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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
This paper shows that a consensus value method can be used to compile on-axis gain measurement data that have a large range of values and uncertainties. A variety of methods are used to analyze multiple data sets such as unweighted averages, weighted averages and other statistical means. The appropriate method is usually dependent on characteristics of the data sets such as, the number of data sets, the spread of the data set values and spread of the uncertainty values for each data set. One method determines a consensus value that is calculated using weighted averages of the inverses of the fractional error of each data set. This consensus value method is compared to methods that remove outlying data sets, as well as unweighted averaging. The results of this comparison show that the consensus value method can be used to calculate an acceptable weighted average of data sets that have a large range of values and uncertainties.
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
Achieving a very large quiet zone across a wide frequency band, in a compact range system, requires a physically large reflector with a suitable surface accuracy. The size of the required reflector dictates attention to several important processes, such as how to manufacture the desired surface across a large area and the practicality of transportation and installation. This inevitably leads to the segmentation of the reflector into multiple panels; which must be fabricated, installed, and aligned to each other to conform to the required geometry. Performance predictions must take into account not only the surface accuracy of the individual panels but also their alignment errors. This paper presents the design approach taken on a recent project for a compact range system utilizing a blended rolled-edge reflector that produces a 5 meter quiet zone across a frequency range of 350 MHz to 40 GHz. It discusses the physical segmentation strategy, the fabrication methodology, the intermediate qualification of panels, the panel alignment technique, and the laser-based metrology methodology employed. Performance analysis approach and results will be presented for the geometry as conceived and then for the realized panelized reflector as machined and aligned.
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.
As printed flexible electronics rapidly emerges with the promise of low cost, light weight, rapid manufacturing of electronic circuits with different form factors, there are extensive efforts towards adopting this cutting-edge technology for RF applications. Advantages of printed electronics usually come at the expense of lower performance. For instance, printed conductors using nano-silver inks have much less conductivity compared to bulk silver conductors (4 times less at best). While compromised performance might not seem problematic for many electronic applications, it is extremely important when it comes to RF applications where the high frequency performance of the material needs to be optimized, characterized and taken into consideration in the design and modeling of the component. In this work, an accurate and repeatable method is introduced to characterize parameter-related dielectric properties at RF and microwave frequencies based on additive manufacturing processes. Two novel simple test circuits are introduced, analyzed, and printed to implement the method in extracting dielectric constant and loss tangent of a printed BST(Barium Strontium Titanate)/polymer dielectric. First circuit is a printed filled cylindrical capacitor that covers a very wide frequency range of 0.045-20GHz. Second circuit is a printed filled coplanar waveguide-interdigital capacitor (CPW-IDC) that is limited in bandwidth (0.045-4.5GHz) due to resonance effects, but more flexible and practical for printed electronic methodologies. Both approaches produce a dielectric constant above 25 and loss tangent below 0.05 for all frequency ranges. The approach presented in this work presents an alternative for determining complex dielectric properties of dielectrics which is both cost-effective and efficient in time and effort, since it bypasses orthodox subtractive processes required for developing test circuits for one-probe reflection measurements. A dielectric characterization approach at RF and microwave regime using additive manufacturing is envisioned where test fixtures are printed on demand and desirable information is extracted in a matter of hours rather than weeks, thus saving time, effort, and cost.
As millimeter-wave applications become more widely available technologies, there is a demand to know material properties for design and application purposes. However, many mass produced materials are either not specified at these frequencies or the price materials can be costly. Therefore the easiest method for characterization is by measurement. Traditional methods of this measurement type involve the reflectivity of a fabric sample placed on a flat metallic reference plate. However, this method has some major difficulties at these high frequencies. For example, the surface of the reference plate must be very flat and smooth and must be carefully oriented such that their surface is precisely facing the transmitting and receive and antennas. Furthermore the electrically large size of the reference plate of this setup makes it difficult to measure in far-field and anechoic range time is expensive. Resistive and conductive fabrics have applications such as shielding, anti-static, and radio wave absorption. Radio wave absorption and radar cross section engineering is currently of high interest to the automotive industry for testing newly emerging automotive radar systems. Such fabric measurement has already been utilized to accurately characterize artificial skin for radar mannequins to recreate the backscattering of human targets at 77 GHz. This paper presents a new and convenient method for measuring the reflective properties of conductive and resistive materials at millimeter wave frequencies by wrapping fabrics around a metallic reference cylinder. This new approach to fabric characterization method is able to obtain higher accuracy and repeatability despite the difficulties of measuring at high frequency.
Rapid advances in material fabrication capability, such as 3D printing, have made the realization of engineered complex media (i.e., anisotropic and bianisotropic materials) possible. One of the primary aspects prompting the interest in complex media is the added control over scattered electromagnetic fields due to the increase in the number of constitutive parameters. Isotropic media are characterized by the 2 well-known scalar parameters of permittivity and permeability. However, in general, it requires 18 and 36 parameters to describe anisotropic and bianisotropic media, respectively. Although the increase in parameter space provides more control over electromagnetic response, the penalty to pay is the added complexity in theoretical analysis when compared to isotropic media. One method that has been developed for the analysis of complex media is the six-vector field formalism which casts Maxwell’s equations into matrix form for ease of manipulation. Although this formalism handles fully populated permittivity and permeability tensors, inversion of a block 3x3 (i.e., 6x6) matrix is required which is mathematically intensive and physical insight can be obscured since a cofactor-based inversion is often employed in the solution process. The goal of this work is to develop a scalar potential formulation capable of handling gyrotropic media. Advantages and limitations of the formulation will be discussed and relevant examples will be provided to demonstrate the simplicity and physically-intuitive nature of the technique. Future work involving the use of the scalar potential formulation in the analysis of antenna, guided wave structures and material characterization of complex media will also be discussed to demonstrate the promising aspects of the technique.
An uncertainty budget for Indoor Radar Cross Section (RCS) measurements contains many contributors. Typically, one of the largest contributors comes from the Quiet Zone quality. To quantify the ripple and the tapper in the Compact Range Quiet Zone of the CEA’s indoor facility CAMELIA, a diagnosis method has been implemented, exploiting the radar response of a moving sphere located on a polystyrene mast. This polystyrene mast is fixed on the top of a linear-translating table over an azimuth positioner. The combination of the two axis capabilities allows to locate the PEC sphere in a horizontal plane cut of the quiet test zone volume. The other cuts at different altitudes are performed by changing the height of the polystyrene mast. This method samples the magnitude of the illuminating field at fixed spatial points (controlled by a laser tracking) in the Test Zone to determine the magnitude of the ripple and thus the Quiet Zone. These experimental data are then statistically processed to determine the measurement uncertainty at a given frequency. This paper introduces and analyses the results of a measurement campaign dedicated to the characterization of the Quiet Zone of the CEA’s indoor facility CAMELIA.
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.
Unlike most antenna performance parameters (directivity, beamwidth, and efficiency, e.g.), phase center is not strictly defined and warrants further clarification when used. Put simply, the phase center is the point at which antenna radiation seems to emanate and is determined as the center of a spherical surface of constant phase in the far field. For practical antennas, however, such a point is fictional and can only be established by minimizing the phase variation on a portion of the spherical surface over a smaller angle of interest, generally where the radiation intensity is greatest (e.g. the 3dB beamwidth). Most commonly, the phase center is defined for a two dimensional planar cut parallel to the direction of propagation, for example the E or H plane of a horn. Knowledge of the phase center is particularly critical in the feeds of reflectors or lenses, where it is required to be located at the focal point of the reflecting or refracting structure to maximize aperture efficiency. Due to its electro-mechanical properties the horn antenna has often been used as the feed for the above mentioned configurations. For wideband applications, the stabilization of the phase center over the entire frequency band poses a significant challenge since this point generally tends from the mouth to the throat of a horn as frequency is increased. The design discussed in this paper involves a feed horn operated in conjunction with a Lunenburg Lens for increased directivity and gain over 18-45 GHz bandwidth. A design overview is discussed with the primary focus on phase stabilization considerations. Methods for determining the phase center of the design are also discussed and compared. These include analytical solutions using the aperture current approximation, simulations using method of moments and finite element method from FEKO and HFSS, respectively, as well as measurements taken in the anechoic chamber at the University of Colorado Boulder.
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.
Highly accurate antenna measurements can require precise alignment and positioning of the probe antenna to the antenna under test. The positioning of the antenna during acquisition can involve the movement of several simultaneous axes of motion. This places a global positioning accuracy requirement on the positioning system. To achieve precision in global positioning and alignment, an understanding of dominant error factors such as load induced deflection/resonance, thermal deflection, positioning error sources and mechanical alignment tolerances is essential. This paper focuses on how global accuracy and stability were achieved, addressing these factors, on a recently delivered large far field antenna measurement system. The system involved eight axes of positioning with the ability to position 950 lbs antenna under test 19.5 ft above the chamber floor achieving 0.007 inch and 0.005 degrees positioning accuracy relative to the global range coordinate system. Stability of the probe antenna after motion was within 0.001 inch. Key Words: Global Position Accuracy, Far Field, Position Stability, Simultaneous Motion, Position Error Correction, High Accuracy, Precise Motion
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.
This paper presents a method for determining the phase center of an antenna based on pattern measurements made over multiple frequencies with a two axis spherical positioning system. Mathematical calculations are used to determine the best-fit sphere for the measured phase data. The origin of the sphere is the phase center of the antenna at the frequency of interest. This method provides fast, flexible multi-frequency measurement of an antenna’s phase center. Results validating the proposed method are presented using both simulated data and measured data. In order to determine the accuracy of this method, a dipole is translated a precise distance within the measurement coordinate system. The difference between phase center measurements made before and after the translation gives an indication of the potential accuracy of the measurements. Also, major contributors to phase center measurement uncertainty are discussed with consideration to reducing the overall phase center measurement uncertainty.
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.
We have successfully designed and developed an innovative “CLose-range Antenna Scanner System” (or CLASS) suitable for measuring the far-field radiation pattern of installed antennae at short distances. The system consists of three key components: (1) a uniquely designed lens horn antenna that generates plane waves in close proximity, (2) a mechanical x-y scanner to scan the antenna-under-test, and (3) a customized stitching software to compute the far-field antenna pattern from the measured field information. The developed system has a scan area of 4.6 x 4.6 m, with resolutions of ±0.1mm in both the x and y traverse directions. The scanner structure is designed in a scalable fashion to cater for measurement of antenna installed at various locations (e.g. front and sides) on a platform. The system is capable of measurement from 1 to 18 GHz and generates far-field radiation pattern with a gain accuracy of ±1 dB.
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.