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A novel technique for estimating the spatial electromagnetic field distribution and its covariance error is presented based on variogram analysis and the statistical interpolation technique known as Kriging. The spatial structure of some field measurements are characterized by variogram analyses and their propagation properties identified. The physical implications of the Kriging interpolator functional fit to measured data is considered and illustrated. It is concluded that with specialist interpretation this new technique can be used as a valuable checking tool, or to reduce the number of field measurement, in a measurement programme, particularly when the costs of the latter are considered.
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.
A large single reflector corner fed rolled edge compact range system, featuring an elliptical cylinder 12’ (H) x 16’ (W) x 16’ (L) quiet zone has been recently installed in a large anechoic chamber [1]. The Compact Range System parameters, such as reflector surface tolerance of better than 0.001” over the Quiet Zone section of the reflector and superior Quiet Zone field performance at frequencies down to 1.0 GHz were verified and validated. As a part of further studies of potential advantages delivered by the compact range system, the study of the compact range application to Antenna and RCS measurements at VHF/UHF frequencies was initiated. Though the reflector surface tolerance is not an issue at the VHF/UHF bands, successful compact range operation at these frequencies would be a significant expansion of the capabilities of the existing compact range system. In order to evaluate the system performance at VHF/UHF frequencies a number of challenging technical issues had to be resolved and performed. They include: Compact Range Quiet Zone Performance Analysis at the VHF/UHF bands Choice of a concept for a broadband feed suitable for the application and installation within the existing feed carousel Feed Design and Performance Validation Feed Installation in the existing feed carousel Quiet Zone Field Probing and Performance Verification All these issues were addressed in the development of a suitable low frequency feed, and are described in more detail below.
Orbit/FR has installed a new compact range for antenna measurements at ACE Antenna Corp. The measurement facility covers a frequency range from 0.8 to 40GHz with a Quiet Zone size of 3 m diameter x 3 m length. The design of the compact range is similar to the one already installed by Orbit/FR at Ericsson (Sweden) with some improvements in the mechanical design and in the system parameters. An intensive simulation of the reflector serrations had allowed for finding its optimal profile, thus improving the quiet zone parameters at entire frequency range, especially at low frequencies, at which a number of base-station and mobile antennas are expected for testing by ACE Antenna Corp. A new design of a feed positioner and a baffle house added more convenience for the compact range alignment and operation. The system was installed and qualified in March 2008. The field probing has been performed within the entire operating frequency range, which then allows for evaluation of the antenna measurement accuracy. A system description as well as results of simulation and excerpt of the qualification data is presented in the paper.
The goal of this research is to develop an unconstrained reconfigurable programmable array antenna that we call the Software Defined Antenna™. We create patch arrays using individual controllable pixels. The aperture of the array is made up of a large group of small (<1/10 ?) pixels. Each computer controlled pixel is a small piston made up of a metal top, a dielectric shaft, and a metal base. When a line of pixels is raised into the up position, a microstrip transmission line is created. A patch antenna is created when multiple pixels are raised into the up position to form a larger rectangle or other shape. In the final design, a set of feed lines and antennas can be created to form an antenna array within 1 millisecond. Under computer control, it is possible to change the beam direction, the beamwidth, the polarization, and the frequency of operation of the array. Theoretical results, experimental results and the implementation of a working prototype will be shown.
Recently, there has been a growing interest in sparse antenna arrays. Formally, it can be shown that their design can be expressed as a constrained multi-dimensional nonlinear optimization problem. Generally, through lack of convex property, such a multi-extrema problem is very tricky to solve by usual deterministic optimization methods. In this paper, a recent stochastic approach, called “Cross-Entropy method”, is applied to the continuous constrained design problem. The method is able to construct a random sequence of solutions which converges probabilistically to the optimal or the near-optimal solution. Roughly speaking, it performs adaptive changes to probability density functions according to the Kullback-Leibler cross-entropy. The approach efficiency is illustrated in the design of a sparse antenna array with various requirements. The results indicate the obtained solution relevance. Furthermore, the software versatility to deal with any design requirements is highlighted.
Paper discusses the design, optimization and implementation of a Circularly Polarized (CP) microstrip 2 x 2 sequentially rotated phased antenna array for an X-band onboard satellite transceiver. In the final design, CP radiation is constructed by using CP elements, having unique sequential rotation along with sequential phase shift feeding–giving wider 3dB Axial Ratio (AR) Bandwidth. CP in each patch element is achieved by a perturbation segment, in this case a pair of truncated corners and with a single point feed–reducing complexity, weight and RF loss of the array feed. First analysis based on cavity model approach for the single CP patch is carried out, which is used to determine the normalized perturbation parameter. The initial dimensions are calculated using perturbation analysis. Optimization initially for individual patch and then for the array is performed using full wave analysis tools based on Method of Moments (MoM), and verified using Finite Difference Time Domain (FDTD). Finally, the measured input impedance and radiation patterns are correlated with the calculated results. It is observed that the measured Gain and 3db Beamwidth agrees well with the simulated results of the array optimized using MoM, while the measured results of Axial Ratio, VSWR and reflection coefficients Sxx follows closely the results from the simulations based on FDTD.
Small GPS antenna elements and arrays are of great interest for future smaller vehicles. The need to cover the lower L5 band presents an additional challenge. To that end, a compact 6-element GPS array is proposed to cover all three GPS bands, namely, L1 (1575 MHz), L2 (1227 MHz) and L5 (1176 MHz). This GPS array consists of six proximity-fed stacked patches (PFSP), each having 1.2’’ in aperture size (?/8 at L5). The overall GPS array has a diameter of 4.5’’, nearly 90% smaller in area size as compared to the standard 14’’ GPS array. A key feature of the GPS element is its single feed to realize the right-hand circularly polarized polarization (RHCP) via a quadrature phase microstrip line splitter. As a result, only 6 coax cables are needed to feed the entire GPS array. Design concepts and procedures are presented, followed by measurements and a performance assessment.
Harmonic radar has recently been shown useful for remote state sensing in high clutter environments. This new application of harmonic radar with chemically sensitive "tags" allows long-range state sensing of individual low-cost passive (battery-less) sensors, such as corrosion indicators for industrial storage tanks. The "tag" response is sensed at the second harmonic of the radar transmitter, eliminating clutter from undesired objects. A new miniaturized planar harmonic radar tag design has been developed from a low-cost switching diode and low-cost laminate, without the use of shorting vias. An 85% cost reduction over the previous tag design has been achieved while maintaining similar performance. Data are presented from field testing and the laboratory environment comparing the new tag design to the old tag design as well as a basic wire dipole.
Microstrip leaky-wave antennas offer a potential low-profile aperture for use over a relatively large bandwidth compared to resonant microstrip patch antennas. The radiation properties of a leakywave antenna can be characterize through the longitudinal wavenumber consisting of attenuation and phase coefficients. Specifically, either the physical length of the antenna or the distance from the feed can define the extent of the aperture such that significant power is transported by the leaky-wave mode (this is primarily determined by the attenuation coefficient), whichever is shorter. The angle of the main radiation lobe can be determined by the normalized phase coefficient. In this work, the longitudinal wavenumber is controlled through the use of reactive edge loads on the antenna. Specifically, the reactive load alters the transverse wavenumber, as dictated by continuity conditions, and consequently the longitudinal wavenumber is also altered from the unloaded state. An approximate theory is presented along with both computational and measured results.
The National Radar Cross Section Measurement Facilities Certification Program seeks to raise collectively the quality bar across the community. A program to accomplish this goal was initiated in 1995. It continues with facilities joining the program every year. The program has now entered the recertification phase for facilities that achieved certification five or more years ago. This paper will briefly cover the history of the program, the participants, the certification process and criteria, the recertification process, status, and the way ahead.
Over the last few years, we have implemented several methodologies pertaining to uncertainty analysis of RF and Optical measurements. These methodologies are currently in use within the radar cross-section, electro-optic/infrared, and material measurement laboratories at the Air Force Research Laboratory. In this paper we discuss from a top level some of the approaches we have implemented, and identify some important issues one needs to address before beginning an uncertainty analysis. We illustrate one such approach as it applies to the estimation of radar cross-section uncertainty.
The measurement accuracy of state-of-the-art RF test facilities like near-field or compact test ranges is influenced due to applied system hardware as well as operational facts which are influenced by human errors. The measurement errors of near-field test facilities were analyzed and published in the past times and are based on the 18-term error model of Newell [1]. For compact test ranges and especially for the cross-polar free compensated compact range a similar error model was established at Astrium GmbH within a study for the satellite service provider INTELSAT [2] in order to define possible facility performance improvements and maximum achievable values for the measurement accuracy. It has to be remarked, that test programs for space applications require very stringent adherence to procedures and documentation of process steps during a test campaign. Within this paper, recommendations for process optimizations and procedures will be presented to guarantee the adherence to the valid error budgets and to minimize the Human Factor. A description of main error contributions in the Compensated Compact Range (CCR) of Astrium GmbH will be performed. Furthermore, the error budgets for pattern and gain measurements and achievable performance improvements will be given.
In this paper, three possible approaches for definition of a highly accurate reference pattern of a reference antenna are described and their pros and contras are discussed. Following the most reliable approach, a dedicated measurement campaign was planned and carried out in 2007-2008 for definition of the highly accurate reference pattern of the VAST12 antenna. In planning the campaign, conclusions from the first comparison campaign with the VAST12 carried out within the ACE network in 2004-2005 were taken into account and these are also presented and discussed. Some typical measurement errors and uncertainties are listed and briefly discussed.
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.
Some compact ranges use two orthogonal linear polarized feed horns for circular polarized antenna measurement. These two feed horns are symmetrically located along the vertical plane through the longitudinal axis (VPTLA). For accurate axial ratio measurement, the CP antenna under test (AUT) should also lie on the VPTLA. However, for some applications, the AUT has to be offset from the VPTLA during measurement. When this happens, rays from two feed horns reaching the AUT are out of phase. This extra phase error causes unwanted test error for the axial ratio measurement. This paper presents an analysis on the error cause, and provides a method to compute and correct the phase error, when the AUT is offset from the VPTLA. The method computes the extra phase difference from two feed horns to the AUT using geometrical optics method. This phase difference is then used to correct the tested data. This paper also shows a successful measurement example using this correction technique.
A first analysis of the equivalences between Multiple Input Multiple Output (MIMO) and physical/synthetic radar arrays is presented. The establishment of these equivalences is addressed to make use of efficient radar imaging algorithms, which were originally conceived for SAR systems, with MIMO arrays. The main advantage of MIMO arrays is that, with a reduced cost and complexity of the antenna feeding network, they offer imaging capabilities very close to those of SAR and physical radar arrays. This makes MIMO radar a very interesting option in real-time imaging applications (e.g., surveillance of small areas). The paper will present some numerical simulations using some reference scenarios where the imaging capabilities of MIMO arrays will be assessed. A comparative analysis with the well-known SAR and uniformly spaced radar arrays will be presented. Here the study is made with one-dimensional radar apertures, and subsequently will be extended to two-dimensional radar apertures. The analysis of the performance of the MIMO arrays is based on a Matlab simulation tool that is used to optimize the array topology and also to form the radar images of a synthetic scenario. The optimization technique is based on a genetic algorithm, using a fitness function measuring the degree of uniformness and uniqueness of the loci of the phase centers of the tx/rx pairs of the MIMO array. Results show that the found topologies show a performance close to uniformly spaced physical radar arrays.
A real-time S-band radar imaging system will be shown in this paper that uses a spatially diverse antenna array connected to a highly sensitive linear FM radar system and uses a synthetic aperture radar (SAR) imaging algorithm to produce real-time radar imagery. The core of this radar system is a high-sensitivity, range gated, radar architecture. Previous work has demonstrated the effectiveness of this radar architecture for applications requiring low-power and high sensitivity for imaging through lossy dielectric slabs at S-band and in free space at both S and X bands. From these results it was decided to develop a real-time S-band SAR imaging system. This is achieved by constructing a spatially diverse antenna array that plugs directly into a pair of S-band transmit and receive radar front ends; thereby providing the ability for real-time SAR imaging of objects. The radar system chirps from approximately 2 GHz to 4 GHz at various rates from 700 microseconds to 10 milliseconds. Transmit power is adjustable from approximately 1 milliwatt or less. The image update rate is approximately one image every 1.9 seconds when operating at a chirp rate of 2.5 milliseconds. This system is capable of producing imagery of target scenes made up of objects as small as 1.25 inch tall nails in free space without the use of coherent integration. Previous applications for this radar system include imaging through dielectric slabs. It will be shown in this paper that this radar system could also be useful for real-time radar imaging of low RCS targets at S-band.
This paper describes how the Dynamic Advanced Radar Test (DART) facility was designed, constructed, integrated and validated within budget in a 12-month time frame using lean engineering techniques. The facility is a world-class Radar seeker test facility. These techniques allowed the DART team to enhance capabilities without adding cost or complexity. The purpose of this paper is to identify a new paradigm in Radio Frequency (RF) range development, whereby all variables are accounted for early in the process, thus preventing and avoiding time consuming and costly mistakes. This process relies on lean engineering Accelerated Improvement Workshops (AIW) and Production, Preparation, Process (3P) workshops to guide the design process. Allowing all stakeholders to be owners through these intense workshops is vital. Additionally since formal evaluation tools and methodologies guide the workshops, improvement opportunities are maximized while minimizing risk.
Hardware-In-The-Loop chambers provide the chamber designer with many difficult obstacles to overcome in order to establish a high performance environment for the measurement of missile seeker systems. One of the most difficult challenges is to overcome the low performance of absorbing materials at low grazing angles. To solve this problem Tapered R-Card Fences have been used in conjunction with Chebyshev absorbers. Last year we reported on the ATK chamber built in Woodland Hills which showed preliminary test results well within the system requirements. This paper will make a direct comparison of chamber performance with and without Tapered R-Card Fences. The establishment of a sister chamber built in the ATK Alliant Techsystems Inc. ABL facility has provided us with the unique opportunity to test the chamber prior to the installation of the R-Cards and then to test it again with the installation of the R-Cards. This unique opportunity has allowed us a direct comparison of an advanced chamber deign with Chebyshev absorbers as would be utilized in a conventional chamber and the performance increase directly attributable to the introduction of the Tapered R-Cards in the anechoic chamber. The chamber evaluation is carried out utilizing The Ohio State developed TDOA measurement method utilizing their proprietary measurement and analysis software.