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In this paper we have revisited the phase retrieval problem for planar near-field antenna measurements. It will be shown that the complexity of retrieval procedures is function of not only the independency of different sets of measurements but also the characteristics of the antenna under test (AUT). Features of antenna like its beam direction will have profound effect on the success of phase reconstruction algorithms. The failure of a well known phase retrieval method, Iterative Fourier Transform (IFT), is investigated for a case where the antenna has a scanned beam. It is found that this is due to the non-judiciary choice of the initial guess. To alleviate the deficiency of the IFT a simple but effective initial guess is sought by Differential Evolutionary Algorithm (DEA). DEA tries to find the best initial phase guess which minimizes an error criterion. Subsequently this best guess will be fed to the phase retrieval IFT routine for further phase refinements. Having done this the far-field can subsequently be constructed. The improvement in the phase reconstruction algorithm is examined, through a series of simulations and measurements.
In this paper a circular planar near-field scan region is considered as an alternative to the commonly used rectangular boundary. It is shown how the selection of this alternative boundary can reduce test time and also to what extent the alternative truncation boundary will affect far-field accuracy. It is also shown how well known single dimensional filter functions can be applied over a two-dimensional region of test and how these attenuate the truncation effect. The boundary and filter functions are applied to measured data sets, acquisition time reduction is demonstrated and the impact on far-field radiation pattern integrity in assessed.
DSO National Laboratories has commissioned a high performance combined near-field and far-field antenna test facility in 2004. This facility supports highly accurate measurement of a wide range of antenna types over 1 – 18 GHz. This combined NF-FF system allows for planar, cylindrical and spherical near-field measurements, as well as far-field measurements. The combined near-field and far-field test facility has undergone meticulous validations making use of a TICRA calibrated “Golden Antenna” (GA). A detailed account of the cylindrical and spherical near-field comparative validation methodology and the test results are the subject of this article. The validation results for planar near-field (PNF), cylindrical near-field (CNF), spherical near-field (SNF) and far-field measurements have clearly shown that the system fulfils all the performance requirements without the use of a calibrated probe. Although dedicated near-field test facilities are generally thought to provide superior measurement accuracies, it will be shown in this article that a well-designed combined NF-FF test facility can deliver highly accurate results without the use of a calibrated probe. This makes the combined NF-FF system a viable and cost-effective antenna measurement solution, without compromising on measurement accuracies.
ABSTRACT An elaborate and effective strategy for estimating the samples external to the measurement region in the planar spiral scanning is developed in this paper. It relies on the nonredundant sampling representations of the electromagnetic field and on the optimal sampling interpolation expansions of central type and uses the singular value decomposition method for extrapolating the outside samples. It is so possible to reduce the inevitable truncation error affecting the near-field reconstruction, thus giving rise to a more accurate far-field prediction. Numerical examples assess the effectiveness of the proposed technique.
For many years now, General Dynamics has described the development, characterization, and performance of an image-based circular near-field-to-far-field transformation (CNFFFT) for predicting far-field radar cross-section (RCS) from near-field measurements collected on a circular path around the target. In this paper, we consider the CNFFFT algorithm as an azimuthal filtering process and develop a formulation capable of transforming data that is not measured over a full 360º. Such a formulation has applications in measurement scenarios where collection of a complete rotation is not practical. As part of the development, we provide guidelines for the near-field data support required to achieve a desired accuracy in the sub-360º CNFFFT result. Numerical simulations are provided to demonstrate that the results of this partial-rotation formulation are consistent with the full-circle CNFFFT results presented in past papers.
For many years now, General Dynamics has described the development, characterization, and performance of an image-based circular near-field-to-far-field transformation (CNFFFT) for predicting far-field radar cross-section (RCS) from near-field measurements collected on a circular path around the target. In this paper, we consider the CNFFFT algorithm as an azimuthal filtering process and develop a formulation capable of transforming data that is not measured over a full 360º. Such a formulation has applications in measurement scenarios where collection of a complete rotation is not practical. As part of the development, we provide guidelines for the near-field data support required to achieve a desired accuracy in the sub-360º CNFFFT result. Numerical simulations are provided to demonstrate that the results of this partial-rotation formulation are consistent with the full-circle CNFFFT results presented in past papers.
This is part three of a series of talks on the development of a new type of near field antenna range. The range is designed to measure the VHF and UHF antenna characteristics of a vehicle over a realistic ground. This means that the spherical symmetry has been lost and the classical spherical mode expansions are not appropriate. We have previously demonstrated a plane wave synthesis approach to the far field transformation, including the lossy dielectric half-space representing the ground. This third yearly presentation will discuss the compensation needed for realistic probe antenna, probe arm and turntable imperfections. Results for actual experimental measurement data with sample space and probe antenna time dispersion compensation will be shown. Comparisons with the theoretical far field computations and the spherical mode expansion results will be included.
Typically radome tests are performed on outdoor far field ranges or compact ranges. ORBIT/FR has designed, build and qualified a unique spherical near-field radome test facility for the nose-mounted radomes of commercial traffic aircraft for the so-called “after repair” tests according to the international standard RTCA/DO-213, as well as the aircraft manufacturers Component Maintenance Manuals. The facility is extremely compact (chamber size 5.7 m x 5.2 m x 3.2 + 0.7 m, L x W x H), can handle radomes as small as used on the Canadair and as large as used on the Airbus-380 and can be installed directly in the repair workshop for such radomes. The tests performed are transmission efficiency and side lobe level increase. The system is completely automated, so that a workshop technician can operate the facility. Utmost attention has been paid to operational aspects and both operator and equipment safety. After the measurements are done, a test report is fully automatically generated according to RTCA requirements and classifications. The facility is equipped to test all standard Airbus, Boeing, Canadair and Dash nose radomes.
Satellite TV reflectors for home use, provided to the public by service companies such as DIRECTV, have many features which must be adequately characterized prior to design release, including: • Multiple Beam Frequency Re-use • FCC Sidelobe Envelope Verification • Circular Polarization Isolation These features must be adequately tested at frequencies up to Ku band and beyond. The use of a far-field range is impractical, as some of the reflectors measure several feet in diameter, and thus requires a range length of several hundred feet at Ku band. Near-field testing requires a full scan to determine a single cut for evaluation of FCC compliant sidelobe performance. Thus, a compact range is a logical alternative for measurement of this class of antennas. The compact range can provide a quick assessment of multiple beam coverage performance and pass/fail analysis against FCC sidelobe curve specifications. In addition, the feeds for these antennas often use Low Noise Block (LNB) Downconverters that are built in as part of the feed assembly. Measuring the output of an LNB does not yield the phase information required to determine all polarization parameters. A spinning linear measurement with some unique processing was implemented on this range to determine the full polarization characterization, using some elementary assumptions about polarization sense. This paper describes the implementation of a compact range based measurement facility for satellite antenna testing, with emphasis on the circular polarization measurement of the LNB assembly, capability for comparison against FCC sidelobe levels, and measurement of offset beams featuring frequency re-use capability.
A computational technique based on near-field to far field transformation is presented. This can be more versatile and accurate than the conventional modal expansions. The established method for near-field to far-field transformation has been the modal expansion method. The primary drawback of the technique is that when a Fourier transform is used, the fields outside the measurement region area is assumed to be zero, particularly in the planar and cylindrical case. Consequently the far-fields are accurately determined only over a particular angular sector which is dependent on the measurement configuration. A simple and accurate integral equation solution which represents an alternate method for computing far-fields from measured near-fields is presented. The basic idea is to replace the radiating antenna by equivalent electric and/or magnetic currents which reside on a fictitious surface and encompasses the antenna. These equivalent currents are assumed to radiate identical fields as the original antenna in the region of interest. Using the surface equivalence principle different types of the E-field integral equation (EFIE) have been developed. The method of moments (MoM) has been utilized to transform the integral equation into a matrix one and the conjugate gradient (CG) procedure has been applied to solve it numerically. Hence, this procedure is not limited by the Nyquist sampling criteria nor by the presence of evanescent waves which may make source reconstruction using current procedures unstable. Accurate far-fields over large elevation and azimuthal ranges have been calculated from simple measurements based on planar and spherical scanning.
A computational technique based on near-field to far field transformation is presented. This can be more versatile and accurate than the conventional modal expansions. The established method for near-field to far-field transformation has been the modal expansion method. The primary drawback of the technique is that when a Fourier transform is used, the fields outside the measurement region area is assumed to be zero, particularly in the planar and cylindrical case. Consequently the far-fields are accurately determined only over a particular angular sector which is dependent on the measurement configuration. A simple and accurate integral equation solution which represents an alternate method for computing far-fields from measured near-fields is presented. The basic idea is to replace the radiating antenna by equivalent electric and/or magnetic currents which reside on a fictitious surface and encompasses the antenna. These equivalent currents are assumed to radiate identical fields as the original antenna in the region of interest. Using the surface equivalence principle different types of the E-field integral equation (EFIE) have been developed. The method of moments (MoM) has been utilized to transform the integral equation into a matrix one and the conjugate gradient (CG) procedure has been applied to solve it numerically. Hence, this procedure is not limited by the Nyquist sampling criteria nor by the presence of evanescent waves which may make source reconstruction using current procedures unstable. Accurate far-fields over large elevation and azimuthal ranges have been calculated from simple measurements based on planar and spherical scanning.
Nowadays near-field measurement techniques are widely used for detecting the characteristics of the radiated pattern for a large variety of antennas. The core of any near-field measurement is the near-field to far-field transformation. Such transformations use different coordinate systems, like planar, cylindrical, or spherical, and may utilize special solutions. They are already well known for many years. The common feature of all mentioned near- to far-field transformations is the usage of regular measurement grids on planar, cylindrical, or, respectively, spherical surfaces. Future applications, like the Airborne Near-Field Test Facility (ANTF) are expected to lack this characteristic of regular measurement grids, since the flying or floating probe platform cannot be guided sufficiently accurate. This requires the utilization of advanced data processing methods for interpolating measured data on an arbitrary irregular grid to a nearby regular grid, or direct transformation to the far-field. It will be shown that this data processing can be performed by using the Stratton-Chu representation formula utilizing equivalent currents on a well-chosen artificial surface or the classical mode expansion method with additional pre-processing. This paper describes briefly the principles of the ANTF, discusses the application of the equivalent current method and compares it with the widely used mode expansion method. Measured and processed data examples will be presented.
A transmission line system has been developed for an electromagnetically transparent antenna array. The goal was to provide equal signal distribution to the array elements while maintaining the transmissivity of the antenna. The transmission lines consist of microstrip directional power couplers which are fed in series. This reduces the transmission line length needed. The transmission line was built, tested, and integrated with an array of circular polarized array elements mounted over a frequency selective surface (FSS) ground plane. Preliminary bench tests performed on the integrated array with a small test dipole indicated that the transmission lines provided uniform signal distribution. Outdoor far field measurements of the integrated antenna indicated that the antenna performance was satisfactory. The integrated antenna array was tested in the compact range located at the ElectroScience Laboratory at The Ohio State University. These tests were used to accurately characterize the antenna performance at S band and the transmissivity properties of the integrated array at L band. The measured antenna pattern and beamwidth were consistent with predictions. Transmissivity of the antenna as viewed by a second antenna was also consistent with predictions.
The R&D testing of antennae today is still an important challenge for many universities. They find it difficult to instrument their antenna labs with equipment that allows the flexibility of re-configuring their test science for their various AUT configurations. Antenna test facilities at educational institutions are typically used sporadically and for a high mix of different antenna types with frequencies ranging up to millimeter wave. Unlike their industry counterparts that build and instrument a production antenna test facility geared to the specifications of the antenna under test. The challenge lies in configuring an antenna test facility to operate within these wide boundaries at a reasonable cost. A flexible RF Sub-System will be discussed that utilizes the Agilent PNA series vector network analyzer and harmonic mixers as the receiver, and a remote PSG series source and multipliers as the stimulus. This paper will examine the steps undertaken to define the requirements necessary to upgrade the existing antenna test facility at the University of Toronto in Toronto Canada. It will also include design considerations necessary to create a power budget in order to estimate the dynamic range of the test system. This paper will also delve into the aspect of selecting and exploring the benefits of the test software requirements.
Antenna pattern and isolation measurements for the B-1 Fully Integrated Data Link (FIDL) Program have been completed at the US Air Force Research Laboratory (AFRL) Antenna Measurements Facility located near the AFRL Rome Research Site (RRS), Rome, NY. This combined satellite and airborne communications upgrade has been performed under the supervision of the B-1 Systems Group, Wright Patterson AFB, Ohio. One eighth scale antenna patterns were collected on a far field range for new Link-16 antennas, a relocated VHF/UHF2/L-Band antenna and the new Satcom transmit antenna, while on a one eighth scale B-1 model. Antenna to antenna isolation measurements were performed with antennas mounted on a full scale front section of the B-1 airframe. The RF Technology Branch (IFGE) has developed techniques for evaluating the effects of airframe and external stores on the radiation pattern characteristics of antenna systems in a simulated flight environment. Data obtained in this manner is used to evaluate antenna radiation characteristics of antenna/systems without the requirement of an extensive flight test program. Using similar techniques, AFRL has developed procedures whereby precision measurements of isolation between aircraft mounted antennas can be accomplished. This paper will present how the measured data was obtained for the antennas involved in the FIDL upgrade.
The UWB radar operates simultaneously over large bandwidth and the antenna parameters must refer to simultaneous performance over the whole of the bandwidth. Conventional frequency domain (FD) parameters like pattern, gain, etc. are not adequate for UWB antenna. This paper describes an UWB radar antenna planar near field (PNF) measurement system under construction to get the impulse response or transient characteristic of the UWB antenna. Unlike the conventional antenna or RCS time domain test system, the UWB radar signal instead of the carrier-free short time pulse was used to excite the antenna that can avoid the decrease of the dynamic range and satisfy the needs of SAR and the other UWB radar antennas measurement. In order to demonstrate the data analysis program, FDTD simulation software was used to calculate the E-field of M×N points in a fictitious plane at different times just like the actual oscilloscope’s sampling signals in the time domain planar near field (TDPNF) measurement. The calculated results can be considered the actual oscilloscope’s sampling output signals. Through non-direct frequency domain near field to far field transform and direct time domain near field to far field transform, we get the almost same radiation patterns comparing to the FD measurements and software simulation results. At last, varied time windows were used to remove the influences of the non-ideal measurement environment.
This paper presents an overview of work carried out in developing the probe-corrected, poly-planar near-field antenna measurement technique [1, 2, 3, 4, 5]. The poly-planar method essentially entails a very general technique for deriving asymptotic far-field antenna patterns from near-field measurements taken over faceted surfaces. The probe-corrected, poly-planar near-field to far-field transformation, consisting of an innovative hybrid physical optics (PO) [6] plane wave spectrum (PWS) [7] formulation, is summarised, and the importance of correctly reconstructing the normal electric field component for each of the discrete partial scans to the success of this process is highlighted. As an illustration, in this paper the poly-planar technique is deployed to provide coverage over the entire far-field sphere by utilising a small planar facility to acquire two orthogonal tangential near electric field components over the surface of a conceptual cube centred about the antenna under test (AUT). The success of the poly-planar technique is demonstrated through numerical simulation and experimental measurement. A discussion into the limitations of the partial scan technique is also presented.
Hemispherical Near-Field (NF) antenna measurement technique has been applied for automotive antenna testing within a chamber dedicated to EMC tests. An existing turntable was used for azimuth rotation of a vehicle and a new portable 90°arch was added for elevation scanning of the radiated NF of the Device Under Test (DUT - vehicle with the antenna). Two antenna types were tested during chamber commissioning, one for GPS and another for XM satellite radio applications at frequencies 1.57 and 2.33 GHz respectively. Test results have shown that the EMC chamber can be successfully used for automotive antenna measurements as well, with accuracies acceptable for automotive applications. For higher operating frequencies, the EMC absorbers must be changed to less reflective material. In the paper, the measurement system is described, and the test results are presented, as well as some considerations on far-field pattern restoration based on measured hemispherical NF data.
This paper describes the use of a two-dimensional chirp z-transform (2D-CZT) to efficiently concentrate a large number of sample points in a single portion of the far zone without interpolation. This work presents the equivalence of transforms calculated from measured near-field data using both the 2D-CZT and 2D-fast Fourier transform (FFT). The paper also shows that the 2D-CZT is computationally more efficient than a zero-padded FFT when one requires a high resolution over a small area of the pattern.
This paper will discuss the development of a VHF/UHF near field test range for the case where there are reflections from a realistic ground surface. We will show the results of a direct computation algorithm where a far field pattern is computed using plane wave synthesis. The performance of a C++ program that implements this algorithm will be discussed.