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Phased Array

A Study of Near-Field Sampling Grid Errors and Their Effect on Phased Array Beam-pointing Error
J. Bowen, November 2011

Large phased arrays have stringent beam-pointing accuracy requirements over their scan volume. Measuring the beam-pointing accuracy of a phased array with a planar near-field scanner is convenient but can lead to erroneous results if the near-field sampling grid is not well controlled. This paper describes numerical experiments that were carried out to assess the impact of various types of grid errors on the measurement of beam-pointing accuracy. The types of grid errors considered include skewing and curvature in the plane of the grid. The numerical experiments use infinitesimal dipoles as the radiating elements and assume an ideal probe. It is shown that beam-pointing errors induced by grid errors that can be described by an affine transformation can be estimated in closed form. For more complicated grid errors, the model is shown to be a useful tool in estimating their impact on measuring beam-pointing error. Finally, the amount of over-scan required for accurate beam-pointing measurements over a large scan volume is examined.

Near-field Antenna Measurement of an Active Phased Array Antenna for a New-Generation Weather Radar
Y. Masuda,T. Kumamoto, F. Mizutani, H. Handa, M. Tanabe, November 2011

We are developing a new-generation weather radar to observe and predict short-term weather phenomena like severe storms, gust and so on. Therefore, an active phased array antenna (APAA) with digital beam-forming (DBF) receivers could be used for the new-generation weather radar to reduce the observation time. In Toshiba Corporation, 33m x 16m vertical near-field antenna range including the digital instrumentation receivers have been working for multi-beam DBF antenna measurements. This near-field antenna range is used to evaluate the performance of an APAA. In this paper, we describe the characteristics of this new-generation weather radar and the APAA. And we demonstrate the antenna measurement set-up using the near-field antenna range and the measurement results of this antenna.

Measurement Techniques for a Transmit/Receive Digital Phased Array
S. Bhatia,W.M. Dorsey, J. Glancy, C.B. Huber, M. Luesse, K. O'Haver, A. Sayers, J.A. Valenzi, November 2011

This paper describes test methods and challenges for performing radio frequency (RF) characterization of a phased array antenna with element-level digital beamforming using planar near-field (PNF) and compact range technologies. The characterization of a digital array requires the synchronization of measurement equipment including positioner controllers, transmitters, and receivers. All hardware and software must remain synchronized with the array clock to achieve accurate amplitude and phase samples and ensure a coherent phase front. This synchronization is achieved through handshake triggers and communication protocols that are managed through external software. The acquisition of element-level data over large PNF scans presents unique challenges in data and post-processing that precipitate the need for optimization of array architecture as well as design of processing software. Advantages of the digital array architecture include being able to generate multiple receive beams from a single near-field scan for each frequency and the ability to compare multiple calibration methods efficiently using off-array processing.

Multiplexed Pulsed Transmit and Receive RF Measurement System for Active Phased Array Testing
K. Hassett,B. Williams, November 2011

Radar antennas are typically required to operate in transmit and receive modes, and may or may not support both CW and pulsed signal operation. In active antenna applications, these modes may require different operating parameters, which currently dictate testing the antenna independently in transmit and receive using different test system configurations. In testing highly-integrated active arrays, electrical and thermal considerations make it preferable to test the antenna in its nominal Tx/Rx (Transmit/Receive) operating mode as opposed to transmit-only or receive-only. An extension to the NSI Panther 9100 RF measurement system has been developed to support multiplexed transmit and receive, pulse-mode measurements with different measurement parameters during the course of a single data acquisition. This capability allows pulsed transmit and receive tests to be interleaved using a single measurement setup, reducing overall test time and improving the real-world accuracy of the test results.

A Large Spherical Near-Field Arch Scanner for Characterizing Low Frequency Phased Arrays
Jeff Fordham, November 2010

The overall measurement system details are presented, along with mechanical accuracies achieved for the scanner system. Details of the chamber and host facility are described. Finally, the paper concludes with measurements of a UHF-band Standard Gain Horn using the system. The challenges and benefits of such a system will be highlighted.

Near-Field Testing of Defocusing Methods for Phased-Array Antenna
Philip Brady,Derrick Mauney, November 2010

The Georgia Tech Research Institute (GTRI) analyzed a phased-array antenna for the purpose of testing phase-only defocusing methods. The array is defocused with the objective of broadening its beam at the cost of lower antenna gain. A design for the beam-steering computer is accomplished which adds the capability of focusing a beam, steering in azimuth and elevation, and performing beam defocusing using only element phase. Widening of the beam is accomplished using only 180° phase shifts in the elements, and it is compared with widening accomplished using gradual phase tapers. The antenna is measured in a near-field range to obtain amplitude and phase information as a function of each element in the array. Near-field testing of the antenna is also used to verify the capability of the beam-steering computer; two-dimensional antenna patterns and near-field hologram projections are compiled to prove this functionality. A software model is designed to mimic the behavior of the phased array antenna in its operational modes; it is also used to predict antenna gain and beamwidth prior to near-field testing. Measured and modeled antenna patterns are compared using focused and defocused modes. Metrics are performed on the near-field data to infer statistics of the individual phase shifters and on the computed far-field patterns to characterize the entire antenna. The defocusing methods under analysis are phase-only methods, due to the inability to control amplitude weighting of elements in this antenna. One method discussed uses only 180° shifting of elements in the antenna to achieve a desired beamwidth. This is compared with another method which gradually spoils the beam by applying a phase taper across the aperture. The results from near-field testing compare the defocusing methods and characterize the relationships between gain, beamwidth, and sidelobe levels for both defocusing methods.

UHF Near Field RCS Facility, comparison between simulation and experimental results
Philippe Berisset,LAURENT BEUNARD, PIERRE MASSALOUX, November 2010

Compact ranges are well suited to perform accurate indoor RCS measurements. These facilities are limited at the lower end of their bandwidth by the size of the parabolic reflector. Therefore, when RCS characterizations are required in the UHF band, RCS measurement facilities usually operate large horns or phased array antennas in a near field measurement layout. However, these calibrated near field measurements cannot directly be compared to the plane wave RCS characteristics of the target. One way to compare simulation and measurement results is to take the near field radiation pattern of the antenna into account. This paper first presents the design of a phased array antenna developed for indoor UHF RCS measurements. Then a model of this antenna is derived and a simulation of the experimental layout is performed. In parallel, near field RCS measurements of a canonical target were performed with this phased array antenna in an anechoic chamber. As a conclusion, a comparison between simulation and experimental results on this particular canonical target is discussed.

An Evaluation of the Aperture Backprojection Technique Using Measurements Made on a Flat Plate Array with a Spherical Near-Field Arch
Doren Hess,Scott McBride, November 2009

We describe two theoretical bases for an algorithm for back-projection. The first is (1) Fourier inversion of the mathematical expression for the far electric field components in terms of the aperture electric field. The second is (2) Fourier inversion of the complete vectorial transmitting characteristic of Kerns' scattering matrix. It is this characteristic that results from the standard process of planar near-field (PNF) scanning and the ensuing reduction of the PNF transmission equation. We demonstrate that the theoretical approaches (1) and (2) yield identical back-projection algorithms. We report on back-projection measurements of an 18 inch X-band flat plate phased array using the far-field obtained from both planar and spherical near-field scanning. The spherical measurements were made on a large arch range.

WIDE BAND DUAL POLARISED V/UHF PHASED ARRAY FOR INDOOR RCS MEASUREMENT
L.J. Foged (SATIMO Italy),Andrea Giacomini (SATIMO Italy), Philippe Berisset (CEA/CESTA), Roberto Morbidini (SATIMO Italy), Thierry Blin (SATIMO Italy), Yannick CHEVALIER (CEA/CESTA), A. Menard (DGA), November 2008

Phased arrays antennas have desirable features in terms of simplicity, compact dimensions and low weight for low frequency applications requiring dual polarization and medium gain such as RCS measurements. However, a fundamental problem with phased arrays technology in wide band applications is grating lobe limitations due to the grid topology of the phased array elements. The spacing of the array elements cannot be to close in order to limit element coupling and not to large to avoid grating lobes. Consequently, conventional phase array antenna applications are generally limited to a useable frequency bandwidth of 1:2. A unique grid topology has recently been developed to overcome this problem [1, 2]. By interleaving three separate phased arrays, each dedicated to a different subband with close to 1:2 bandwidth, the useable bandwidth of the combined phased array antenna can be extended to as much as 1:7 while maintaining the nice performance features of the basic phase array technology. Based on this technology a large dual polarized phase array antenna has been designed for indoor RCS testing in the frequency range from 140MHz to 1000MHz. The operational bandwidth of the array is split into three subbands: 140-260 MHz, 260-520 MHz and 520-1000 MHz. The array is 6.34 x 6m and weighs less than 250Kg. Due to the element spacing and topology the phased array is sensitive to excitation errors so the beam forming network (BFN) feeding the elements must be wellbalanced. A uniform amplitude and phase distribution for the array excitation coefficients has been selected to simplify the BFN design and minimize possible excitation errors throughout the bandwidth. This paper describe the antenna electrical design and performance trade-off activity, the manufacturing details and discuss the comprehensive validation/testing activity prior to delivery to the final customer.

Active Array Antenna Noise Temperature Measurement
S.A. Rawson (Callisto),Nelson Fonseca (CNES (The French Space Agency) ), November 2008

Active phased array antennas are often considered for many applications in radar and communications, particularly in millimeter wavelengths. The ability of active phased array antennas to be reconfigured with different beam shapes and pointing directions makes them attractive to increase the flexibility of the next generation of communications satellites so that they can adapt to the needs of a fast changing communications market. The antenna noise temperature of an active array is an important performance parameter, which is difficult to measure compared to a classical passive antenna. Moreover, for a satellite antenna, which has to be evaluated in an anechoic chamber before integration on the spacecraft, the ability to characterise the noise contribution of the antenna itself independent of the environment noise would be very interesting as it would allow better prediction of the antenna performance when it is deployed in orbit on the spacecraft. The paper describes the results of a study undertaken for the French Space Agency (CNES) to devise a new method for the measurement of the noise temperature of a Ka band active phased array antenna when mounted in a Compact Antenna Test Chamber (CATR). An important objective of the study was to find a method which did not rely on the substitution of the antenna under test with a reference antenna, which is the method often used in practice. The method of measurement of noise was based on digital processing of signal to noise ratio rather than analogue detection of noise level, which improves the measurement precision.

A Standalone RF System for Solid-State Phased Array Antenna Measurements
Dave Fooshe (Nearfield Systems Inc.),Chris Smith (Lockheed Martin Corp.), November 2008

Lockheed Martin MS2 has a long history of utilizing antenna ranges for calibration, test and characterization of the phased array antennas. Each range contains an integrated RF receiver subsystem for performing antenna measurements, typically on the full array. For solid-state phased array testing, what is often needed, however, is a test station capable of performing complex S-parameter measurements on a subarray or subset of the full antenna system without incurring the expense of a test chamber. To address this requirement, Lockheed Martin, working with Nearfield Systems, has developed a portable standalone RF measurement system. The standalone system consists of an Agilent PNA, automated transmit/receive unit (TRU) and a waveform generation (WFG) subsystem for interfacing to the phased array beam-steering computer. This paper will discuss the capabilities of the Standalone RF System including the TRU and WFG subsystems. The TRU is used to tailor the RF signal by automated switching of amplifiers and programmable step attenuators for various test scenarios. The WFG is an automated pattern generator used to present many digital waveforms in arbitrary sequences to the phased array beam steering computer. The design features of the standalone RF system will be presented along with the COTS hardware utilized in assembling the station.

Transformational Element Level Arrays (TELA) Testbed
Jonathan Buck (Air Force Research Laboratory),Peter Buxa (Air Force Research Laboratory), Thomas Dalrymple (Air Force Research Laboratory), David Kuhl (Air Force Research Laboratory), Matthew Longbrake (Air Force Research Laboratory), John McCann (Air Force Research Laboratory), Daniel Spendley (Air Force Research Laboratory), November 2008

There is a desire for antenna technologies that will support surveillance needs in a complex Radio Frequency (RF) environment. There are many current technologies that support these needs, including individual components such as broadband phased array antennas, broadband RF components, and miniaturized digital receivers. A testbed has been established to develop systems combining these elements, resulting in wideband phased arrays encompassing multiple receiver channels and capable of forming multiple independent beams through digital beamforming. This effort revolves around phased array calibration and testing, RF component characterization, system integration, system testing, and digital beamforming. The Transformational Element Level Arrays (TELA) Testbed allows for the integration of these technologies so that they can be tested and verified as a system. What will be described here is recent and current work taking place in this testbed. Some of this work includes system integration and testing and subsequent digital beamforming of a four-channel recieve system. Also included is the calibration process of an 8:1 bandwidth, 256-element phased array, and integration and testing of the 16-channel recieve system corresponding to this array.

An Ethernet Based Controller for Managing Complex Antenna Measurements with a Vector Network Analyzer
Marion Baggett (MI Technologies),James Langston (MI Technologies), November 2008

Vector Network Analyzers (VNA’s) are finding increasing utilization in antenna measurement ranges. At the same time, complex measurement scenarios involving many data channels in the antenna under test along with integration to beam steering computers for phased array antennas require management of the data collection beyond the VNA. Traditional methods have added control cards in the measurement control computer, increasing software complexity and reducing measurement throughput. The MI-788 Networked Acquisition Controller is designed to manage the hardware handshakes between position controllers, external sources and VNA’s, control up to 16 channels of multiplexed data from the antenna under test and/or interface with a beam steering computer. The MI-788 tremendously increases system throughput, particularly in these more complex measurement scenarios by removing real time data collection responsibilities from the measurement control computer. In addition, this unit makes all instrument communication Ethernet based, eliminating the spacing and operational limitations of GPIB based measurement systems. This paper will describe the operation of the MI-788 and demonstrate the increased measurement capabilities while using VNA’s in antenna measurements.

Indoor Spherical 3D RDC Near-field Facility
Y. Chevalier, P. Minivielle,F. Degery, P. Berisset, November 2007

Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor near field monostatic RCS assessment. The experimental layout is composed of a motorized rotating arch (horizontal axis) holding the measurement antennas. The target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. Two bipolarization monostatic RF transmitting and receiving antennas are driven by a fast network analyser : - an optimised phased array antenna for frequencies from 800 MHz to 1.8 GHz - a wide band standard gain horn from 2 GHz to 12 GHz. This paper describes the experimental layout and the numerical post processing computation of the raw RCS data. Calibrated RCS results of a canonical target are also presented and the comparison with compact range RCS measurements is detailed.

Comparison of RCS Measurement of a NASA Almond Using Classical Compact Indoor Facility and a new Phased Array Antenna
J. De Kat, November 2006

CEA-Cesta has developed a new phased array antenna for RCS dual polarization wide bandwidth measurement in V/UHF bands. This array enables us to enhance signal to noise ratio especially at low frequencies. It is composed of 3 sub arrays dedicated each to one frequency band. The innovative design allows installing it in one of CEA/CESTA RCS facilities called “CAMELIA”. In order to validate this array in the highest sub-band [700 to 2000MHz], we measured in both HH and VV polarizations the near field RCS of a 2.5m long NASA almond target. This canonical object has been made of polystyrene coated with conducting nickel varnish. It has been hung on an eight wires rotating positionner. The results are compared with the data acquired in a classical RCS compact range and with the output of the 3D finite element code called ODYSSEE developed at CEA.

A Generalized Method to Diagnose Faults in Antenna Arrays Using Neural Networks
N. Sarma,D. Vakula, November 2006

A generalized method to diagnose a defective element of an antenna array using neural networks is presented. A defective element with no excitation is classified as on off faults (i.e., total failure) and with current variation from designed values are current magnitude and phase faults. A uniform linear array of 101 isotropic elements with half wave distance between them and 1 amp current excitation is considered. Complex deviation pattern is determined which is the difference between the measured radiation pattern of the array under normal condition and degraded radiation pattern of the array with any one defective element. One radial basis function neural network is trained with all possible angle values of deviation pattern to determine the number of the faulty element. Other radial basis function neural network is trained with all possible absolute value of deviation pattern to determine current in defective element. The trained network showed high success rate. Key words:-Artificial neural networks, Phased array, Radial basis function (RBF), Radiation pattern

Evaluation of Low-Cost Phased Array Antenna Design
J. Kemp,B. Mitchell, L. Corey, R. Cotton, November 2006

In the early 1990's, Georgia Tech Research Institute (GTRI) was able to acquire an unclassified phased-array antenna from the former Soviet Union. Since that time, GTRI personnel have analyzed the antenna for design features that enabled the production of low-cost phased-array antennas. Antenna pattern data collected on the GTRI planar near-field range of a working and errored antenna will be presented. Also, modeled antenna pattern data will be presented as a comparison to show the particular effects of the low-cost design versus an ideal antenna. Finally, the original control mechanism of the phased-array antenna will be analyzed and compared with a modern control mechanism developed by GTRI researchers. Control data for the original and new control systems was captured with a logic analyzer and will be presented for comparison.

Method for Selecting Sources to Calibrate a Non-Rigid Phased Array
G. Brown,D. Wright, G. Wilson, R. Parks, November 2005

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.

Influence of Truncation of Near-Field Data in Calibration of Phased Array Antennas
A. Boomstra, November 2005

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.

Determination of Element Locations in a Wideband Phased Array
H. Aumann,F. Willwerth, K. Tuttle, November 2004

In this paper the electrical displacement of phased array elements along the axis of a linear array, and in the direction normal to the array are examined. A closed-form solution is presented for determining the location of phased array elements from the first and second derivatives of the phase measured on a near-field antenna range. The technique is applied to swept CW measurement patterns of a 20-element, S-band array of open-ended waveguides. It is shown that the electrical location of edge elements differs significantly from the physical location in both x-dimension and z-dimension. The effects of wide array bandwidth on the phase center displacement are illustrated.







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