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Holographic back-projections of planar near-field measurements to a plane have been available for some time. It is also straightforward to produce a hologram from cylindrical measurements to another cylindrical surface and from spherical measurements to another spherical surface1-7. In many cases the AUT is approximately a planar structure and it is desirable to calculate the hologram on a planar surface from cylindrical or spherical near-field or far-field measurements. This paper will describe a recently developed spherical hologram calculation where the farfield pattern can be projected on any plane by specifying the normal to the plane. The resulting hologram shows details of the radiating antenna as well as the energy scattered from the supporting structure. Since the hologram is derived from pattern data over a complete hemisphere, it generally shows more detail than holograms from planar measurements made at the same separation distance.
This paper presents a first-principles algorithm for estimating a target’s far-field radar cross section (RCS) and/or far-field image from extreme near-field linear (1- D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. Wavenumber migration (WM) is an approach that was first developed for the problem of geophysical imaging and was later applied to airborne SAR imagery [1], where it is often referred to as the “Range Migration Algorithm (RMA)”[2]. It is based on rigorous inversion of the integral equation used to model SAR/ISAR imagery, and is closely related to processing techniques for near-field antenna measurements. A derivation of WM and examples of approximate farfield RCS and image reconstructions are presented for the one-dimensional (1D) case, along with a discussion of the angular extent over which the far-field estimates are valid as a function of target size, measurement standoff distance, and near-field aperture dimensions.
A novel broadband dielectric rod probe design that has the characteristics of broad bandwidth; symmetric probe pattern; low RCS; low antenna clutter and dual polarization operation is discussed. The RCS level reduces the interaction between the probe and antenna under test (AUT). The lower antenna clutter level improves the sensitivity in detecting responses from wide angles with greater time delays. During the transmission mode, the rod is excited with a broadband microwave launcher from one end. The radiation then occurs at the other terminal of the rod. Measurement results of the far-field patterns, RCS and reflection coefficient for a prototype rod probe (DRP) are presented.
Two ways of modelling a compact range design are presented, and the coupling to a given antenna under test (AUT) is determined and compared to the AUT far field. The compact range models are both based on physical optics (PO). The first model applies a simple presentation of the serrations of the range reflector while the second model is based on a new feature of GRASP8, which allows a detailed description of the triangles of the range serrations. The AUT measurement is modelled by an accurate coupling analysis between the current elements on the compact range reflector and the antenna under test. This coupling pattern is compared to the real far-field pattern and the differences are discussed. By including known range imperfections in the AUT-torange coupling a better agreement to the measured patterns may be obtained. All computations are carried out by GRASP8.
Two ways of modelling a compact range design are presented, and the coupling to a given antenna under test (AUT) is determined and compared to the AUT far field. The compact range models are both based on physical optics (PO). The first model applies a simple presentation of the serrations of the range reflector while the second model is based on a new feature of GRASP8, which allows a detailed description of the triangles of the range serrations. The AUT measurement is modelled by an accurate coupling analysis between the current elements on the compact range reflector and the antenna under test. This coupling pattern is compared to the real far-field pattern and the differences are discussed. By including known range imperfections in the AUT-torange coupling a better agreement to the measured patterns may be obtained. All computations are carried out by GRASP8.
Log Periodic Dipole Arrays (LPDAs) are widely used for certain metrology applications including site attenuation measurements. To accurately make such measurements, the location of the phase center of the antenna is required. However, the LPDA does not, in the strictest sense, exhibit a phase center. Approximate phase centers can be defined by computing the local curvature of a far-field constant-phase surface on the antenna’s principal lobe. However, because the E- and H-plane patterns are different, the phase centers computed from each pattern (or any two-dimensional cut) are not co-located at a given frequency and, moreover, track differently with frequency. An H-plane array of LPDAs with an appropriate taper can be made to exhibit very similar E and H plane patterns over a very broad frequency range. Such an antenna exhibits a much better defined phase center (the phase center still moves as a function of frequency) and is therefore much better suited for metrology applications. Here we present phase center calculations and measurements for two different H-plane arrays of LPDAs. One array is composed of two highly compressed LPDAs (ô=.88, ó=.05) fed with a corporate feed network, while the other is composed of two high gain LPDAs using the so-called “optimum” parameters (ô=.88, ó=.16) and fed with a hybrid feed network. Numerically predicted and experimentally measured results for the phase center loci are presented and compared with those of the component LPDAs.
Near-field ground-to-ground imaging systems are widely used to discover damage that could degrade the radar signature of low observable vehicles. However, these systems cannot presently assess the impact of this damage on the far-field signature of these vehicles. We describe progress made on a method to accurately project the near-field data from these to the far field. Near-field data for the algorithm development is provided by the hybrid finite element/integral equation RCS computer code SWITCH. The near-field data is processed to extract the near-field scattering centers using imaging. The imaging algorithm used differs from the usual far-field imaging formulation in that it incorporates some near-field physics. The processing algorithm, which incorporates a modified version of the CLEAN technique, verifies that the scattering centers that were extracted reproduce the original data when illuminated in the near-field. These near-field scattering centers are then illuminated by a plane wave to produce far-field data. This procedure was tested using VHF band scattering data for a full size treated planform. The near field data was projected to the far-field and then compared to data from a far-field SWITCH computation.
This paper addresses the sensitivity of the cylindrical near-field technique to some of the critical alignment parameters. Measured data is presented to demonstrate the effect of errors in the radial distance parameter and probe alignment errors. Far-field measurements taken on a planar near-field range are used as reference. The results presented here form the first qualitative data demonstrating the impact of alignment errors on a cylindrical near-field measurement. A preliminary conclusion is that the radial distance accuracy requirement may not be as crucial as was stated in the past. This paper also shows how the NSI data acquisition system allows one to conduct such parametric studies in an automated way.
In this paper, a versatile indoor antenna measu rement facility in Nokia Resea rch Center is presented Two measurement systems have been implemented into a rectangular, shielded anechoic chamber having dimensions of 10 m * 7 m * 7 m. The first configuration is an in-house developed 3D radiation pattern measurement system that uses a rotating elevation arm. The primary application of this system is characterization of terminal antennas including the effect of a test person or a human body phantom. The elevation arm can be easily removed and the chamber then used as a conventional 5-m far-field range. This configuration is applied mainly for directive antennas. The facility has been found out to be very useful in research and development of wireless com munications antennas. The 3D spherical scanning system opens up a much wider perspective than before on how the human body interacts with different kinds of terminal antennas and what are the radiation and receiving performance characteristics under realistic usage conditions.
A novel, combined far-field and cylindrical near-field tapered anechoic chamber was designed for RACAL Antennas (UK). Advanced ElectroMagnetics Inc. (AEMI) and ORBIT/FR-Europe collaborated in the design and the facility was completed in April 2000. The far-field tapered chamber performance was verified by Shielding Integrity Services. The tapered chamber far field facility performance after construction is compared with the original design predictions at several cellular band frequencies. Near-field measurements, in the rectangular section, compare well with outdoor measurements. There is discussion of the installation of the shielded facility and the absorbers intended for engineers interested in the cellular antenna test and measu rement arena.
A novel, combined far-field and cylindrical near-field tapered anechoic chamber was designed for RACAL Antennas (UK). Advanced ElectroMagnetics Inc. (AEMI) and ORBIT/FR-Europe collaborated in the design and the facility was completed in April 2000. The far-field tapered chamber performance was verified by Shielding Integrity Services. The tapered chamber far field facility performance after construction is compared with the original design predictions at several cellular band frequencies. Near-field measurements, in the rectangular section, compare well with outdoor measurements. There is discussion of the installation of the shielded facility and the absorbers intended for engineers interested in the cellular antenna test and measu rement arena.
The NRTF and MRC have recently completed the first bistatic RCS test utilizing the Bistatic Coherent Measurement System (BICOMS). BICOMS is the first true far-field, phase coherent, bistatic RCS measurement system in the world and is installed at the NRTF Mainsite facility. The test objects include a 10 foot long ogive and a 1/3 scale C-29 aircraft model. Full pol rimetric, 2-18 GHz monostatic and bistatic RCS measurements were performed on both targets at 17 degree and 90 degree bistatic angles. BICOMS data demonstrates excellent agreement to method-of moments RCS predictions (ogive) and indoor RCS chamber measurements (monostatic, ogive). This paper describes the BICOMS system and the test process, highlights some process improvements discovered during testing, assesses the quality of the collected data set, and analyzes the accuracy of the bistatic equivalence theorem.
Radar Cross Section measurements require the target to be in the far field of the illuminating and receiving antennas. Such requirements are met in a compact range in the SHF band, but problems arise when trying to measure at lower frequencies. Typically, below 500 MHz, compact ranges are no more efficient, and one should only rely upon direct illumination. In this case, the wavefront is spherical and the field in the quiet zone is not homogeneous. Furthermore, unwanted reflections from the walls are strong due to the poor efficiency of absorbing materials at these frequencies, so the measurement that can be made have no longer something to see with RCS, especially with large targets. We first propose a specific array antenna to minimize errors caused by wall reflections in the V-UHF band for small and medium size targets. Then an original method based upon the same array technology is proposed that allows to precisely measure the RCS of large targets. The basic idea is to generate an electromagnetic field such that the response of the target illuminated with this field is the actual RCS of the target. This is achieved by combining data collected when selecting successively each element of the array as a transmitter, and successively each other element of the array as a receiver. Simulations with a MoM code and measurements proving the validity of the method are presented.
Composite Optics Inc (COI) has designed and constructed a Portable Far-Field Antenna Test Chamber to complement their Large Compact Range. The need for this chamber arose after COI won a contract to design, build, and test hundreds of small broadband antenna elements. Because of the portability requirement, COI chose to procure and modify an industrial container, suitable for transportation on a standard flatbed trailer. This paper discusses the design, fabrication, and installation of a chamber, suitable for pattern measurements of small (<2 feet) antennas in the 6-18 GHz frequency range.
Composite Optics Inc (COI) has designed and constructed a Portable Far-Field Antenna Test Chamber to complement their Large Compact Range. The need for this chamber arose after COI won a contract to design, build, and test hundreds of small broadband antenna elements. Because of the portability requirement, COI chose to procure and modify an industrial container, suitable for transportation on a standard flatbed trailer. This paper discusses the design, fabrication, and installation of a chamber, suitable for pattern measurements of small (<2 feet) antennas in the 6-18 GHz frequency range.
For greatest efficiency and accuracy in measuring patterns of a circularly polarized antenna on a planar near field range (NFR), a recommended procedure is to use a fast switched, dual circularly polarized probe. With such equipment one obtains complete pattern and polarization data from a single scan of the antenna aperture. For our task of measuring high gain shaped beam apertures, measurement efficiency is further improved by using a moderately high gain (about 12 dBi) probe that has been accurately calibrated for patterns, polarization, and gain over the test frequency band. Such a probe allows scan data point spacing to be typically at least one wavelength, thus keeping scan time minimized with acceptably small aliasing (data spacing) error. The measured near field amplitude and phase data is transformed via computer to produce the angular spectrum that is further processed to remove the effect of the probe patterns, i.e. probe correction. The final output is a set of (principal and cross) circular polarized far field patterns. However on one occasion, due to fast breaking changes in requirements, we were unable to obtain a calibrated circular polarized probe in the available time. For this test we used an available calibrated 12 dBi fast-switched dual linear-polarized probe with software capable of processing principal and cross circular-polarized far field patterns. As anticipated, we found from preliminary tests that the predicted low cross-polarized shaped beam pattern was not achieved when using the calibrated fast Ku band probe switch. Further tests showed the problem to be due to small errors in calibration of the probe switch. This paper will discuss test and analysis details of this problem and methods of solution.
When mounted on spacecraft , pattern of some antennas are perturbed by the presence of satellite body. The prediction of antenna performances including satellite structure effect is generally done at early stage of antenna design but is limited in terms of model complexity. The test on full spacecraft & in far field condition is then necessary. This solution is very expensive as it means for test at satellite level to use Compact antenna Test Range in order to satisfy cleanliness aspects. For the Meteosat Second Generation (MSG) program test on the toroidal antennas need to be performed on different model including a flight model. A good compromise was to use the external omnidirectional antenna range and a part of satellite structure representing the major contributor for the antenna pattern as identified via numerical analysis. The external range offer possibilities that cannot be reached in Compact range, e.g. low cost, full sphere pattern, low frequency range.
Obtaining far-field radiation patterns of high frequency antennas (>80Ghz) from near-field measurements has been an important issue in the last twenty years. However with frequencies increasing into the millimetre and sub-millimetre bands, questions have been raised about possible limitations on the assessment of such antennas and in particular the measurement of phase. The PTP phase retrieval algorithm addresses the problem by extracting the phase from the knowledge of two amplitude data sets in the near-field. The accuracy of the algorithm is studied by simulation and measurement by means of a numerical/statistical approach. Pseudo-random phase apertures can be generated using Zernike polynomials, which in turn can be used as initial estimates for the algorithm. This paper shows some simulated and measured results for various separations. It can be seen that different pseudo-random phase functions can affect the accuracy of phase retrieved results in particular when the distance between planes is considerably small in relation to the AUT size.
In this paper, an iterative algorithm for the retrieval of the radial component of the electric field is described to be used in matrix source reconstruction methods that deal with spherical measurement. A source-field decoupled integral equations are presented, making it necessary the use of a radial field retrieval algorithm to calculate the equivalent magnetic currents (EMC) in the antenna plane from the angular components of the electric field. The technique is applied in near field to far field (NF-FF) transformations using spherical ranges. With the presented technique, some drawbacks, inherent to the intensive resolution of the integral equations that appears in the methods based on equivalent currents, are overcome. Verification with simulated results as well as measurement results are presented.
The complexity of modern antennas has resulted in the need to increase the productivity of ranges by orders of magnitude. This has been achieved by a combination of improved measurement techniques, faster instrumentation and by increased automation of the measurement process. This paper concentrates on automated measurement systems, and describes the requirements necessary to make such systems effective in production testing, and in research and development settings. The paper also describes one such implementation - the MI Technologies Model MI-3000 Acquisition and Analysis Workstation - that was designed specifically to cmnply with these requirements The paper discusses several important factors that must be considered in the design of automated measurement systems, including: (1) Enhancing range productivity; (2) Interfacing with instrumentation from a large number of suppliers; (3) Providing a uniform front-end for the measurement setup and operation that must be largely independent of the choice of the hardware configu rations or the type of range (near-field or far-field); (4) Making the test results available in a format that simplifies transition to external commercial and user program med applications; (5) Providing powerful scripting capability to facilitate end-user program ming and customization; (6) Using a development paradigm that allows incremental binary upgrades of new features and instruments. The paper also discusses computational hardware issues and software paradigms that help achieve the requirements.