UCLA's Millimeter-Wave Bi-polar Planar Antenna Measurement System: A Novel Portable Design
As new antenna designs reach higher frequencies and smaller sizes, traditional large scale antenna chamber systems become ill-suited for measurement. External mixing, room-sized chambers, and expensive test equipment add large costs and burden to antenna measurement systems. A smaller, more cost effective system is proposed. Using the bipolar planar scanning technique developed at UCLA, a portable and movable millimeter-wave antenna chamber is currently under development. The chamber is being designed to fit on the end of a standard optical table and enjoys the space-saving and accuracy inherent to the bipolar planar configuration. Simple construction of the chamber will allow relatively easy assembly and disassembly and allow movement of the chamber from one table to another, if needed. Antenna of diameters up to 40cm can be accommodated and scan planes of up to ~160cm can be measured. Millimeter-wave frequencies from around 30GHz to 67GHz can be measured. Antennas measured will use planar near-field to far-field techniques. In particular, the post-process will follow the OSI/FFT method and will incorporate the phase retrieval techniques developed for the bipolar configuration. These phase-less measurements will allow the use of scalar millimeter-wave test equipment with much lower cost than comparable vector test equipment.
Extracting the Polarization from Bi-polar Phaseless Nearâ€“Field Measurements
The polarization extraction in the phaseless near-field measurement is investigated. Sensing the antenna polarization based on the implementation of phase-retrieval methods like IFT (Iterative Fourier Technique) will not result to a unique solution. It is shown how a single extra point measurement can provide the complete vectorial representation of the field in a two-component representation. This means for the first time by the application of phaseless methods, one not only can get an understanding of the dominant polarization of the antenna in terms of linearity, ellipticity or circularity but also the true representation of the co- and cross polarized components in the far-field based on any definition (like Ludwig’s definitions). The applicability of the method is shown through a near-field measurement of a right-hand elliptically polarized antenna array in UCLA bi-polar near-field facility.
Conventional compact ranges use a reflector antenna’s near field to produce the plane wave illumination needed to measure a second antenna under test (AUT). The quasi-compact range described here uses a conventional reflector antenna at a greater range separation than conventional compact ranges, but still within the reflector’s near field. Its illumination allows the antenna evaluations at smaller range separations than the AUT’s far-field distance and allows modification of a current far-field range with a reflector range antenna to measure larger test articles than normally acceptable. This approach preserves many advantages of a standard compact range including reduced multipath and high measurement sensitivity that result from the collimated near field of the illuminating reflector antenna. Additionally, a conventional reflector antenna is used without requiring edge treatments. Experience with a four-foot prime focus parabola operating at 18 GHz illustrates this technique. The measured quiet zone fields compare favorably with calculated values using the GRASP codes. Likewise, measurements of a 20”-diameter offset reflector antenna compare favorably with GRASP results.
Coordinate System Plotting For Antenna Measurements
Antenna measurement data is collected over a surface as a function of position relative to the antenna. The data collection coordinate system directly affects how data is mapped to the surface: planar, cylindrical, spherical or other types. Far-field measurements are usually mapped or converted to spherical surfaces from which directivity, polarization and patterns are calculated and projected. Often the collected coordinate system is not the same as the final-mapped system, requiring special formulas for proper conversion. In addition, projecting this data in two and three-dimensional polar or rectangular plots presents other problems in interpreting data. This paper presents many of the most commonly encountered coordinate system formulas and shows how their mapping directly affects the interpretation of pattern and polarization data in an easily recognizable way.
Mission to MARS - In Search of Antenna Pattern Craters
Reflections in anechoic chambers can limit the performance and can often dominate all other error sources. NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been demonstrated to be a useful tool in the fight against unwanted reflections. MARS is a post-processing technique that involves analysis of the measured data and a special mode filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical near field or far-field range. It has also been applied to extend the useful frequency range of microwave absorber down to lower frequencies. This paper will show typical improvements in pattern performance, and will show results of the MARS technique using data measured on numerous antennas.
Cross Polarization Uncertainty in Near-Field Probe Correction
The probe correction of near-field measured data can be considered as being composed of two parts. The first part is a pattern correction that corrects for the effects of the aperture size and shape of the probe and can be analyzed in terms of the far-field main component pattern of the probe. The second part is due to the non-ideal polarization properties of the probe. If the probe responded to only one vector component of the incident field in all directions, this correction would be unnecessary. But since all probes have some response to each of two orthogonal components, the polarization correction must be included. The polarization correction will be the focus of the following discussion. Previous studies have derived and tested general equations to analyze polarization uncertainty12. This paper simplifies these equations for easier application. The results of analysis and measurements for Planar, Cylindrical and Spherical near-field measurements will be summarized in a form that is general, easily applied and useful. Equations and graphs will be presented that can be used to estimate the uncertainty in the polarization correction for different AUT/Probe polarization combinations and measurement geometries. The planar case will be considered first where the concepts are derived from the probe correction theory and computer simulation and then extended to the other measurement geometries.
DATA REDUCTION IN THE NF-FF TRANSFORMATION TECHNIQUE WITH SPHERICAL SPIRAL SCANNING
An effective near-field – far-field transformation technique with spherical spiral scanning tailored for antennas having two dimensions very different from the third one is here proposed. To this end, an antenna with one or two predominant dimensions (as, e.g., an elongated or quasi-planar antenna) is no longer considered as enclosed in a sphere, but in a prolate or oblate ellipsoid, respectively, thus allowing one to remarkably reduce the number of required data. Moreover these source modellings remain quite general and contain the spherical one as particular case. Numerical tests are reported for demonstrating the accuracy of the far-field reconstruction process and its stability with respect to random errors affecting the data.
Microwave holography as diagnostic tool: an application to the GALILEO Navigation antenna
This paper describes an application of well known microwave holography to the practical case of the space antenna for the European Navigation System GALILEO. The antenna consists in an array of 45 patch elements, divided into six sectors, fed by a two level beam forming network. In fact, the procedure described in this paper has been used in the frame of the development of the GALILEO Navigation antenna to identify element feeding errors. A planar hologram on the aperture plane of the array has been obtained by a set of spherical near field measurements. Sampling the resulting aperture field distribution (in amplitude and phase) allowed reconstructing the excitation law and identifying errors. The developed procedure was validated with a number of test cases assessing numerical errors introduced by the process. Applying the back-projection to the measured far-field led to discover that some sectors of the array were overfed and that errors were present in the central power divider responsible of the first power distribution in the antenna. A new power divider was then manufactured and integrated into the array leading to a well performing antenna.
NEAR-FIELD TO QUASI-FAR-FIELD TRANSFORM THROUGH PARALLAX
Practical ISAR measurements must often be made in the near-field. Scatterers are illuminated by a spherical wavefront, generating a continuum of incident angles due to parallax. Ignoring this, radar image processing produces geometrically distorted images whose utility diminishes the more deeply into the near-field the measurements are made. The underlying assumption that a target may be accurately modeled as a collection of isotropic point scatterers can enormously widen in angle. Yet, by considering parallax (with attention to phase), near-field measurements can produce quasi-far-field images, whose Fourier transform bears a greater likeness to a far-field RCS signature. A technique is presented and explored whereby each image pixel is focused at angles normal to the incident spherical wavefront by compensating for parallax. The focused coordinates are spatially variant, but for a pixel exactly containing a point scatterer, the resulting focused IQ pairs are identical with those in the far-field.
Advances of the Source Reconstruction Method and its applications for the diagnostics of antennas of arbitrary geometry
An Integral Equation-based method for Near-to-Far Field Transformation method and antenna diagnostics is presented. This technique, called the Sources Reconstruction Method (SRM) makes use of the Equivalence Principle jointly to the Integral Equations in order to find an equivalent problem so that the fields radiated by the original problem and by the equivalent one are the same. While most of the antenna diagnostics techniques limit their application to canonical geometries (planar, cylindrical, spherical), the SRM extends the diagnostics capabilities to arbitrary geometries. Thus, if the surface where the equivalent electromagnetic currents are reconstructed fits the Antenna-Under-Test (AUT) geometry it is possible to diagnose the fields and currents distribution over the AUT surface. This generalization for arbitrary geometries increases the SRM computational cost if compared to other diagnostics methods. The paper describes the latest SRM improvements, which are mostly related to the computational cost reduction by means of the Fast Multipole Method (FMM). Examples showing the SRM capabilities for antenna diagnostics are included.
Focal-Plane Scanner for Measuring Radome Beam Deflection in a Compact Range
Measurement of radome beam deflection and/or Boresight shift in a compact range generally requires a complicated set of positioner axes. One set of axes usually moves the radome about its system antenna while the system antenna remains aligned close to the range axis. Another set of axes is normally required to scan the system antenna through its main beam (or track the monopulse null) in each plane so the beam pointing angle can be determined. The fidelity required for the beam pointing angle, combined with the limited space inside the radome, usually make this antenna positioner difficult and expensive to build. With a far-field range, a common approach to the measurement of beam deflection or Boresight shift uses a down-range X-Y scanner under the range antenna. By translating the range antenna, the incident field's angle of arrival is changed slightly. Because the X-Y position errors are approximately divided by the range length to yield errors in angle of arrival, the fidelity required of the X-Y scanner is not nearly as difficult to achieve as that of a gimbal positioner for the system antenna. This paper discusses a compact-range positioner geometry that approximates the simplicity of the down-range-scanner approach commonly used on far-field radome ranges. The compact-range feed is mounted on a small X-Y scanner so that the feed aperture moves in a plane containing the reflector's focal point. Translation in this 'focal plane' has an effect very similar to the X-Y translation on a far-field range, altering the direction of arrival of the incident plane wave. Measured and modeled data are both presented.
Characterization of measurement systems through extensive measurement campaigns
Within the European Union network "Antenna Center of Excellence" – ACE (2004-2007), a first intercomparison campaign among different European measurement systems, using the 12 GHz Validation Standard (VAST12) antenna, were carried out during 2004 and 2005. One of the challenges of that campaign was the definition of the accurate reference pattern. This was the reason why a dedicated measurement campaign for definition of the accurate reference pattern was hold during 2007 and beginning of 2008. This second campaign is described in the companion paper “Dedicated measurement campaign for definition of accurate reference pattern of the VAST12 antenna”. This dedicated measurement campaign was performed by Technical University of Denmark (DTU) in Denmark, SAAB Microwave Systems (SAAB) in Sweden and Technical University of Madrid (UPM) in Spain. This campaign consisted of a large number of measurements with slightly different configurations in each of the three institutions (2 spherical near field systems and one compact range). The purpose of this paper is to show the process to achieve the reference pattern from each institution and the evaluation of the accuracy. The acquisitions were performed systematically varying in applied scanning scheme, measurement distances, signal level and so on. The results are analyzed by each institution combining the measurement results in near or far field and extracting from these measurements: a “best” pattern, an evaluation of possible sources of errors (i.e. reflections, mechanical and electrical uncertainties) and an estimation of the items of the uncertainty budget.
EVALUATION OF ISOFILTER™ FIDELITY IN SELECTED APPLICATIONS
The IsoFilterTM technique was originally demonstrated to operate by rejecting secondary signals that derive from reflections off of a nearby metallic object – namely, the ground plane surface supporting a small pyramidal horn.[1,2] The aperture of the horn was located several wavelengths above the ground plane and the sidelobes and backlobes of the horn illuminated the ground plane itself. The success of this demonstration has been sufficient to encourage us to pursue further the question of how well the IsoFilterTM technique will work to suppress other types of secondary signals– such as signals coming from other elements of an array antenna or another individual first-order primary radiator nearby. Here we report on some of the results of that investigation. We have calculated the far-field patterns of a sparsely populated array and applied the IsoFilterTM technique. The goodness of the suppression is judged by how well the “IsoFiltered” result agrees with the calculated pattern of the individual radiator.
CONICAL NEAR-FIELD ANTENNA MEASUREMENTS
A near-field measurement technique for the prediction of asymptotic far-field antenna patterns from data obtained from a modified cylindrical, or plane-polar, near-field measurement system is presented. This technique utilises a simple change in facility alignment to enable near-field data to be taken over the surface of a conceptual right cone [1, 2], or right conic frustum [3, 4] thereby allowing existing facilities to characterise wide-angle antenna performance in situations where hitherto they would perhaps have been limited by truncation. This paper aims to introduce the measurement technique, describe the novel probe-corrected near-field to far-field transform algorithm which is based upon a cylindrical mode expansion of the measured fields before presenting preliminary results of both computational electromagnetic simulations and actual range measurements. As this paper recounts the progress of ongoing research, it concludes with a discussion of the remaining outstanding issues and presents an overview of the planned future work.
EXPERIMENTAL VALIDATION OF THE NF-FF TRANSFORMATION WITH HELICOIDAL SCANNING SUITABLE FOR ELONGATED ANTENNAS
In this work an experimental validation of the nearfield – far-field transformation technique with helicoidal scanning tailored for elongated antennas is provided.
Such a transformation relies on the theoretical results relevant to the nonredundant sampling representations of the electromagnetic fields and makes use of an optimal sampling interpolation algorithm, which allows the reconstruction of the near-field data needed by the near-field – far-field transformation with cylindrical scan. In such a case, a prolate ellipsoid is employed to model an elongated antenna, instead of the sphere adopted in the previous approach. It is so possible to consider measurement cylinders with a diameter smaller than the source height, thus reducing the error related to the truncation of the scanning surface.
The comparison of the reconstructions obtained from the data directly measured on the classical cylindrical grid with those recovered from the nonredundant measurements on the helix assesses the validity of this innovative scanning technique.
On the Robustness of Planar Phaseless Near-field Measurements to Probe Positioning Errors
The Phaseless techniques have gained considerable attention during the past two decades in the antenna measurements community. The removal of the phase measurements has some immediate advantages over the common vectorial measurements. They are cost effective, well-adapted for higher frequencies and insensitive to phase instabilities. The phaseless techniques have been discussed in the antenna measurements community and the theories behind these techniques are well explained in the literature. Unfortunately the issue of the noise and the presence of measurement errors are not investigated in details to provide strong impetus to the importance of phaseless measurements. In this paper the near field of a number of different types of antennas with high, medium and low side lobes is simulated to create as realistic case as possible. The effects of the probe positioning errors are investigated by injecting random errors in the position of the probe samples along x-, y- and z-axis. It is also illustrated how the positioning errors can distort the phase distributions. Through detailed characterizations of the constructed far field patterns, robustness of the Iterative Fourier technique even at the presence of very high probe positioning errors is demonstrated. It is shown how the utilization of the phaseless techniques will significantly reduce the probe positioning error effects when it is compared to the commonly used amplitude and phase near field measurement techniques.
USE OF VECTOR ISOMETRIC ROTATIONS IN THE MEASUREMENT OF IMPERFECTLY ALIGNED ANTENNAS
Most traditional antenna measurement techniques presume that the antenna under test (AUT) is accurately aligned to the mechanical axes of the test range. Sometimes, however, it is not possible to achieve such a careful antenna alignment . In these cases, standard post processing techniques can be used to accurately correct antenna-to-range misalignment. Alternatively, similar results may be obtained by approximation in the form of piecewise polynomial interpolation. When carefully employed, this method will result in only a small increase in uncertainty, but with a significant reduction in computational effort. This paper describes this far-field alignment correction method, which is closely related to standard active alignment correction methods . This paper then proceeds to use numerical simulation as well as actual range measurements to demonstrate the effectiveness of this method. Finally, the utility of this technique in the presentation of far-field antenna pattern functions is illustrated.
Band Rejection and Feeder Effects on the Far-field Purity of Log-Periodic Antennas
Full-wave modeling and far-field measurements are utilized to study the effects of a microstrip feeder on the band rejection and the far-field purity of planar log-periodic antennas. Three different configurations are investigated. Specifically, band rejection by relevant teeth removal (near/far-field), integration of the band-stop filter (near-field only), and the combination of the two are studied. Far-field contamination effects due to a microstrip feed line, and coupling to the antenna radiator, are evaluated for both radiating and band rejection regions. Important guidelines regarding the position and distancing of the feed to the radiator, as well as the trade-offs between substrate and superstrate configurations are derived. Antennas are developed to have a VSWR better than 2.5:1 in the 2-4 GHz and 7-11 GHz bands, and band rejection centered at 6 GHz. It is clearly shown that log-periodic antennas can be readily designed to have arbitrary, even reconfigurable, band rejection regions where overall realized gain is notched for more than 20 dB. A computer aided analysis was performed using commercial finite element and method of moments software tools. The measurements were conducted at Lockheed Martin in Denver, Colorado.
HOW RANGE LENGTH AFFECTS THE MEASUREMENT OF TRP
Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) are the two metrics most commonly used to characterize the over the air (OTA) performance of a wireless device. The minimum range length for these measurements has usually been determined using the arbitrary far-field criteria of R>2D2/?. This paper quantifies the changes in measured TRP as the range length is increased from D2/? to infinity (or thereabouts). TRP measurements on a UMTS dipole combined with a phantom head have been made at different range lengths. Additionally, numerical simulations of TRP measurements on an array of point sources have been made at different range lengths. The result is a theoretical determination of TRP measurement errors versus range length supported by actual measurement results.
Uncertainty evaluation through simulations of virtual acquisitions modified with mechanical and electrical errors in a cylindrical near-field antenna measurement system
An error simulator based on virtual cylindrical near-field acquisitions has been implemented in order to evaluate how mechanical or electrical inaccuracies may affect the antenna parameters. In outdoor ranges, where the uncertainty could be rather important due to the weather conditions, an uncertainty analysis a priori based on simulations is an effective way to characterize measurement accuracy. The tool implemented includes the modelling of the Antenna Under Test (AUT) and the probe and the cylindrical near-to-far-field transformation. Thus, by comparing the results achieved considering an infinite far-field and the ones obtained while adding mechanical and electrical errors, the deviations produced can be estimated. As a result, through virtual simulations, it is possible to determine if the measurement accuracy requirements can be satisfied or not and the effect of the errors on the measurement outcomes can be checked. Several types of results were evaluated for different antenna sizes, which allowed determining the effect of the errors and uncertainties in the measurement for the antennas under study.