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Phased arrays antennas are designed to control their radiation characteristics by accurately setting the phase and amplitude distribution of the elements. Inaccurate control of the phase and amplitude can significantly alter the radiation pattern of an array. In fact, the operating principle of scanning arrays of elements for applications such as target tracking or mobile satellite communications, where the requirements for low side lobes and high gain are of very high importance, is primarily based on precise control of the phase and amplitude of the elements. For these reasons, the complexity of antenna measurement system design for phased array antennas measurements involves high accuracy and precise time synchronization between all the components of the system. This paper presents a comprehensive solution for accurate and reliable measurement of very large phased array antennas at high frequencies. The presented solution addresses the following issues: • Accurate positioning of the RF sensor / probe. • High-speed multi – frequency data collection. • High-speed multi - port data collection. • Programmable and real-time TTL position event triggers. • Pulse measurement. • Multi beam measurement. • Synchronization with the radar computer.
Techniques for measurement of the phased-array antenna system include ambient temperature measurements in a compact antenna range, thermal vacuum testing, and spacecraft level testing. There have been two novel developments in the characterization of the phased-array system. The first is a “gain envelope” response, which is a measurement of the gain of the array at the intended scan angle as the array is electrically scanned in 1° increments. This response was produced through a combination of hardware and test software to synchronize the gain measurement with the mechanical and electrical scanning. The second is a phase steering verification test that utilizes couplers in each steered element in conjunction with previously measured element patterns to confirm that the antenna beam is steered properly. This method allows functional verification of the phased-array system while radiating into an RF absorber-lined hat during spacecraft-level tests.
For enhancing the performance of existing near field antenna test facilities it is quite reasonable to use both conventional (the amplitude and phase measurements) and the phaseless measurements techniques during electrically scanning phased array antennas (PAA) testing. This simple yet critical approach helps to improve the quality of PAA alignment and testing reducing measurement errors and saving costs. In this way many difficulties related to precise phase measurements are overcome. Both simulation and measurement results will be presented to demonstrate the utility of such approach to PAA alignment and determination of its parameters. Comparison will be made between the PAA patterns for electrically scanned beams calculated using traditional near field - far field (NF/FF) transformations, the phaseless methods and the results obtained applying both measurement techniques.
CAMELIA is one of the three anechoïc chambers of the French Atomic Energy Center (CEA). It is equipped with a compact range reflector and a pulsed radar allowing antenna and RCS measurements from 800 MHz to 18 GHz. Below 800 MHz, measurements are made with different kind of antennas (log- periodic, horns, arrays…). Nevertheless, measurements at such low frequencies suffer from serious artifacts due to coupling effects. This paper describes a particular array we designed, realized and characterized to cover the 100 MHz – 2000 MHz bandwidth. Although the antenna diagram shape was the most constraining factor, the ability to cover the whole bandwidth with as few handling as possible was the major issue.
As satellite communication systems grow increasingly complex, so has the need for spacebased phased array antennas. After these antennas have been designed and assembled, they need to be tested. This paper describes the new antenna measurement facility that NGST (Northrop Grumman Space Technology) has installed to that end. This includes descriptions of near-field and compact ranges that are integral parts of the Phased Array Assembly, Integration and Test Area.
Qualification measurements of phased array antennas for communication satellite uplink applications present new measurement challenges. These measurement challenges include verifying array element excitation accuracies, amplitude and phase tracking over environmental conditions, and corrections for antenna noise temperature that are not required for conventional aperture antenna designs. Additionally, the usual antenna characterization parameters must be established as well. These measurement issues are discussed.
A technique is presented for determining the insertion phase of array elements directly from time domain measurements. It is shown that the Inverse Discrete Fourier Transform (IDFT) commonly used in swept frequency time delay measurements may yield unreliable phase results. A compensation to the IDFT is proposed which allows the phase of an array element to be accurately estimated from time domain data without gating and without taking a second DFT. The technique is applied to determine the insertion attenuation and phase of the elements in a linear L-band phased array. Compared to conventional array calibrations, the removal of extraneous range reflections implicit to the time domain technique resulted in a significant improvement in measurement accuracy.
In order to diagnosis array antennas we implemented the back projection technique in our bi-polar near field laboratory. Using capabilities of microwave holography we investigated actual distribution of excitation coefficient values in a variety of antenna arrays operating at 5 GHz and around 10 GHz. We investigated phase and amplitude alignment of the linear MC-8 phased array with eight elements operated in the band centered at 5000 MHz. The reconstructed phase distribution images reveal phase distributions consistent with the design values. A major technical impairment is that the resolution at the element level can not be easily assured and it is related to the element spacing.
The installation and operation of large horizontal UHF phased array antennas in arctic regions is challenged by severe environmental conditions. It can be shown that large and moderate snowfall can impact the operation of exposed dipole array elements and reduce aperture efficiency. Reflection coefficients measured at the antenna terminal of an embedded array element can vary drastically as a function of snow depth. In this paper we will describe several measurements that show embedded array element impedance variations versus snow height. Measured results will be presented for an array operating from 400 MHz to 470 MHz.
Ideal phased array antennas offer advantages for communication systems, such as wide-angle scanning and multibeam operation, which can be utilized in certain NASA applications. However, physically realizable, electronically steered, phased array antennas introduce additional system performance parameters, which must be included in the evaluation of the system. The NASA Glenn Research Center (GRC) is currently conducting research to identify these parameters and to develop the tools necessary to measure them. One of these tools is a testbed where phased array antennas may be operated in an environment that simulates their use. This paper describes the development of the testbed and its use in characterizing a particular K-Band, phased array antenna.
A Wideband Optically Multiplexed Beamformer Architecture (WOMBAt) was developed and characterized at the Crane Naval Surface Warfare Center Active Array Measurement Test Bed (AAMTB) facility. The project includes development and integration of the true-time delay (TTD) WOMBAt photonic beamformer with the Active Array Measurement Test Vehicle (AAMTV). The AAMTV is a 64-channel transmit-receive (TR) module based phased array beamformer that is integrated with the AAMTB facility 12’x9’ planar near-field scanner. The AAMTV provides phase trimming and a small amount of delay using electrical components while the WOMBAt provides longer delays using commercial-off-the-shelf (COTS) optical components typically manufactured for the telecommunication industry. By integrating the WOMBAt with the AAMTV, a highly flexible test environment was achieved that includes system calibration, multi-frequency scanning, and antenna pattern analysis. Phase I receive tests for this system were previously described and presented to AMTA[1] in 2002. This paper will describe the results of reconfiguring the AAMTV into a transmit architecture for Phase II. WOMBAt successfully demonstrated wideband TTD in both receive and transmit configurations at angles greater than the system goal of ±65º while exceeding all other system level performance goals. System level performance included a beam squint of less than 1.1º for receive and 0.5º for transmit, a worse case amplitude variation of 2.4 dB receive and 1.6 dB transmit and differential delays of less than 3.5 picoseconds.
The recent launch and successful orbiting of the EO-1 Satellite has provided an opportunity to validate the performance of a newly developed X-Band transmitonly phased array aboard the satellite. This paper will compare results of planar near-field testing before and after spacecraft installation as well as on-orbit pattern characterization. The transmit-only array is used as a high data rate antenna for relaying scientific data from the satellite to earth stations. The antenna contains distributed solid-state amplifiers behind each antenna element that cannot be monitored except for radiation pattern measurements. A unique portable planar near-field scanner allows both radiation pattern measurements and also diagnostics of array aperture distribution before and after environmental testing over the ground-integration and pre-launch testing of the satellite. The antenna beam scanning software was confirmed from actual pattern measurements of the scanned beam positions during the spacecraft assembly testing. The scanned radiation patterns on-orbit were compared to the near-field patterns made before launch to confirm the antenna performance. The near-field measurement scanner has provided a versatile testing method for satellite high gain data-link antennas.
A Wideband Optically Multiplexed Beamformer Architecture (WOMBAt) was developed and characterized at the Crane Naval Surface Warfare Center Active Array Measurement Test Bed (AAMTB) facility. The project included development and integration of the WOMBAt photonic beamformer with the Active Array Measurement Test Vehicle (AAMTV). The AAMTV is a 64-channel transmit-receive (TR) module based phased array beamformer that is integrated with the AAMTB facility 12’x9’ planar near-field scanner. The AAMTV provided phase trimming and a small amount of electrical delay while the WOMBAt provided longer optical delays using commercial-off-the-shelf (COTS) components typically manufactured for the telecommunication industry. By integrating the WOMBAt with the AAMTV, a highly flexible test environment was achieved that included system calibration, multi-frequency scanning, and antenna pattern analysis. This paper presents antenna pattern results showing less than 0.7 dB of amplitude variation over the frequency range from 9 to 10 GHz at each of the measured nominal steering angles. The beamformer was steered to greater than ±69 degrees with an observed beam squint from 9 to 10 GHz of less than 1 degree.
Software GPS receiver development has been undertaken. We are particularly interested in improving the GPS signal-to-noise/interference ratio using a beam forming techniques. The phase relationship among the antenna array elements requires careful calibration. In this study, we will report a phase calibration technique for a 2 by 2 GPS antenna array using both simulated and real GPS signals. This technique is based on the GPS signalprocessing algorithm developed for the software GPS receiver. A four-channel digital data collecting system was used in the experiment. For a simulated GPS signal, the experiment was conducted in an anechoic chamber in which a GPS simulation system was facilitated. For real GPS signals, we conducted the experiment on a rooftop to receive the signal from GPS satellites. The calibration verified the coherent nature of the signals among the elements. The results also allowed the source's direction to be determined.
Accurate UHF phased array antenna patterns are difficult to achieve due to high level multipath present in the far field measurement test range. Special range geometry’s and source arrangements have been devised over the years to mitigate the measurement errors produced by test range multipath. In this paper we will describe new measurement results achieved using Aperture Synthesis illumination method designed to optimize and control the influence of ground reflections and in turn reduce quietzone amplitude ripple. Measured phased array patterns at 418, 434, 449, and 464 MHz will be shown for a 64- element array.
Two cylindrical phased array antennas were characterized at the NAVSEA Crane.s Active Array Measurement Test Bed (AAMTB) facility. The antennas include the Full Performance Antenna (FPA) and the Ultra Light Antenna (ULA) that are intended for land mobile test sites for the United States Department of Defense. These air breathing, low-cost antennas are candidates for a new communication system. Crane.s role as the program Technical Advisor (TA) includes integration and performance testing at the component level, antenna level, and system level. This paper discusses issues related to the antenna-testing phase including pattern measurements, G-F, and high power safety concerns. The final goal of the integration and testing phase was to verify that the antenna RF performance specifications were met. To this end, conventional cylindrical near-field pattern testing was adequate for many items such as beam width, pointing angle, and side lobe levels. However, two issues required additional effort: G-F measurement and high-power transmit safety concerns. Since the majority of required measurements could be made using the near-field chamber and the antenna required special controllers and prime power sources, it was desirable to make all measurements in the same location. Hence, a new measurement process was required for G-F using a near-field range and the high-power safety concerns needed to be addressed.
An empirical study on Planar Near-Field Scan Plane Truncation applied to the measurement of a large phased array radar antenna saves test time per antenna. Lockheed Martin has been manufacturing, aligning, and verifying the AEGIS SPY-1B/D phased array radar antenna for the past 17 yrs . A custom built planar nearfield scanner system (ANFAST II) was designed and built specifically for this purpose. Existing raw near-field measured data sets were cropped in both the X and Y scan planes, processed to the far field, and compared with the un-truncated data to determine the error sensitivity vs near-field amplitude level truncated. Near-field measurements were then acquired at the truncated scan plane dimensions and compared. It was demonstrated that 100 hrs of test time could be saved by applying this technique without adversely effecting the antenna measurement uncertainty. This paper discusses the application of the truncation technique, results of the experiments, and practical limitations.
Simulated data is presented for a planar array to demonstrate the limitations of planar near-field back projections. It is well known that the result obtained in this way is of limited resolution and accuracy and these limitations are further illustrated through the data presented here. The impact of probe to AUT separation distance is shown as well as the correspondence between array excitation perturbations and that detected through the back projection technique. Results are shown for a simple iterative array excitation adjustment process. The purpose of this paper is to provide guidelines for the application of the planar near-field back projection technique.
This paper presents an accurate method for diagnosis of element failures in large phased arrays. The method is based on the reconstruction of the excitation from measured near-field data by solving the linear system relating the excitation coefficients to the field at measurement points. The dimension of the linear system is reduced by adopting sampling strategies with minimal redundance. The strongly ill-conditioned system is solved using an iterative generalized Landweber algorithm. Numerical simulations on a 2225 elements planar array confirm the effectiveness of the approach.
Analysis of in-orbit phased array antenna patterns measured from earth station requires a considerable examination of the in-orbit antenna operation. The antenna analysis should take into account the constant change of both observation angles and scan angles. The in-orbit phased array antenna pattern characteristics are mathematically analyzed. The coordinate transformation technique to calculate the time-varying trajectory of the observation angle in the antenna coordinate system is presented. The technique also encompasses the satellite track angle calculation as seen from the ground antenna. Data processing procedure of the dynamic antenna patterns and several test issues are discussed.