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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
Driven by the mobile data communications needs of market broadband antennas at the upper frequency bands are already state-of-the-art, e.g. at the Ka-Band. For the characterization of an antenna the antenna gain is one of the major test parameters. This measurement task is already challenging for standard applications at the Ka-Band. However, for the calibration of remote station antennas utilized in high precision test facilities, e.g. the compact range, even higher measurement accuracies are typically required in order to fulfil the overall system performance within the later test facility. Therefore the requirement for this investigation is to improve the measurement set-up and also the steps to get a failure budget which is better than ± 0.15 dB. Every antenna gain measurement technique is affected by required changes in the measurement setup, e.g. the Device under Test (DUT) or the remote station, respectively. This results for example in a variation of mismatch with resulting measurement errors. To determine and compensate the occurred mismatches, the scattering parameters of the involved components have to be measured and be evaluated with a corresponding correction formula. To quantify the effect for the gain measurement accuracy the remaining uncertainty of the mismatch correction values is examined. Another distortion is caused by multiple reflections between the antenna apertures. To reduce this error source, four additional measurements each with a decreased free space distance should be performed. In addition to the common methods, this paper explains in detail an advanced error correction method by using the singular value decomposition (SVD) and compares this to the standard mean value approach. Finally the restricted distance between both antennas within the applied anechoic far-field test chamber has to be analysed very critically and optionally corrected for the far-field gain at an infinite distance in case the measurement distance is fulfilling the minimum distance requirement, only. The paper will discuss all major error contributions addressed above, show correction approaches and verify these algorithms with exemplary gain measurements in comparison to the expected figures.
When performing far field or near field antenna measurements on large antennas, it is often necessary to have various types of mechanical positioning systems to achieve the required hemispheric scans. Measurement systems employing a single-arm gantry, a dual-arm gantry, a fixed arch moving probe, or a fixed arch multi-probe have been paired with either an azimuth positioner or a vehicle turntable to provide hemispheric scanning of the object being tested. This paper will highlight the key characteristics of various scanning methods making comparisons between the different techniques. Positioning and system accuracy, speed, stowing ability, calibration, frequency range, upgradability, relative cost and other key aspects of the various techniques will be discussed in detail to help the end user during the system design and selection process. In addition, the paper will highlight novel hemispheric and truncated spherical scanning approaches. In many applications, the success of the entire project often centers on the judicious selection of the positioning subsystem. This paper will provide guidance toward making the proper selection of the scanning concept as well as of the positioning system.
A three-step equiangular (120º) phase shifting holography (EPSH) technique is proposed for THz antenna near-field/far-field prediction. The method is attractive from the viewpoint of receiver sensitivity, phase accuracy over the entire complex plane, simplified detector array architecture, as well as reducing planarity requirements of the near-field scanner. Numerical modeling is presented for the holographic receiver performance, using expected phase shift calibrations errors and phase shift noise. The receiver model incorporates responsivity and thermal noise specifications of a commercial Schottky diode detector. Additionally, simulated near-field patterns at 372GHz demonstrate the convenience of the method for accurate and high dynamic range THz near-field/far-field predictions, using a phase-shifter calibrated to ±0.1°.
Legacy methods for testing the performance of radome panels and finished radomes have always been in isolation from the system antenna, for many reasons. The legacy method of testing employed horn antennas at relatively close distances, a fixed-frequency signal source, and primitive receivers. More modern systems used much better receivers capable of measuring both phase and amplitude, and these gave way to automatic network analyzers. The network analyzer system also replaces the fixed-frequency source, because it has its own step-frequency source. The rest of the setup remains the same. A network analyzer can itself be calibrated, but that calibration cannot include the probe antennas, nor can it account for interactions, particularly at normal incidence. With increasing demands on performance, it is essential that the interaction effects of the probe antennas with the radome be removed. The micorwave integrated circuit industry has the identical problem. The circuit probes that are used to reach into the circuit assemblies have very small tips, and the internal elements to accomplish this size reduction make probe mataching difficult. Thus the probe parameters become embedded into the overall measured response. The circuit testing community has developed a process to de-embed these probes, yielding the S-parameters of the circuit under test in isolation from surroundings. This paper investigates a method for applying this closed-system technique to open-system testing, such as panel-measuremsnt tables, by using a secondary calibration technique that is adapted to open systems. This effectively extends the calibration of the analyzer system to encompass the probes, thus improving accuracy.
Broad band, dual polarized probes are becoming increasingly popular options for use in near-field antenna measurements. These probes allow one to reduce cost and setup time by replacing several narrowband probes like open-ended waveguides (OEWG) with a single device covering multiple waveguide bands. These probes are also ideal for production environments, where chamber throughput should be maximized. Unfortunately, these broadband probes have some disadvantages that must be quantified and corrected for in order to make them viable for high accuracy near-field measurements. Most of these broadband probes do not have low cross polarization levels across their full operating bandwidths and may also have undesirable artifacts in the main component of their patterns at some frequencies. Both of these factors will result in measurement errors when used as probes. Furthermore, the use of a dual port RF switch adds an additional level of uncertainty in the form of port-to-port channel balance errors that must be accounted for. This paper will describe procedures to calibrate the pattern and polarization properties of broad band, dual polarized probes with an emphasis on a newly developed polarization correction algorithm. A simple procedure to measure and correct for amplitude and phase imbalance entering the two ports of the near-field probe will also be presented. Measured results of the three calibration procedures (pattern, polarization, channel balance) will be presented for a dual-polarized, broad band quad-ridged horn antenna. Once calibrated, this probe was used to measure a standard gain horn (SGH) and will be compared to baseline measurements acquired using a good polarization standard open-ended waveguide (OEWG). Results with and without the various calibration algorithms will illustrate the advantage to using all three routines to yield high accuracy far-field pattern data.
In classical probe-corrected spherical near-field measurements, one source of measurement errors, not often given sufficient consideration is the probe [1-3]. Standard near-field to far-field (NFFF) transformation software applies probe correction with the assumption that the probe pattern behaves with a µ=±1 azimuthal dependence. In reality, any physically-realizable probe is just an approximation to this ideal case. Probe excitation errors, finite manufacturing tolerances, and probe interaction with the mounting interface and absorbers are examples of errors that can lead to presence of higher-order spherical modes in the probe pattern [4-5]. This in turn leads to errors in the measurements. Although probe correction techniques for higher-order probes are feasible [6], they are highly demanding in terms of implementation complexity as well as in terms of calibration and post-processing time. Thus, probes with high azimuthal mode purity are generally preferred. Dual polarized probes for modern high-accuracy measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port isolation. As a desired feature of modern probes the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign. Consequently, the probe design is a trade-off between performance requirements and usable bandwidth. High performance, dual polarized probe rely on balanced feeding in the orthomode junction (OMJ) to achieve good performance on a wide, more than octave, bandwidth [5-7]. Excitation errors of the balanced feeding must be minimized to reduce the excitation of higher order spherical modes. Balanced feeding on a wide bandwidth has been mainly realized with external feeding network and the finite accuracy of the external components constitutes the upper limits on the achievable performance. In this paper, a new OMJ designed entirely in waveguide and capable of covering more than an octave bandwidth will be presented. The excitation purity of the balanced feeding is limited only by the manufacturing accuracy of the waveguide. The paper presents the waveguide based OMJ concept including probe design covering the bandwidth from 18-40GHz using a single and dual apertures. The experimental validation is completed with measurements on the dual aperture probe in the DTU-ESA Spherical Near-Field facility in Denmark. References: [1]Standard Test Procedures for Antennas, IEEE Std.149-1979 [2]Recommended Practice for Near-Field Antenna Measurements, IEEE 1720-2012 [3]J. E. Hansen (ed.), Spherical Near-Field Antenna Measurements, Peter Peregrinus Ltd., on behalf of IEE, London, UK, 1988 [4]L. J. Foged, A. Giacomini, R. Morbidini, J. Estrada, S. Pivnenko, “Design and experimental verification of Ka-band Near Field probe based on wideband OMJ with minimum higher order spherical mode content”, 34th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2012, Seattle, Washington, USA [5]L. J. Foged, A. Giacomini, R. Morbidini, “Probe performance limitation due to excitation errors in external beam forming network”, 33rd Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2011, Englewood, Colorado, USA [6]T. Laitinen, S. Pivnenko, J. M. Nielsen, and O. Breinbjerg, “Theory and practice of the FFT/matrix inversion technique for probe-corrected spherical near- eld antenna measurements with high-order probes,” IEEE Trans. Antennas Propag., vol. 58, no. 8, pp. 2623–2631, Aug. 2010. [7]L. J. Foged, A. Giacomini, R. Morbidini, "Wideband dual polarised open-ended waveguide probe", AMTA 2010 Symposium, October, Atlanta, Georgia, USA. [8]L. J. Foged, A. Giacomini, R. Morbidini, “ “Wideband Field Probes for Advanced Measurement Applications”, IEEE COMCAS 2011, 3rd International Conference on Microwaves, Communications, Antennas and Electronic Systems, Tel-Aviv, Israel, November 7-9, 2011.
The US NEXRAD weather surveillance Doppler radar (WSR-88D) was recently upgraded to polarimetric capability. This upgrade permits identification of precipitation characteristics and type, thus providing the potential to significantly enhance the accuracy of radar estimated rainfall, or water equivalent in the case of frozen hydrometeors. However, optimal benefits are only achieved if errors induced by the radar hardware are properly accounted for through calibration. Hardware calibration is a critical element in delivering accurate meteorological information to the forecast and warning community. The calibration process must precisely measure the gain of the antenna, the Polarimetric bias of the antenna, and the overall gain and bias of the receive path. The absolute power measurement must be accurate to within 1 dB and the bias between the Polarimetric channels must be known to within 0.1 dB. These requirements drive a need for precise measurement of antenna characteristics. Engineers and scientists with the NEXRAD program employ solar scanning techniques to ascertain the absolute gain and bias of the 8.53 m parabolic center fed reflector antenna enclosed within a radome. They are also implementing use of daily serendipitous interference strobes from the sun to monitor system calibration. The sun is also used to adjust antenna gain and pedestal pointing accuracy. This paper reviews the methods in place and under development and identifies some of the challenges in achieving the necessary calibration accuracies.
Within RF measurement systems, engineers commonly wish to know how much phase ripple will be present in a signal based on a given signal-to-noise ratio (SNR). In a past AMTA paper (Measurement Considerations for Antenna Pattern Accuracy, AMTA 1997), John Swanstrom presented an equation which demonstrated how the bound on the phase error could be calculated from the peak SNR value. However, it can be shown that the Swanstrom bound is broken when the signal has a peak SNR value of less than approximately 15 dB. This paper introduces a new equation that bounds the maximum phase error of a signal based on the signal’s peak SNR value. The derivation of this new bound is presented, and comparisons are made between the old Swanstrom bound and the new bound. In addition, the inverse relationship (i.e., calculating the SNR value of a signal from phase-only measurements) is investigated. In the past, analytical equations for this relationship have been presented by authors such as Robert Dybdal (Coherent RF Error Statistics in IEEE Trans. on Microwave Theory and Techniques) and Jim P.Y. Lee (I/Q Demodulation of Radar Signals with Calibration and Filtering in a Defense Research Establishment Ottawa publication). The analytical equations for calculating the SNR value using phase-only measurements are reviewed and discussed, and a brand new numerical relationship based on a polynomial curve fitting technique is proposed.
In array signal processing; i.e. direction of arrival estimation, digital beam forming/ nulling one needs to know the antenna array manifold (relative gain and phase of individual antenna elements) as well as the relative response of RF front end of the various channels. Here the RF front end is defined as LNA, filters, various down convertors and A/Ds. Since RF front end response is highly dependent on the physical environment, in general, nearly real time calibration of the RF front end is carried out. To accomplish this, a pilot signal is injected in various channels or a strong signal is received from a known direction. The received pilot is isolated, and the isolated signal is processed for relative calibration of the various front ends. One simple processing technique is to divide the signal received at a given frequency by the channel of interest with the signal received by the reference channel at that frequency. This technique works fine when the pilot signal has high SNR (20 dB or so). For low SNR, the front end calibration will have large variance and also bias, which is undesired. One can also cross correlate the signal received by the two channels at the frequency of interest, and normalize the cross correlation with the autocorrelation of the signal received by the reference channel. We will show that for low SNR of the pilot signal, this approach can lead to bias in the estimated relative magnitude. We will also present a novel approach that leads to unbiased estimate with small variance. The approach is based on signal space idea. At the given frequency, we generate a covariance matrix that contains the correlation between the signals received by various channels. Next, the principle eigenvector (corresponding to the largest eigenvalue) of the covariance matrix is calculated. The eigenvector is adjusted such that its element corresponding to the reference channel is unity. Then the other elements of the eigenvector yield the relative response of the front end of various channels. We have applied the suggested approach to real world data with very good results. Some examples are discussed in the paper.
Abstract – This paper introduces a method for calibrating the gain and the frequency dependent phase center locations of Log Periodic Dipole Arrays (LPDAs). The method builds upon the three antenna method, but is conducted over a PEC ground plane in an Open Area Test Site (OATS). Similar to the traditional three antenna method, three pairings of transmission measurements are taken. In each measurement, one antenna is set at a fixed height above the ground plane, while the other antenna is scanned in height over 1 to 4 m heights. Magnitude and phase responses between the two antennas are taken at multiple heights. Measured results are fit to a theoretical model using a complex fit algorithm. From this process, the gain and frequency dependent phase center locations of each antenna can be solved. Measurement data show that it is effective in reducing systematic uncertainties associated with assuming fixed phase center locations. In addition, unlike other calibration methods over a conducting ground plane, no assumptions are made about the antenna patterns. This method provides an accurate, versatile and fast method for calibrating LPDAs from as low as 100 MHz.
In this paper we describe the NGS calibration facility and calibration hardware, and discuss the motivation for providing calibration services. We provide the time-difference, single-difference carrier phase observable models and estimation strategy currently used to generate NGS absolute calibrations. Calibration examples are provided in the accompanying poster presentation.
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Abstract—In order to optimize accuracy of near field measurements, it is required not only to acquire data for two orthogonal polarizations, but the relative amplitude and phase balance between the two channels must also be accurately matched. This can be difficult at millimeter wave frequencies because of the transmission lines and other components involved. ORBIT/FR has explored multiple methods of achieving optimum vertical and horizontal polarization matching and found a very simple solution to achieve acceptable results. Some of the methods investigated included the use of dual-polarized feeds, dual single-polarized feeds mounted adjacently, waveguide rotary joints with a mechanically rotated feed, and a mechanically-rotated feed using a 1.0 mm coaxial-based cable. Interestingly, the mechanically-rotated feed with coaxial cable provided acceptable results on par with or better than the other methods, which moreover results in a very simple implementation in the measurement system. Measured results are presented for the chosen implementation demonstrating the near field data quality is adequate for a variety of antennas.
Abstract— Antenna arrays works at their peak performance when they are well calibrated at the factory. Once they are employed in a real environment, they might be subject to unpredictable disturbances. That’s why recalibration after operational deployment is required but is usually not done due to practical difficulties. In some applications such as Direction Finding (DF), direction of arrival estimation is susceptible to the antenna model errors. However, the evolution of Direction finding antenna, as the strong integration of an antenna array mounted on a vehicle and the use of more efficient antennas tend to increase this type of disturbances. This paper proposes to evaluate the benefit of an in-situ measurement system for detecting and compensating the disturbance of antenna radiation. The influence of permanent scatters on one hand and variables (open door…) on the other hand in the vicinity of antenna array is investigated. We present a quantitative study of a biased calibration using a model combining 3D electromagnetic simulation, a complete receiver model and a MUSIC direction of arrival algorithm characterization. Two antennas arrays with same height are compared: a standard dipole array and an electrically small UWB antenna array.
In the present paper, a non-dedicated mass market GNSS antenna calibration method is discussed, with a special focus on the significant error component due to phase variations of receiving antennas in precise GNSS applications. Different calibration methods are compared from the literature; the indoor (anechoic chamber) calibration has been selected. The algorithm used to compute the mean Phase Center (PC) and its associated Phase Center Variation (PCV) for all angular directions is also described and has been validated on simulated canonical antennas. PC and PCV are then computed when four antennas are placed near the command unit of an unmanned aerial vehicle (UAV), which emulates the final application scenario. The impact of this structure is evaluated thanks to PCV cartographies. Two low-cost COTS antennas have been selected and their PCV maps are compared with regards to their geometry. Lastly, a reproducibility study based on the PCV characterization of ten copies of one of the selected COTS antennas concludes on the robustness of the PCV calibration.
The improved calibration model proposed in this paper is based on the traditional capacitance model which suffers from errors caused by the assumption that the capacitance is independent of frequency and the permittivity of the ambient medium under test. By analyzing the near-zone field of the coaxial opening, we introduce the new near-field capacitance to account for the dependency on the external permittivity. Simulation results show that the calibration error is significant reduced for low and moderate loss medium. And the calibration of the unknown coefficients simply requires the premeasurement of three known material including air, which provides convenience for the real field measurement. Measurement results obtained by a novel wideband in-situ coaxial probe are included to prove the accuracy improvement improved calibration model. by using this
Radar Cross Section (RCS) and Antenna measurement ranges share many common features and are often used for both purposes. Calibration of these dual-purpose ranges is typically done using the substitution method for both RCS and antenna testing, but with separate RCS and antenna standards. RCS standards are typically based on a geometric shape having a well known theoretical value – and corresponding small uncertainty. By contrast, antenna standards typically must be “calibrated” in a separate antenna calibration system to be used as a gain standard, often yielding higher uncertainties. This paper presents an efficient method for transferring an RCS measurement calibration to an antenna measurement range configuration, allowing a range to be used for both purposes with a single calibration. Insight into the best ways to re-configure the instrumentation between RCS and antenna testing is included. Validation measurements from a compact range are included along with an uncertainty analysis of the method.
Many modern antennas are incorporating LNAs into the aperture to maximize system receive performance. G/T (Gain over Temperature) quantifies the performance of these antenna systems. Historically, G/T measurements needed knowledge of absolute effective temperature of multiple noise sources, which is not practical in an anechoic chamber. A Y-factor method is presented which uses a reference antenna system with a known G/T to determine the G/T of the Antenna Under Test (AUT). This paper will review G/T, describe the measurement process, cover calibration of the reference antenna system and discuss error sources and their mitigation.