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A portable, handheld imaging verification radar (HIVeR) system is designed to verify the RCS integrity of a low observable (LO) platform. The HIVeR is the latest generation to a previously designed and field-tested system (SARBAR) that produced radar images of targets in real-time. For applications with LO aircraft, an objective of the present technology is to extend the first-generation SARBAR system performance to easier use, higher sensitivity, and effective pass/fail decisions for selected regions on the aircraft outer mold line (OML). A novelty of the HIVeR design is an automatic registration scheme incorporated into the radar set. The location and orientation of the HIVeR unit is continually recorded using a precision position and orientation monitoring system. This registration process locates the handheld radar antenna position and orientation with respect to a fixed coordinate system. Similarly, the region-of-interest (ROI) on the aircraft surface is registered in this fixed coordinate system. An important feature of the new HIVeR is its capability to form calibrated radar images along a surface defined by the OML of the LO aircraft. This enables the radar to produce images that can be related to the RCS integrity of the ROI. The image along the OML can be used for pass/fail decision-making by comparing the image with a “gold standard” image for the same region.
DSO National Laboratories (DSO) has commissioned a state-of-the-art 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. The overall system accuracy allows for future extensions to 40GHz and higher. The 11.0m x 5.5m x 4.0m (L x W x H) shielded facility houses the anechoic chamber and the control room. As the proffered location for this indoor facility is on top of an existing complex instead of the ground floor, antenna pickup is facilitated by a specialized loading platform accompanied by a heavy-duty state of the art fully automated 2.0m x 3.0m (W x H) sliding door, as well as an overhead crane that spans the entire chamber width. Absorber layout comprises 8-inch, 12-inch, 18-inch and 24-inch pyramidal absorbers. The positioning system is a heavy-duty high precision 3.6m x 2.9m (W x H) T-type planar scanner and AUT positioner. The AUT positioner system is configured as roll over upper slide over azimuth over lower slide system. This positioning system configuration allows for planar, cylindrical and spherical near-field measurements. A rapidly rotating roll positioner is mounted on a specialized alignment fixture behind the scanner to facilitate far-field measurements. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 4.0 package. A Real-Time Controller (RTC), accompanied by an 8-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.
ABSTRACT We have developed a new compact spherical near-field measurement system using a photonic sensor as a probe and successfully measured the 3D antenna patterns of a double-ridged horn antenna from 1 GHz to 10 GHz. This system consists of a compact spherical scanner and a photonic sensor that is used for the probe of the spherical near-field measurements. In our system, only one probe can be used for the wide frequency range measurements and the probe compensation is not needed in the measurements. For the system, we propose a simple calibration method using a double-ridged horn antenna for our system. We calibrate the system by measuring the double-ridged horn antenna on the reasonable assumption that the antenna efficiency is 100 %. Comparing the absolute gain obtained by the proposed calibration method with the one decided by using three-antenna method at far-field range, we show that the agreement is good within 1 dB over the whole frequency range.
In making accurate measurements of antenna gain one must correct for the impedance mismatches between (1) the signal generator and transmitting antenna, (2) between the receiving power sensor and the receiving antenna and (3) between the signal generator and receiving power sensor. This is true for both far-field gain measurements and near-field gain measurements. It has recently come to our attention that there is a lack of clarity as to the form the mismatch factor should take when correcting near-field measured data. We show that a different form of impedance mismatch factor is to be used with the voltage domain equations of near-field than has been used with the power domain Friis transmission equation.
Precise mechanical alignment of motion axes of both cylindrical and spherical near-field systems is critical to producing accurate data. Until recently the only way to align these types of systems was to employ traditional optical tooling (i.e. jig transits, theodolites). Alignment by these methods is difficult, time consuming, and requires specialized training. More recently, laser trackers have been used for this type of alignment. Unfortunately, these devices are expensive and demand an even higher level of operator training. This paper describes the use of low cost alignment tools and techniques that have been developed by Nearfield Systems, Inc. (NSI) that greatly simplify the alignment process. Setup and alignment can be performed in a very short period of time by technicians that have been given minimal training. Suitable optical alignment procedures when followed by the use of electrical alignment techniques [7] yield sufficient alignment accuracy to permit testing up to Ku-band.
We examine how accurately the transmit and receive parameters of a radar cross section measurement system can be determined by use of a rotating dihedral as the polarimetric calibration device. We derive expressions for the errors due to misalignment in the angle of rotation. We obtain expressions for the angles a0,hv and a0,vh for which the measured cross-polarization ratios of a target vanish. Since the theoretical cross-polarization of a cylinder is 0, we can .nd the calibration bias-correction angles. We use simulated and real data to demonstrate the robustness of this bias-angle correction technique. We derive expressions for the uncertainty in the polarimetric system parameters.
wall or ceiling. The resulting motion changes the angle of the The incident field in the test zone can be measured using one-way string with respect to the incident field direction, during which probe antennas or passive two-way reflectors. In most cases time the coherent radar echo is recorded as a function of the string neither the probe antenna nor the passive reflector is very large, angle. The coherent signal-vs-angle data are then transformed to usually only a few wavelengths at best. It must be rolled across the cross-range domain using the fast Fourier transform (FFT), the range on carriages or raised up and down on towers, and the whence we obtain a chart of the incident field amplitude as a distance moved must somehow be measured. If a passive function of cross-range distance. Numerical examples are reflector is used as a probe, the carriage system must be shielded presented that show how variations in the incident field influence with absorbers and the reflector must be mounted on low-the string echo. A sample of experimental data shows that the reflectivity support structures. In addition to the hazard of processed data are readily interpreted. contaminating reflections, the mere assembling of all this equipment can be a significant task.
An upgrade to the large 75 foot radius spherical arch range at the U.S. Army Electronic Proving Ground at Ft. Huachuca, AZ has presented a complex design challenge in order to accommodate multiple test requirements, including both far-field and near-field measurements, as well as antenna under test (AUT) mode switching, over a wide frequency range. The range features a 60 foot diameter turntable (capable of supporting 80 tons) for azimuth positioning of large vehicles. The large arch/turntable positioning system combination presents a number of design issues in the implementation of a high performance, wideband RF subsystem. In addition, a significant requirement for this range is to allow either the probe mounted on the arch or the AUT mounted on the vehicle to transmit. The RF subsystem design utilizes the Agilent PNA in conjunction with the Agilent 85310 Distributed Downconverter system. Location of all the primary RF components are key issues in achieving sufficient transmit power, LO power, and receive sensitivity. Moreover, the selection and placement of the long RF cable runs has a significant impact on system level performance, and required thorough investigation. A unique utilization of available synthesizers provides a compact physical configuration and also provides an increase in speed over other multiple source configurations. This paper examines the design considerations for the RF subsystem and the configuration for achieving both near-field and far-field measurements for the case of the AUT transmitting as well as receiving.
In this paper is presented the first European mapping of the antenna measurement expertise. This initiative is conducted in the framework of the Antenna Centre of Excellence (ACE) of the European Community. This mapping has been set-up with the intent to provide all the European scientific and industrial community with a valid means to improve and facilitate the research and development activities in the field of antennas. The collected information will serve to create a valid and new service for all potential users of antenna measurements, in particular from the wireless communication industry, by witch it will be identify and contact antenna measurement facilities. The first results indicated a significant and encouraging reaction to this initiative with more then 50 European facilities registered into the database. The next step is the integration of non European institutions.
In the frame of Activity 1.2 "Antenna Measurement Techniques and Facility Sharing" of the EU network "Antenna Center of Excellence. (ACE) an activity on comparative measurements has been performed. The ongoing activity involves different test facilities and a 0.8-12GHz dual ridge horn. The facility comparison activities will be extended to more facilities within and outside Europe in the following two years. The participating facilities in this campaign where: SATIMO, the technical university of Denmark (DTU), The technical university of Madrid, Spain (UPM) and Saab Ericsson Space, Sweden (SES).
In this paper is presented an experimental investigation of conventional array calibration in the presence of various kinds of joint discontinuities between array panels. Two rigid array panels were positioned such that the element lattice was continuous across a narrow joint. Three kinds of discontinuities were applied to the joint: (1) an angle, (2) a gap (including an edge), and (3) a step between panels. Each type was investigated for joints oriented in the E-plane and the H-plane. Each discontinuity was also varied in magnitude so as to observe parametric effects. Planar near-field-range (NFR) measurements were made in a conventional array calibration mode and a near-field pattern mode. Processing included separating the pattern component due to element transmission (impedance) change from that due to pattern shape change. Results show that conventional calibration methods quickly become inadequate to calibrate these discontinuities because they change element pattern shapes.
Abstract— A method is proposed that will optimally select the placement of sources to aid in the calibration of a phased array of scalable panels that is mounted on a stationary, ground-based, non-rigid frame. A cost function based on the Cramer-Rao Lower Bound is optimized through constrained minimization. The array is constructed from idealized (non-deforming) subarray panels that have unknown perturbations in orientation and location. To demonstrate the proposed method, several case studies are investigated involving combinations of known calibration sources.
In this paper, reduction of the near-field scanplane in calibration of phased array antennas is discussed. In general, truncation of near-field data can give a considerable reduction of acquisition time. This particularly applies in a larger extent to phased array measurements, where a high number of channels is measured in the calibration process. Also, relative small equipment can be used to measure relative large antennas, which can be cost-effective. In this paper, it is shown that under certain conditions the scanplane, and therefore acquisition time, can be reduced substantially. Based on an example, different scanplane sizes and reduction techniques are considered to investigate and estimate the influence of truncation size on the error in the calibration parameters.
A system for measuring large linear arrays of antennas has been developed, fabricated and tested. The system consists on a 12 meters structure where the antenna under test (a L band array of dipoles in this case) is positioned. The measurement probe (another dipole) moves on a linear slide and stops in front of each element of the array to acquire the electric field. All the system is installed on an semi-anechoic chamber, that can be lifted (with two synchronized stepped motors). This semi-anechoic chamber covers the top and side parts of the structure. The bottom part consists on a metallic reflector, that controls the reflections from each antenna element. Once the data is acquired, the data are processed to obtain the far field patterns and parameters of the antenna array (element amplitude and phase, beam width, side level, beam pointing …) All the results are presented in a windows environment, and all the system is integrated in a friendly user interface.
For many years now, GDAIS 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 present an improved version of the algorithm that avoids a stationary phase approximation inherent in earlier versions of the technique. The improvement is realized by modifying the range-domain weighting used to implement the frequency derivative in the existing method. A similar modification was presented in the context of linear near-field measurements in an earlier AMTA paper. Numerical simulations are presented that demonstrate the improvement afforded by the technique in predicting far-field RCS patterns from near-field data collected using typical bandwidths and standoff distances. An additional benefit of the revised algorithm is that it readily admits a formulation that includes antenna pattern compensation, as described in a companion paper.
In previous work [1], we presented an antenna pattern compensation technique for linearly-scanned near field measurements. In this paper, we present a similar technique to mitigate the errors from uncompensated azimuthal antenna pattern effects in circular near-field monostatic radar measurements. The antenna pattern co mpensation is implemented as part of an improved algorithm for transforming the near-field measurements to the far-field RCS. A description of this improved circular near field-to-far field transformation CNFFFT technique for isotropic antennas is presented in a companion paper [2]. In this paper, we formulate the near-field signal model in the presence of an azimuthal antenna pattern under the same scattering approximation used in the isotropic CNFFFT. Using this model, we derive a modified version of the CNFFFT that includes antenna pattern compensation. Numerical simulations are presented that demonstrate the ability of the technique to remove antenna pattern errors and improve the accuracy of the far field RCS patterns and sector statistics.
The linear phase interferometer (LPI) has long been a popular means for performing direction finding (DF). Most texts treat the error terms associated with LPI-DF determination with approximation, such that the error formulae given are a generalized bound, useful for system engineering and design. For certain applications, however, it is important to understand the error associated with LPI-DF in more detail. Related work has accomplished this via Monte Carlo simulations for specific comparisons [5, 6]. To provide an improved (and more general) understanding, we have formulated a rigorous receiver noise error distribution that enables direct determination of bias and variance in LPI-DF. The approach can be generalized to an arbitrary number of simultaneous antenna element apertures.
ABSTRACT A fast and accurate technique is proposed in this work for the far field evaluation from a nonredundant number of voltage data collected by using the planar wide-mesh scanning (PWMS). It relies on the nonredundant sampling representations of the electromagnetic field and on the optimal sampling interpolation expansions of central type. By using a very flexible source modelling, which fits very well a lot of actual antennas, a new sampling technique is developed to recover the plane-rectangular data from the knowledge of the PWMS ones. It must be stressed that the so developed near-field–far-field transformation requires a number of data remarkably lower than that needed by the standard plane-rectangular scanning. Some numerical tests, assessing the accuracy of the technique and its stability with respect to random errors affecting the data, are reported.
ABSTRACT Electronic devices designed for purposes other than transmitting and receiving electromagnetic fields nonetheless act as unintentional antennas. Measurements methods are needed to characterize these antennas for electromagnetic compatibility tests; however, the rigor of precision antenna measurements is typically too costly and time consuming for electromagnetic compatibility applications. Alternate approaches are needed. This paper presents analytical estimates for the directivity of unintentional antennas based on the assumption that unintentional antennas will only randomly excite the available propagating spherical modes at a given frequency. This directivity estimate is then compared to simulated and measured data. Good agreement is shown. Directivity estimates combined with simple total radiated power measurements represent a useful alternative to direct antenna measurements for electromagnetic compatibility tests.
Antenna systems are increasing in complexity at a rapid pace as advances are made in electronics, signal processing, communication, and navigation technologies. In the past, antenna design requirements have focused on parameters such as gain, efficiency, input impedance, and radiation pattern (e.g., beamwidth and sidelobe level). For some new systems, the group delay characteristics of the antenna are important, where the group delay is proportional to the derivative of the insertion phase as a function of frequency. The group delay is required to stay within certain bounds as a function of frequency and pattern angle. Unfortunately, there are not well established methods or standards for calibrating antenna group delay like the standard methods used for gain and input impedance. This paper presents a method for calibrating the group delay of three antennas based on an extension of the widely used three-antenna gain and polarization calibration methods. No prior knowledge of the gain or group delay of the three antennas is required. The method is demonstrated by a measurement example where it is shown that multipath errors and time gating can be critical for calibrating the group delay.