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Obtaining far-field radiation patterns of high frequency antennas (>80Ghz) from near-field measurements has been an important issue in the last twenty years. However with frequencies increasing into the millimetre and sub-millimetre bands, questions have been raised about possible limitations on the assessment of such antennas and in particular the measurement of phase. The PTP phase retrieval algorithm addresses the problem by extracting the phase from the knowledge of two amplitude data sets in the near-field. The accuracy of the algorithm is studied by simulation and measurement by means of a numerical/statistical approach. Pseudo-random phase apertures can be generated using Zernike polynomials, which in turn can be used as initial estimates for the algorithm. This paper shows some simulated and measured results for various separations. It can be seen that different pseudo-random phase functions can affect the accuracy of phase retrieved results in particular when the distance between planes is considerably small in relation to the AUT size.
In this paper, an iterative algorithm for the retrieval of the radial component of the electric field is described to be used in matrix source reconstruction methods that deal with spherical measurement. A source-field decoupled integral equations are presented, making it necessary the use of a radial field retrieval algorithm to calculate the equivalent magnetic currents (EMC) in the antenna plane from the angular components of the electric field. The technique is applied in near field to far field (NF-FF) transformations using spherical ranges. With the presented technique, some drawbacks, inherent to the intensive resolution of the integral equations that appears in the methods based on equivalent currents, are overcome. Verification with simulated results as well as measurement results are presented.
Antenna measurement systems have unique requirements, which must be properly evaluated and understood in order for the antenna engineer to be successful in specifying an RF system that meets his needs. Antennas are characterized by specific operating and performance parameters that will determine the requirements for a measurement system. Aperture size, frequency range, bandwidth, side-lobe nulls, beamwidth, and polarization characteristics are a few of the more important parameters. As with most engineering problems, system performance often requires a trade-off of equally important, but conflicting characteristics. Sensitivity and measurement time are well-known examples of this trade-off. Other examples include local vs remote mixers, receiver speed vs sensitivity, range size vs system dynamic range, and there are many others. The antenna engineer must be able to identify his most important system performance parameters in order to make compromises with confidence, since they are inevitably required. Once the system performance requirements have been determined, the antenna engineer can begin to select equipment, cables and components with the desired performance characteristics for his range. This paper will describe the process for analyzing requirements, performing system trade-offs and specifying equipment and components for several antenna measurement system types.
The complexity of modern antennas has resulted in the need to increase the productivity of ranges by orders of magnitude. This has been achieved by a combination of improved measurement techniques, faster instrumentation and by increased automation of the measurement process. This paper concentrates on automated measurement systems, and describes the requirements necessary to make such systems effective in production testing, and in research and development settings. The paper also describes one such implementation - the MI Technologies Model MI-3000 Acquisition and Analysis Workstation - that was designed specifically to cmnply with these requirements The paper discusses several important factors that must be considered in the design of automated measurement systems, including: (1) Enhancing range productivity; (2) Interfacing with instrumentation from a large number of suppliers; (3) Providing a uniform front-end for the measurement setup and operation that must be largely independent of the choice of the hardware configu rations or the type of range (near-field or far-field); (4) Making the test results available in a format that simplifies transition to external commercial and user program med applications; (5) Providing powerful scripting capability to facilitate end-user program ming and customization; (6) Using a development paradigm that allows incremental binary upgrades of new features and instruments. The paper also discusses computational hardware issues and software paradigms that help achieve the requirements.
The majority of precision spherical positioner alignment techniques used today are based on procedures that were developed in the 1970's around the use of precision levels and auto-collimation transits. Electrical alignment techniques based on the phase and amplitude of the antenna under test are also used, but place unwanted limitations on accurately characterizing an antenna's electrical/mechanical boresight relationship. Both of these techniques can be very time consuming. The electrical technique requires operator interpretations of data obtained from amplitude and phase measurements. The autocollimation technique requires operator interpretations of optically viewed measurement data. These results are therefore typically operator dependent and the resulting error quantification can be inaccurate. MI Technologies has recently developed a mechanical alignment technique for Spherical Near-Field antenna measurements using a tracking laser interferometer system. Once the laser system has been set-up and stabilized in the operational environment; the entire spherical near-field alignment may be completed in a few hours, as compared to the much more lengthy techniques used with level/transit or electrical techniques. This technique also simplifies the quantification of the errors due to the inaccuracy of the alignment. This paper will discuss the effect of the alignment error on results obtained from spherical near-field measurements, and the procedures MI Technologies developed using a tracking laser interferometer system to obtain the precision alignment needed for a spherical near-field measurement.
This paper summarizes a joint NIST-Industry measurement effort. The purpose of this effort was to use a NIST developed ultrawideband measurement system to assess the performance improvement of ferrite tile anechoic chamber after a partial retrofit. Measurements were performed in the 30-1200 MHz frequency range before and after treatments were applied and excellent results were obtained. The system exhibited good sensitivity and the results highlight the effects of various retrofitting treatments. The effort also demonstrates that the NIST ultra wideband system is an efficient tool for the evaluation for both current and proposed anechoic EMC compliance test chambers.
A freely available raytracing code was used to render the environment of a number of antenna ranges at the Air Force Research Laboratory (AFRL) Rome Research Site (RRS) for the purposes of determining specular points contributing to reflections. The program POV-Ray (Persistence Of Vision), which is available for free, has advantages over raytracing techniques in that it renders more realistic images, can model surface attributes of objects, and create a series of images to be used to create animations of the environment as the antenna is being measured.
This paper will detail the implementation and results of a gain calculation performed on standard gain horns (SGHs) in the LS and XN microwave bands. The three-antenna method was used to ensure the highest accuracy possible, and extensive efforts were made to minimize the error budget. The measurement was performed in a large anechoic chamber, with the receive and transmit antennas placed 4.6 meters high in opposing corners. The resulting fifteen meters of aperture separation (approximately 10D2/l. for LS band and 15D2/l for XN band) eliminated all measurable aperture interactions and greatly reduced multipath interference from chamber reflections. Rigorous analysis of the error terms proved this method to be both accurate and reliable. Typical values of measured error terms will be presented.
This paper considers the use of Cramer-Rao bound (CRB) to aid in providing accurate and quantitative system-level trades for antenna direction finding (DF). Past work has focussed on the use of spectral estimation techniques (e.g., MLM and MUSIC) to obtain needed DF accuracy. Here, the CRB is used to quickly assess tradeoffs in determining optimal antenna array positioning on a platform system. We develop the necessary CRB mathematical relations and demonstrate the potential advantage of using multimode spiral antennas over a standard linear phase interferometer (LPI). The standard LPI configuration is used as a baseline for comparison.
This paper describes the initial pointing control possible with large deployable mesh reflectors that use a control mechanism and gives the results of a control experiment. In the experiment, the pointing error is measured by an RF sensor and reduced to within designated value by altering the support structure. The result indicates that such alteration can appropriately reduce the initial pointing error for a large deployable reflector.
Toshiba Corporation, working with Nearfield Systems Inc., has a fully digital antenna measurement system for digital beam-forming (DBF) antennas. The DBF test facility is integrated with the large 35m x 16m vertical near-field range installed at Toshiba in 1997 [3], and includes the NSI Panther 6500 DBF Receiver as the primary measurement receiver. The DBF system was installed in March 1999 and has been used extensively to test and characterize a number of complex, high performance DBF antennas. A DBF antenna typically incorporates an analog-to digital (AID) converter at the IF stage of the transmit/receive (T/R) module. The digital IF signals are transferred to a digital beam-forming computer, which digitally constructs, or forms, the actual antenna pattern, or beams. Since the interfaces to the DBF antenna are all digital, the usual microwave mixers and down-converters are incompatible. The NSI Panther 6500 is designed to interface directly with DBF antennas and allows up to 8 channels of I and Q digital input (16 bits each) with 90 dB dynamic range per channel. The NSI DBF receiver solves the DBF interface problem while providing enhanced performance over conventional microwave instrumentation. [2].
A software's user-interface design determines how productive someone will be in a accomplishing a given task. This is particularly true in the area of antenna measurement and analysis. The MiDAS software package is used as an example of how software can be specifically designed to focus on enhancing efficiency by implementing an advanced human-machine interlace. Simple yet critical aspects such as minimized menu access, integrated, user friendly error checking and help, and clear, consistent, and integrated features help to improve productivity, reduce errors and save time. In addition, design principles such as having only one interface for all antenna measurement disciplines (e.g., near-field and far-field), reduces the time needed for training which, in turn, lowers costs. This paper explores how the implementation of such a user interface can be used as a paradigm for increasing efficiency in the field of antenna measurement and analysis.
We present a method for evaluating broadband absorber in a non-ideal testing environment. Using broadband, short impulse TEM horns, a frequency-rich spectrum (equivalent pulse length < 0.5 ns) iHuminates a sample of the material under test and the reflections are recorded. Unwanted reflections from the sample edges, room environment, antenna and other systematic events are mathematically removed by a combination of time gating, background subtraction and systematic deconvolution. The result is an estimate of the reflection characteristics of the center at the sample.
In the early 1990's, one of the Air Force's satellite ground communication systems was experiencing an unexpected failure-rate. This was due in a large part to the assumption that the communication system's RF front-end was a low maintenance item. As a result, the ground system's subarrays were succumbing to moisture problems. Over time, the Air Force developed a subarray repair process and improved the external moisture sealing of the arrays. However the inexperience of the initial maintenance crews in handling RF systems led to the deterioration of the interconnecting cabling that combined the individual subarrays into three larger arrays. This paper will provide a description of the ground satellite communication's RF system and chronicle the development of the repair process. The interconnecting RF cable phase-matching procedure development will be discussed, including the engineering process, repair team training, and the transferring of this workload to other organizations. The effect of the challenging field work environment on developing the phase-matching process will be discussed. Quantitative evaluations of the repair team's field efforts will be provided, showing the improvement of the team and process over time.
The purpose for this advanced development program was to design, fabricate and test a physically small, broadband, dual-polarized, phased array antenna aperture using Flare Notch elements. The array was designed to operate in the 4 to 18 GHz frequency spectrum, having a VSWR of less than 2:1 and capable of handling 10 watts per element. The array was configured with polarization diversity, essentially, dual cross elements are used which are excited in phase or out of phase depending on the application. One of the significant accomplishments of this research effort was the elimination of grating lobes and the reduction of the size of the elements. Another significant accomplishment is the feeding of dual flare notch elements with a broadband microstrip match network. The antenna elements were implemented using Rogers 4003 materials. Fabrication of the elements and assembly of the elements is being done in a configuration of two rows by twelve elements of which only eight elements are normally excited. The remaining elements are used as parasitics to support the desired radiation pattern. The research work is being done in support of the next generation of solid state broadband radiation systems presently under development for ECM applications.
During the last 6 years scientists at NIST have been focusing on radar cross section (RCS) measurements to improve RCS uncertainty analysis, and to develop new measurement and calibration artifacts and procedures. In addition, NIST has been asked to provide technical support to the DoD RCS self-certification effort. In this talk I review the technical accomplishments of the program, and will make suggestions for future research to improve RCS calibration and measurement technology. I will also present the structure of the certi fication process, and discuss NIST's role in the ongoing certification activities.
As a result of Government and Industry RCS Teaming, initial RCS range certification exercises are underway. One critical element of certification exercises is the modeling and characterization of error terms according to the unique properties and requirements of individual RCS ranges, and the development of a method for propagating these errors into overall RCS measurement uncertainty. Previously, we presented the statistical model for the case where errors are grouped into multiplicative and additive classes, as well as a robust methodology for the propagation of errors in both the signal space and RCS (dBsm) domains [1-3]. Initial data at the National RCS Test Facility (NRTF) RAMS site located in the White Sands Missile Range near Holloman AFB, NM, have been collected for range certification exercises. Preliminary analysis has been accomplished on certain dominant error terms for calibration uncertainty characterization only. A general approach [7] has been followed here, with the exception that multiplicative and additive error terms are treated separately. In addition, only variance effects are treated (not bias). This paper is a status of work in progress. The ultimate goal of this work is the full implementation of previously described concepts [1-3]. We plan to demonstrate an improved ability to capture the effects of both error bias and variance (as has been demonstrated mathematically to date) using a more complete set of data collections.
This is a report on work in progress to understand the wide variations in sphere calibration data observed on dynamic radar cross section measurement ranges. The magnitude of these fluctuations indicate an uncertainty of greater than 2 dB in some cases. The range of fluctuations in the received power (which is well beyond fluctuation due to received noise) underlines the need for a thorough understanding of sources of uncertainties in dynamic radar cross section measurements. In addition to the fluctuations, we observe a systematic error with respect to the mean of the data segments, possibly due to drift, pointing errors and I or target-background interactions. Understanding the error mechanisms in these measurements allows us to reduce the overall uncertainty and to improve data quality.
(U) Precise radar cross-section (RCS) calibration are needed for all RCS measurement facilities. In 1996, AFRL began to advocate the use of a series of precision, short cylinder RCS calibration standards, demonstrating consistently greater accuracy than traditional sphere targets. Previous AMTA publications [1,2,3,4] demonstrated the overall measurement fidelity of these targets. However, questions regarding the accuracy and stability of the numerical RCS solutions to these cylinders continue to be raised. This paper will strictly and thoroughly examine the accuracy of several numerical techniques used to predict the AFRL calibration cylinder RCS, and will examine such "real world" issues as gridding sensitivity, conductivity vanat1ons, frequency bandwidth, and practical manufacturing tolerances.
In order to have definitive measurement traceability according to, ANSI-Z-540, a radar cross section measurement facility must have solid traceability to a known and accepted measurement standard. The Air Force Research Laboratory choose short right circular cylinders as calibration standards for their facilities. We describe a general RCS uncertainty analysis technique, and apply the method to our calibration standards to establish absolute traceability to a known standard. Though applied to cylinders in the current paper, the uncertainty method is general enough for any arbitrary target