inside a gondola that is positioned by an overhead crane. The gondola is equipped with one or several probes to scan the near-fields of the antenna under test. With a modified crane control the gondola can be placed anywhere within the working space of the crane, which is considered as being giant in comparison to measurement volumes of existing large antenna test facilities. The whole system supports but is not limited to common classical near-field scanning techniques. Thanks to new near-field to far-field transformations the system can deal with arbitrary free form scanning surfaces and probe orientations allowing measurements that have been constrained by the classical near-field theory so far. The paper will explain the PAMS concept on system level and briefly on sub-system level. As proof of concept, study results of critical technologies are discussed. The paper will conclude with the status about on-going development activities.
In recent years a number of analyses and simulations have been published that estimate the effect of using a probe with higher order azimuthal modes with standard probe corrected spherical transformation software. In the event the probe has higher order modes, errors will be present within the calculated antenna under test (AUT) spherical mode coefficients and the resulting asymptotic far-field parameters [1, 2, 3, 4] where the simulations were harnessed to examine these errors in detail. Within those studies, a computational electromagnetic simulation (CEM) was developed to calculate the output response for an arbitrary AUT/probe combination where the probe is placed at arbitrary locations on the measurement sphere ultimately allowing complete near-field acquisitions to be simulated. The planar transmission equation was used to calculate the probe response using the plane wave spectra for actual AUTs and probes derived from either planar or spherical measurements. The planar transmission formula was utilized as, unlike the spherical analogue, there is no limitation on the characteristics of the AUT or probe thereby enabling a powerful, entirely general, model to be constructed. This paper further extends this model to enable other measurement configurations and errors to be considered including probe positioning errors which can result in ideal first order probes exhibiting higher order azimuthal mode structures. The model will also be used to determine the accuracy of the Chu and Semplak near-zone gain correction [5] that is used in the calibration of pyramidal horns. The results of these additional simulations are presented and discussed. Keywords: near-field, antenna measurements, near-field probe, spherical alignment, spherical mode analysis. REFERENCES A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction”, Antenna Measurement Techniques Association (AMTA) 34th Annual Meeting & Symposium, Bellevue, Washington October 21-26, 2012. A.C. Newell, S.F. Gregson, “Higher Order Mode Probes in Spherical Near-Field Measurements”, 7th European Conference on Antennas and Propagation (EuCAP 2013) 8-12 April 2013. A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction”, Antenna Measurement Techniques Association (AMTA) 35th Annual Meeting & Symposium, Columbus, Ohio, October 6-11, 2013. A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Azimuthal Modes in Spherical Near-Field Probe Correction”, The 8th European Conference on Antennas and Propagation (EuCAP 2014) 6-11 April 2014. T.S. Chu, R.A. Semplak, “Gain of Electromagnetic Horns,’’ Bell Syst. Tech. Journal, pp. 527-537, March 1965
Applications using millimeter-wave antennas have taken off in recent years. Examples include wireless HDTV, automotive radar, imaging and space communications. NSI has delivered dozens of antenna measurement systems operating at mm-wave frequencies. These systems are capable of measuring a wide variety of antenna types, including antennas with waveguide inputs, coaxial inputs and wafer antennas that require a probing station. The NSI systems are all based on standard mm-wave modules from vendors such as OML, Rohde & Schwarz and Virginia Diodes. This paper will present considerations for implementation of these systems, including providing the correct RF and LO power levels, the impact of harmonics, and interoperability with coaxial solutions. It will also investigate mechanical aspects such as application of waveguide rotary joints, size and weight reduction, and scanner geometries for spherical near-field and far-field measurements. The paper will also compare the performance of the various mm-wave solutions. Radiation patterns acquired using some of these near-field test systems will be shared, along with some of the challenges encountered when performing mm-wave measurements in the near-field.