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NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been used to improve performance in anechoic chambers and has been demonstrated over a wide range of frequencies on numerous antenna types. MARS is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical or far-field range or Compact Antenna Test Range (CATR). It has also been applied to extend the useful frequency range of microwave absorber to both lower and higher frequencies than its normal operating band. This paper will demonstrate the use of the MARS capability in evaluating the performance of anechoic chambers used for spherical near-field measurements, as well as in improving chamber performance.
A probe-compensated near-field-to-far-field transform algorithm has been developed that can generate far-field patterns from near-field measurements made on an arbitrary surface. We present the concept, the algorithm, and computer simulated and measured test results for measurements on a conical surface. The prototype conical near-field measurements were made in a planar near-field range on a horn antenna under test (AUT) mounted on an azimuth-over-elevation positioner to produce a conical measurement surface. This system is especially applicable for producing full-hemisphere far-field patterns for antennas mounted on vehicles where other standard measurement systems may not adapt to the profile well, may not provide full-hemisphere coverage, or may require large, mechanically complex systems.
The DGA/CELAR (France) (Centre d'Electronique de l'Armement: French Center for Armament Electronics) is able to measure targets in order to get their RCS (Radar Cross Section). Yet CELAR RCS measurement facilities are not compact bases and therefore the measured field is a near field. This article proposes a solution allowing the transformation of this near field to a far field and this in the three dimensions of space without limiting any dimension with Fraunhöfer criterion. Thanks to this method the RCS of a target is able to be known in any direction of space and moreover the calculation of a three-dimensional ISAR (Inverse Synthetic Aperture Radar) picture is thus possible. At first the theoretic part of our work is presented. Then a fast method in order to calculate the transformation of a near field to a far field by optimising the calculation time thanks to signal processing theory is given. Finally obtained results from simulated bright points are presented.
ABSTRACT A new probe compensated near-field – far-field transformation technique with planar spiral scanning is here proposed. It is tailored for quasi planar antennas, since an oblate ellipsoid instead of a sphere is considered as surface enclosing the antenna under test. Such an ellipsoidal modelling is quite general (containing the spherical one as particular case) and allows one to consider measurement planes at a distance smaller than one half the maximum source size, thus reducing the error related to the truncation of the scanning surface. Moreover, it reduces significantly the number of the needed near-field data when dealing with quasi planar antennas. 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.
Modern day remote sensing spacecraft often feature multiple payloads sharing a common bus (spacecraft platform). RE02 emission testing (1, 2) characterizes the emission signature of a given payload in order to assess electromagnetic compatibility with respect to other payloads (i.e. “victims”) on the bus. Typically, a simple path loss model based on 1/r2 power variance (ref: Friis path loss equation) is used to account for the distance between the emitting and victim payloads using measured amplitudes taken during RE02 measurements. RE02 measurement technique (2) dictates that emissions testing take place at a fixed radial distance of one meter from the radiating instrument. At certain frequencies, however, this measurement takes place in the near field of the emitter. In general, power density amplitudes are greater in the near field than its far field counterpart. This paper investigates any potential error incurred by not accounting for this effect. A simple math model for a common mode radiator is developed to estimate this error and attempt to better understand the field relationships at lower frequencies where the near field predominates.
The Microwaves and Radar Institute regularly performs calibration campaigns for spaceborne synthetic aperture radar (SAR) systems, among which have been X-SAR, SRTM, and ASAR. Tight performance specifications for future spaceborne SAR systems like TerraSAR-X and TanDEM-X demand an absolute radiometric accuracy of better than 1 dB. The relative and absolute radiometric calibration of SAR systems depends on reference point targets (i. e. passive corner reflectors and active transponders), which are deployed on ground, with precisely known radar cross section (RCS). An outdoor far-field RCS measurement facility has been designed and an experimental test range has been implemented in Oberpfaffenhofen to precisely measure the RCS of reference targets used in future X-band SAR calibration campaigns. Special attention has been given to the fact that the active calibration targets should be measured under the most realistic conditions, i. e. utilizing chirp impulses (bandwidth up to 500 MHz, pulse duration of 2 µs for a 300 m test range). Tests have been performed to characterize the test range parameters. They include transmit/receive decoupling, background estimation, and two different amplitude calibrations: both direct (calibration with accurately known reference target) and indirect (based on the radar range equation and individual characteristics). Based on an uncertainty analysis, a good agreement between both methods could be found. In this paper, the design details of the RCS measurement facility and the characterizing tests including amplitude calibration will be presented.
A new antenna diagnostics technique has been developed for the DTU-ESA Spherical Near-Field Antenna Test Facility at the Technical University of Denmark. The technique is based on the transformation of the Spherical Wave Expansion (SWE) of the radiated field, obtained from a spherical near-field measurement, to the Plane Wave Expansion (PWE), and it allows an accurate reconstruction of the field in the extreme near-field region of the antenna under test (AUT), including the aperture field. While the fundamental properties of the SWE-to-PWE transformation, as well as the influence of finite measurement accuracy, have been reported previously, we validate here the new antenna diagnostics technique through an experimental investigation of a commercially available offset reflector antenna, where a tilt of the feed and surface distortions are intentionally introduced. The effects of these errors will be detected in the antenna far-field pattern, and the accuracy and ability of the diagnostics technique to subsequently identify them will be investigated. Real measurement data will be employed for each test case.
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 handheld wireless device. The minimum range length for these measurements has usually been determined using the far-field criteria of R>2D2/.. Since the devices are relatively small (<30cm) and the frequencies relatively low (<2GHz), the range length required to meet the far-field criteria is less than 120 cm. However, wireless devices are being designed that operate at the higher frequencies of the IEEE 802.11 standards, and many of these devices are no longer small handheld devices but rather notebook computers, appliances or even vehicles. Applying the far-field criteria to testing such devices can generate requirements for large and expensive chambers. This paper demonstrates through both numerical simulations and actual measurements that accurate TRP and TIS measurements can be made at range lengths significantly shorter than those indicated by R>2D2/..
Comparisons of the far-field results from two different ranges are a useful complement to the detailed 18 term uncertainty analysis procedure. Such comparisons can verify that the individual estimates of uncertainty for each range are reliable or indicate whether they are either too conservative or too optimistic. Such a comparison has recently been completed using planar and spherical near-field ranges at Nearfield Systems Inc. The test antenna was a mechanically and electrically stable slotted waveguide array with relatively low side lobes and cross polarization and a gain of approximately 35 dBi. The accuracies of both ranges were improved by testing for, and where appropriate, applying small corrections to the measured data for some of the individual 18 terms. The corrections reduce, but do not eliminate the errors for the selected terms and do not change the basic near-to-far field transformations or probe correction processes. The corrections considered were for bias error leakage, multiple reflections, rotary joint variations and spherical range alignment. Room scattering for the spherical measurements was evaluated using the MARS processing developed by NSI. The final results showed a peak equivalent error signal level in the side lobe region of approximately -60 dB for both main and cross component patterns for angles of up to 80 degrees off-axis.
Comparisons of the far-field results from two different ranges are a useful complement to the detailed 18 term uncertainty analysis procedure. Such comparisons can verify that the individual estimates of uncertainty for each range are reliable or indicate whether they are either too conservative or too optimistic. Such a comparison has recently been completed using planar and spherical near-field ranges at Nearfield Systems Inc. The test antenna was a mechanically and electrically stable slotted waveguide array with relatively low side lobes and cross polarization and a gain of approximately 35 dBi. The accuracies of both ranges were improved by testing for, and where appropriate, applying small corrections to the measured data for some of the individual 18 terms. The corrections reduce, but do not eliminate the errors for the selected terms and do not change the basic near-to-far field transformations or probe correction processes. The corrections considered were for bias error leakage, multiple reflections, rotary joint variations and spherical range alignment. Room scattering for the spherical measurements was evaluated using the MARS processing developed by NSI. The final results showed a peak equivalent error signal level in the side lobe region of approximately -60 dB for both main and cross component patterns for angles of up to 80 degrees off-axis.
Now-a-days, far-field ranges are being used to measure antenna radiation patterns. Two main types of ranges used are used for these measurements: direct and indirect illumination. In either case, the accuracy of the measurement is dependent upon the quality of the range quiet-zone fields. In direct illumination, phase and amplitude taper cause discrepancies in the fields. For indirect illumination, only amplitude taper must be accounted for. Additionally, stray signals and cross-polarization will further distort the quiet-zone fields and lead to measurement errors. This new methodology starts with the measured antenna data and a priori knowledge of the incident fields and estimates an Effective Aperture Distribution (EAD). The EAD compensates for these sources of error and can be used to predict the far-field radiation pattern of the antenna under test. Analytical results are presented for taper and stray signal analysis.
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.
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.
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.
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.
In this paper we present a direct optimization procedure which utilizes phase-less electric field data over arbitrary surfaces for the reconstruction of an equivalent magnetic current density that represents the radiating structure or an antenna under test. Once the equivalent magnetic current density is determined, the electric field at any point can be calculated. Numerical results using experimental data are presented to illustrate the applicability of this approach for non-planar near field to far field transformation as well as in antenna diagnostics.
In this paper we present a direct optimization procedure which utilizes phase-less electric field data over arbitrary surfaces for the reconstruction of an equivalent magnetic current density that represents the radiating structure or an antenna under test. Once the equivalent magnetic current density is determined, the electric field at any point can be calculated. Numerical results using experimental data are presented to illustrate the applicability of this approach for non-planar near field to far field transformation as well as in antenna diagnostics.
The Air Force Research Laboratory (AFRL), RF Technology Branch at the Rome Research Site, Rome NY provides a capability of far field antenna testing on full scale aircraft. A computer program, APATS – Antenna Pattern Analytical Tool Set, was developed in conjunction with the Information Systems Research Branch to provide a better way to visualize and understand the antenna pattern data taken during testing. The program is written in Java and relies on JView, developed by the Information Systems Research Branch, to process and display the 3D, three-dimensional, elements of the program.
The CTIA (The Wireless Association – www.ctia.org) were the first to publish a widely accepted test plan for antenna performance testing of “live” mobile phones[1]. The test plan describes the use of phantom heads and involves recording transmitted power and receiver sensitivity information over a full sphere to derive parameters such as Total Radiated Power (TRP) and Total Integrated Sensitivity (TIS). The test plan, has until now, assumed that testing is performed in the far-field at test distances greater than 2D2/.. For typical mobile phone frequency and device test diameters (assumed 300mm in the CTIA test plan), this has not been a constraint. However, as such testing evolves to include the various versions of IEEE 802.11 combined with new devices such as larger laptops and other consumer electronics, a far-field test requirement would lead to very large test facilities. Using experiments and rigorous simulations, this paper will show that for the commonly accepted performance criteria, the far-field requirement is unnecessarily strict. A minimum distance requirement based on the geometry and probe pattern is proposed which will ensure that the performance parameters (TRP, TIS, and others) are obtained with insignificant loss of accuracy.