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The design of active electronically steered arrays (AESA) is a challenging, time-consuming and costly endeavor. The design process becomes much more sophisticated in the case of dual-band circularly polarized active phased arrays, in which CP radiating elements at two different frequency bands occupy a common shared aperture. A design process that takes into account various inter-element and intra-element coupling effects at different frequency bands currently relies solely on computer simulations. The conventional near-field scanning systems have serious limitations for quantifying these coupling effects mainly due to the invasive nature of their metallic probes, which indeed act as receiving antennas and have to be placed far enough from the antenna under test (AUT) to avoid perturbing the latter’s near fields. In recent years, a unique, versatile, near-field mapping/scanning technique has been introduced that circumvents most of such measurement limitations thanks to the non-invasive nature of the optical probes. This technique uses the linear Pockels effect in certain electro-optic crystals to modulate the polarization state of a propagating optical beam with the RF electric field penetrating and present inside the crystal. In this paper, we will present near-field and far-field measurement data for 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 patched together to create larger apertures.
Calibrating a passive, space-fed, phased array antenna is more difficult and time consuming then calibrating corporate-fed arrays because individual elements cannot be activated or deactivated. We will present our method of determining element state-phase curves and insertion phase bias between elements. We will also explain this method’s theoretical basis and validate it by comparing data measured in an anechoic chamber with data measured in a planar near field range. The anechoic chamber data will be compared with the typical, proven, but more time-consuming planar near field calibration method.
This paper explores the accuracy capabilities of a two element phase interferometer measurement in a planar near-field scanner. Traditional phase interferometer applications utilize wide field of view antennas such as spirals making the utilization of planar near-field measurements less than ideal. In this application, high directivity antennas were utilized which allowed us to consider a planar near-field measurement solution. Leaving the AUT stationary and the stability of the planar near-field coordinate system were primary considerations in deciding to utilize a planar near-field measurement system. Typical interferometer performance metrics include comparing measured phase differences to ideal element phase differences at the same locations. Often the nominal drawing locations are used to generate the ideal element phase difference curves. The sensitivity of actual element vector displacement values versus ideal displacements can be reduced by deriving the best-fit displacement vector from the measured data and is utilized in the processing and reporting of results. This paper reviews the measurements, analysis techniques and results from this investigation and illustrates the capabilities of a planar near-field scanner to perform these types of measurements with a high degree of measurement fidelity.
In planar and cylindrical near-field antenna measurements the probe pattern correction is essential, since the used angular sector of the probe pattern extends over large part of the forward hemisphere. But in spherical near-field measurements, the probe is always looking towards an antenna under test (AUT) and the used angular sector of the probe pattern is relatively small: it usually does not exceed some ±30deg, but typically is much smaller, depending on the size of the AUT and the distance to the probe. For this reason, for low-directive probes with little pattern variation in the used angular sector, it is often said that the probe pattern correction can be omitted without introducing significant error in the calculated far-field AUT pattern. However, no specific guidelines on the value of the introduced error have been presented so far in the literature. In this paper, the error in the calculated far-field AUT pattern due to omitted probe pattern correction is investigated by simulations and confirmed by selected measurements. The investigation is carried out for two typical probes, an open-ended waveguide and a small conical horn, and for aperture-type AUTs of different electrical size with different distance to the probe. The obtained results allow making a justified choice on including or omitting the probe pattern correction in practical situations based on the estimated error at different levels of the AUT pattern.
There is a lot of interest in measuring the scattered fields from a panel. The panel could be a frequency selective surface (FSS), could consist of lossy dielectric material, resistive material, etc. For these measurements, the panel is mounted in a large ground plane (perfectly conducting) that mimics an infinite ground plane and the back scattered/bistatic scattered fields are measured. These measured fields contain the scattering from the panel under test as well as the diffracted fields from the junction between the panel and the ground plane, and it is quite difficult to discern the two field components. Alternatively, one can measure the scattered fields over a frequency band in the near zone using a fixed transmitting antenna while the receiving antenna is displaced to scan a planar surface or a linear scan. Note that the measurements are similar to one-way probing. The total measured scattered fields can be processed to isolate the scattering from the panel of interest. In this paper, we will present various signal processing techniques that can be applied to the measured scattered field data. These techniques include high resolution down range processing (tie domain), time domain near field focusing, etc. We will also show that it is straight forward to obtain the reflection and transmission coefficient of the panel from the near field measured data.
This paper discusses the application of modern NF measurements and statistical analysis techniques to efficiently characterize the FF radiation pattern statistics of antennas and other EM emitters whose radiated EM fields vary erratically in a seemingly random manner. Such randomly-varying radiation has been encountered, for example, in measurements involving array antenna elements and reflector feed horn(s) containing active or passive devices that affect the relative phases and/or amplitudes of the pertinent RF signals in a non-deterministic manner [1-2]. In-Band (IB) as well as Out-Of-Band (OB) signals may be involved in some cases. Other possible randomly varying EM radiations include leakage from imperfectly-shielded equipment, connectors, cables, and waveguide runs [2- 4] Previous work at GTRI [5-7] has shown that computations of key FF radiation pattern statistics can be made based on NFFF transformations involving a) the sample average value of the complex electric field at each NF measurement point, b) the sample average value (a real number) of the standard deviation of the complex electric field at each NF measurement point, and c) the measured complex cross-covariance functions at all different NF measurement points. The key FF radiation pattern statistics of most interest are typically a) the statistical average FF radiation pattern, b) the standard deviation, c) the probability density function (p.d.f.), and d) the cumulative probability distribution (C.P.D.). Simulated data measurement protocols and the requisite statistical processing of the NF measured data will be presented and discussed in detail at the symposium. The NF cross-covariance functions introduce a new level of complexity in NF measurements and analysis that is absent for “deterministic” EM field measurements because the cross covariance functions must be measured and processed for all different NF measurement points on the NF surface to compute valid Pattern FF statistics. However, pairs of linear or circular probe arrays can be used to great advantage to achieve tolerable NF measurement times for the cross covariance functions and the aforementioned NF statistical quantities, thereby enabling valid computations of the FF pattern statistics. The use of dual probe arrays will be presented and discussed in detail and compared with mechanical scanning of two “single” probes over two NF measurement surfaces. A technique for estimating the cross-covariance functions will be presented and compared with exact values.
MIL STD 461 is the Department of Defense standard that states the requirements for the control of electromagnetic interference (EMI) in subsystems and equipment used by the armed forces. The standard requires users to measure the unintentional radiated emissions from equipment by placing a measuring antenna at one meter distance from the equipment under test (EUT). The performance of the antenna at 1m distance must be known for the antenna to measure objects located at this close proximity. MIL STD 461 requires the antennas to be calibrated at 1 m distance using the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 958. This SAE ARP 958 document describes a standard calibration method where two identical antennas are used at 1m distance to obtain the gain at 1m for each antenna. In this paper the authors show using simulations that the SAE ARP 958 approach introduces errors as high at 2 dB to the measured gain and AF. To eliminate this problem the authors introduce a new method for calibrating EMC antennas for MIL STD 461. The Method is based on the well-known extrapolation range technique. The process is to obtain the polynomial curve that is used to get the far field gain in the extrapolation gain procedure, and to perform an interpolation to get the gain at 1 m. The results show that some data in the far field must be collected during the extrapolation scan. When the polynomial is calculated the antenna performance values at shorter distances will be free of near field coupling. Measured results for a typical antenna required for emissions testing per the MIL STD 461 match well with the numerical results for the computed gain at 1 m distance. Future work is required to study the use of this technique for other short test distances used in other electromagnetic compatibility standards, such as the 3 m test distance used by the CISPR 16 standard. Keywords: Antenna Calibrations, EMC Measurements, Extrapolation Range Techniques
Methods for determining the uncertainty in antenna measurements have been previously developed and presented. The IEEE recently published a document [1] that formalizes a methodology for uncertainty analysis of near-field antenna measurements. In contrast, approaches to uncertainty analysis for antenna measurements on a compact range are not covered as well in the literature. Unique features of the compact range measurement technique require a comprehensive approach for uncertainty estimation for the compact range environment. The primary difference between the uncertainty analyses developed for near-field antenna measurements and an uncertainty analysis for a compact range antenna measurement lies in the quality of the incident plane wave illuminating the antenna under test from the compact range reflector. The incident plane wave is non-ideal in amplitude, phase and polarization. The impact of compact range error sources on measurement accuracy has been studied [2,3] and error models have been developed [4,5] to investigate the correlation between incident plane wave quality and the resulting measurement uncertainty. We review and discuss the terms that affect gain and sidelobe uncertainty and present a framework for assessing the uncertainty in compact range antenna measurements including effects of the non-ideal properties of the incident plane wave. An example uncertainty analysis is presented. Keywords: Compact Range, Antenna Measurement Uncertainty, Error Analysis References: 1. IEEE Standard 1720-2012 Recommended Practices for Near-Field Antenna Measurements. 2. Bingh,S.B., et al, “Error Sources in Compact Test Range”, Proceedings of the International Conference on Antenna Technologies ICAT 2005. 3. Bennett, J.C., Farhat, K.S., “Wavefront Quality in Antenna Pattern Measurement: the use of residuals.”, IEEE Proceedings Vol. 134, Pt. H, No. 1, February 1987. 4. Boumans, M., “Compact Range Antenna Measurement Error Model”, Antenna Measurement Techniques Association 1996 5. Wayne, D., Fordham, J.A, Mckenna, J., “Effects of a Non-Ideal Plane Wave on Compact Range Measurements”, Antenna Measurement Techniques Association 2014
Spherical spiral scanning involves coordinating the motion of two simultaneous axes to accomplish near-field antenna measurements along a line on a sphere that does not cross itself. The line would ideally start near a pole and trace a path along the sphere to the other pole. An RF probe is moved along this path in order to collect RF measurements at predefined locations. The data collected from these measurements is used along with a near-field to far-field transformation algorithm to determine the radiated far-field antenna pattern. The method for transforming data collected along spherical spiral scan has been previously presented [1]. Later laboratory measurement studies have shown the validity of the spherical spiral scanning technique [2]. Here, the authors present a review of the spherical scanning technique and present recent advances and the applicability of the method to testing antennas mounted on automobiles. The method has the advantages of a reduction of the overall number of data points required in order to meet a minimum sampling requirement determined using non-redundant sampling techniques. This reduction in the number of data points and the advantage of moving two axes simultaneously result in a significant reduction in the time required to collect a set of measured data. Keywords: Spherical Near-Field, Telematics, Automotive References: [1] F. D’Agostino, C. Gennarelli, G. Riccio, and C. Savarese, “Theoretical Foundations of near-field to far-field transformations with spiral scannings,” Prog. In Electromagn. Res., vol. PIER 61, pp 193-214, 2006. [2] F. D’Agostino, F. Ferrara, J. Fordham, C. Gennarelli, R. Guerriero, and M. Migliozzi, “An Experimental Validation of the Near-Field to Far-Field Transformation with Spherical Spiral Scan,” Proc. Of the Antenna Measurement Techniques Association, 2012.
MI Technologies has recently developed and installed two separate real-time laser correction mechanisms for large planar scanners. One mechanism employs a spinning laser, while the other uses a tracking laser with multiple SMR constellations. The spinning laser system is limited to planarity correction, and is appropriate for any planar scanner up to a diagonal of about 15 meters. The tracking laser system compensates X, Y, and Z, and is intended for a horizontal planar scanner of larger size or when X and Y positions also require dynamic correction. This paper will provide an overview of the two correction mechanisms, contrast the two approaches, and include measured performance data on scanners employing each mechanism. Keywords: Laser Correction, Spinning Laser, Tracking Laser, Planar Scanner, Planarity Correction
We previously demonstrated that 60 GHz planar near-field antenna measurements without external frequency conversion can provide far-field radiation patterns in good agreement with spherical near-field antenna measurements in spite of the cable flexing and thermal drift effects [P.I.Popa, S.Pivnenko, J.M.Nielsen, O.Breinbjerg, ”60 GHz Antenna Measurement Setup Using a VNA without External Frequency Conversion ”36thAnnual Meeting and Symposium of the Antenna Measurements Techniques Association, 12-17 October, 2014]. In this work we extend the validation of this 60 GHz planar near-field set-up to antenna diagnostics and perform a detailed systematic study of the extreme near-field of a standard gain horn at 60 GHz from planar and spherical near-field measurement data. The magnitude and phase of all three rectangular components of the electric and the magnetic aperture fields are calculated, as is the main component of the Poynting vector showing the power flow over the aperture. While the magnitude of the co-polar electric field may seem the obvious object for antenna diagnostics, we demonstrate that there is much additional information in those additional quantities that combine to give the full picture of the aperture field. The usefulness of the complete information is illustrated with an example where the horn aperture is disturbed by a fault. We compare the results of the planar and spherical near-field measurements to each other and to simulation results.
During the last years, new algorithms, based on time filtering, spatial or modal filtering, have been designed for echo reduction techniques applied to antenna measurements. These algorithms have been used for different applications where the effect of the echoes is important, as far field system, VHF or UHF applications, automotive systems, small antennas, etc. The authors, in previous papers, have analysed the effect of different algorithms: time filtering (fft, non uniform dft or matrix pencial), modal filtering based on Spherical modes (MV-Echo) and spatial filtering based on Integral Equations (Insight) and holographic techniques (fft and dft) to cancel the effect of the reflections. This comparison has been applied to the measurements of a dipole antenna (SD1900) using a StarLab system. It is observed that each of the algorithms is better for different situations, depending on the source of the echo. For instance, time filtering techniques are good for reflections coming from different distances with respect the direct ray, but not so good for close reflections. In addition hey need a large frequency band to work properly. Spatial algorithms can correct the effect of positioners or other structures close to the antenna under test, but they are better for planar near field acquisitions and worse for classical single probe spherical near field where the antenna is rotated and probe is fixed (e.g. roll-over-azimuths systems). Moreover, they require extra information of the AUT geometry. This paper presents first a comparison of each algorithm and then, a combination of time and spatial techniques based on uniform or non-uniform DFT to take advantage of the benefits of each algorithm for different origins of the reflections.
Compensated Compact Range Facilities are the state-of-the-art RF test facilities for spacecraft payload modules and/or antennas. The outstanding features of the compact range technique are the (a) real-time testing capability, (b) easy to use far-field measurement technique, (c) extremely high frequency capability, (d) end-to-end payload testing at multiple test zones due to scanning features, and last but not least the (e) considerable low cross-polar contribution over the full frequency band between 1 - 200 GHz which is one of the important parameters for telecommunication antenna testing. Upcoming spacecraft antennas with single feed per beam configuration and broadband transponder requirements (up to 500 MHz) need rapid test environments for antenna and payload (end-to-end) measurement campaigns. For the desired wide frequency spectrum the Ka-Band and even higher bands (U, and V) are of interest for the next generation of telecommunication spacecraft antennas. Compensated Compact Ranges provide an excellent test environment for such scenarios. Recent developments for the range feeds up to 200 GHz, a new heavy load and highly accurate specimen positioner design, and the easy enlargeable reflector system within the existing chamber complete the picture of a state-of-the-art test facility for present and future spacecraft testing. The paper will explain the advantages of the selected system design and preferred technology with its resulting features to optimally cover the future requests focusing to new developments in the high frequency range. For typical spacecraft antenna scenarios a comparison between Compact Range and Near-Field facilities will demonstrate the applicability in the frequency range from 1 to 200 GHz. Beside the developed test set-up for the required measurement parameters, typical measurement times and achievable performance with its related error budget will be depicted.
Antenna measurements at increasing working frequencies carry the difficulty of reliably measuring the signal phase, due to effects of cable bending, thermal drift, etc, and the resulting impedance mismatch which introduces uncertainty in the measurement results. In this paper we investigate the problem of phaseless measurements and phase retrieval for planar near-field measurements, together with the application of probe correction of the retrieved results, to the best of our knowledge the first experimental case of probe correction in phaseless near-field antenna measurements. A phase retrieval method based on an iterative Fourier technique (IFT) is proposed and tested with measurements of a Standard Gain Horn at 60GHz acquired at the planar near-field (PNF) scanner facility at the Technical University of Denmark. The obtained results indicate good agreement with a measured reference pattern within the region of validity when the probe correction is applied after performing the phase retrieval from a pair of uncorrected probe signals. Application of the probe correction before the phase retrieval, on the other hand, shows not satisfactory results. Additional improvements are obtained by introducing spatial filtering at the AUT aperture, thus enhancing performance of the algorithm by reducing phase noise of retrieved fields. Also, a “double-iterated” approach is explored, with additional phase-retrieval iterations after probe correction, with the aim of introducing true electric fields into the IFT.
A multi-probe array (MPA) spherical near-field antenna measurement system, comprised of COTS equipment, has been developed for testing UHF antennas mounted in an aircraft rotodome. The spherical probe radius is 5 meters, which accommodates a 24 ft. diameter rotodome. The probe array, arranged in a circular arc about the test zone center, provides rapid time multiplexed samples of dual polarized spherical theta angle measurements. These measurements are collected at incremental steps of spherical phi angles, provided by a floor azimuth turntable. The rotodome is mounted on the azimuth turntable, and is rotated 360 degrees during a data collection. During one azimuth rotation, completed in a few minutes, a full set of 3D, dual polarized, multi-frequency near-field pattern data is collected. The data is transformed to full 3D far-field patterns in another few minutes, providing a complete rotodome test time within 15 minutes. The entire system is contained within a room 42’ x 42’ x 25’. This paper will describe the test requirements, physical requirements of the DUT, size constraints of the facility, and measurement speed goals. Alternate solutions and range geometries will be discussed, along with why the MPA solution is best given the requirements and size constraints. The system will be described in detail, including discussion of the room design, RF instrumentation, multi-probe array, positioning equipment, and controllers. Measurement results will be presented for test antennas of known pattern characteristics, along with other performance metrics, such as test times.
For today's sophisticated antenna applications, the accurate knowledge of 3D radiation patterns is increasingly important. To measure the antennas under far-field conditions over a broad frequency band is hereby hardly impossible. By near-field to far-field transformation, one can overcome the difficulties of limited measurement distances. In common spherical near-field antenna measurement software, the transformation based on spherical mode expansion is typically implemented. These software tools only provide to correct the influence of first order azimuthal probe modes. The influence of the probe’s higher order modes though increases with shorter measurement distances. To measure a broad frequency range in one measurement set-up and to save time, dual ridged horns are popular candidates since they operate over a wide frequency range. The drawback is that they are probes of higher order. In this contribution, we will present an investigation on near-field measurements which are transformed into the far-field deploying the transformation technique based on spherical modes which is extended by a higher order probe correction capability. The resulting diagrams comparing first and higher order probe correction show that a correction is important in particular for the cross polarization In addition, the near-field data is transformed with an algorithm which employs a representation by equivalent currents. In this method, a higher order probe correction based just on the probe’s far-field pattern is integrated. The equivalent currents supported by an arbitrary Huygens surface allows to reconstruct the current densities close to the actual shape of the AUT which is mandatory for precise antenna diagnostics. Another issue needs to be accounted for regarding limited measurement distances and spherical modal expansion. While representing the AUT and the probe in spherical modes the radii of the spheres grow the more modes are included which depends on the sizes of the TX and the RX antennas. It has to be ensured that both spheres do not interfere. All measurements were carried out in the anechoic chamber of our laboratory in which measurements starting at 1 GHz are practicable according to the dimension of the chamber and of the absorbers. Due to our restricted measurement distance of 0.57 m, all the above mentioned rules need to be considered. In conclusion, small anechoic chambers are also capable of delivering precise antenna measurements over a broad frequency range due to algorithms capable of higher order probe correction.
The spherical multipole based near-field far-field transformation is one of the most widespread algorithms for field transformation due to its very low computation time achieved by employing the fast Fourier transform (FFT) and imposing the utilization of first order probe antennas which obtain regularly distributed near-field samples on a spherical surface. Thus, huge efforts in highly accurate scanner system and antenna design are invested to fulfill the transformation algorithm requirements. In comparison, the recently developed inverse source reconstruction methods are very undemanding as they allow to use arbitrary probe antennas and arbitrarily shaped measurement surfaces as long as the probe’s relative position and orientation with respect to the device under test (DUT) is accurately known. Furthermore, the diagnostics capabilities of the algorithms give insight into the radiation mechanisms of the antenna. Although multilevel fast multipole boosted inverse source reconstruction algorithms such as the fast irregular antenna field transformation algorithm (FIAFTA) provide an excellent linearithmic complexity, their computation time is still higher than the one of the spherical transformation. The flexibility to process near-field samples on an irregular grid is yet only of interest for some challenging measurement scenarios where it is easier to determine the exact position and orientation of the probe than to accurately position it at certain grid points. Moreover, most antenna measurement facilities are already equipped with positioner systems for spherical scans. Therefore, a spherical multipole based transformation with higher order probe correction capability is proposed to perform a fast near-field far-field transformation. Once the far-fields and thus, the plane wave representation of the antenna has been obtained, a hierarchical plane wave representation is utilized to efficiently determine the equivalent sources of the antenna. For best sources localization and diagnostic features, equivalent surface currents on a Huygens’ surface enclosing the antenna are used. Their organization in a hierarchical octree is the key to a fast transformation from the antenna far-field to its equivalent sources. In this way, the blend of the spherical multipole based transformation and the hierarchical plane wave based field representation allows to profit from the benefits of both transformation approaches.
For satellite applications payload measurements are a crucial part of the radio frequency validation campaign before the launch. Parameters like Equivalent Isotropic Radiated Power (EIRP), Input Power Flux Density (IPFD), Gain over Noise Temperature (G/T), Gain over Frequency (G/F), Group Delay, and Passive Intermodulation (PIM) are to be measured in suitable facilities on satellite level. State-of-the-art payload measurements are conducted in compensated compact range facilities which offer a real-time test capability which is easy to setup and use. Closed link tests are straightforward to realize with two compact range feeds employing feed scanning. The measurement techniques as well as the error budgets are well known. Near-field facilities are widely used for antenna pattern measurements. However, there is not much literature available discussing in particular measurements of G/T, G/F, and Group Delay in the near field. Measurements of the above parameters in the near field seem to be feasible, however, the processing of the measured data has to be adapted and further calibration measurements are required. In this paper methodologies for payload parameter measurements in compact range and near field facilities will be described. A comparison of payload measurement campaigns in near field and compact range facilities will be drawn. The techniques will be compared in terms of measurement timing and effort, practicability for satellite applications, and achievable accuracies.
In this paper an advanced analysis regarding the interaction between antennas installed on a spacecraft is presented. In particular, data coming from a GNSS satellite near field measurement campaign have been considered and the MV-INSIGHT software has been used to perform the analysis. Such a software, starting from the measured field of a DUT, computes equivalent currents on a surface conformal to the test object. The availability of the equivalent currents is a key point for an in depth analysis of DUT such as a spacecrafts since it allows to obtain exclusive diagnostic information like coupling between antennas and satellite structure. The near-field data have been collected in the Hybrid ESA RF and antenna Test Zone (HERTZ) at ESTEC. Such a hybrid NF/FF system has recently been installed in the existing dual reflector CPTR. The installed system has been designed to perform spherical, cylindrical and planar NF measurements in the broad frequencies range 0.4-50 GHz.
The source reconstruction or equivalents source method provides an accurate near-field representation of any radiating device in terms of equivalent electric and magnetic currents. The equivalent currents can be determined from measured near or far field data through a post-processing step involving the solution of an integral equation. The currents constitutes an accurate 3D electromagnetic model, maintaining near and far field properties of the measured device. A newly created link, enable the export of the model to a number of commercial computational electromagnetic (CEM) solvers in the form of a near-field Huygens box. Of special interest to the EMC community, equivalent current representation of measured devices are directly applicable in diagnostics/hot-spot finding and in the determination of radiated emission at any distance. The Huygens box, derived from measurements, is applicable in the simulation of emission in different scenarios when the device is in vicinity of different objects such as shielding, cables etc. This papers shows examples of diagnostics and emission analysis of a representative printed circuit board (PCB) based on commercially available near field measurement systems, post-processing and CEM tools.