Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
This paper describes the alignment procedure for using a spherical near-field measurement facility to determine incident fields throughout a spherical volume. This information can be used, for example, to characterize an anechoic chamber or the quiet zone of a compact range. A probe is mounted on a standard roll-over-azimuth positioner and aligned looking out of the sphere so its aperture maps out the surface of a sphere. The probe measures the amplitude and phase of the fields incident on the sphere. This method differs from the standard spherical near-field measurement where the source antenna serves as the probe and is looking into a sphere containing the test antenna.
MI Technologies has developed a technique to achieve very high accuracy cross-polarization measurements using a single reflector compact range. The technique, known as the "Error Correction Code Algorithm" (ECCA) leverages the "ideal" performance of a single parabolic reflector when the feed axis is aligned to the parabola axis. ECCA mathematically corrects for the amplitude taper induced by the feed axis alignment. Historically, 'conventional' compact range polarization purity has been limited to »-30 dBi. The ECCA technique, however, lowers the cross-polarization error to »-48 dBi. This performance has been verified in two separate inter-range measurement comparisons with the National Institute of Standards and Technology. The results of these tests prove ECCA is an extremely accurate technique for low cross polarization measurements and provides a lower cost, superior performance alternative to dual reflector systems when low cross-polarization measurements are required.
Papers were presented at the last two AMTA meetings reporting on the effect of rotator system alignment on the results of spherical near-field measurements. When quantifying the effect of non-intersection errors on the AUT directivity, these two papers presented very different results. One AMTA paper 1 and an earlier study at The Technical University of Denmark 2 found that the directivity error was extremely sensitive to non-intersection errors while the other AMTA paper3 found a very small sensitivity. During the past year, scientists at the Technical University of Denmark, The National Institute of Standards and Technology, and Nearfield Systems Inc. have been working together to determine the reasons for these differences. It now appears that the major reason for the difference is due to the method used to acquire data on the sphere. Theta scans that pass through the pole, or equivalently, phi spans of 180 degrees, produce both plus and minus phase errors that tend to cancel in the on-axis direction. Theta scans that do not pass through the pole, or equivalently phi spans of 360 degrees, produce phase errors of the same sign over the sphere which are concentrated in the on-axis direction. Examples of measurements and recommendations for using this information in spherical measurements will be presented.
Traditional measurement methods assume that very accurate antenna to range alignment of the antenna under test (AUT) is convenient or possible. It has recently been shown that the use of non-rectilinear co-ordinate systems are of particular use for the purpose of correcting antenna to range misalignment. Additionally, this misalignment correction can be used to construct an extended composite measurement plane from a series of mis-aligned scans that themselves can be considered as constituting a polyhedral measurement surface. This paper describes the additional processing that is required to yield corrected near and far field data from an acquisition of a mis-aligned AUT. This technique is then illustrated with example results. The agreement of the corrected results is determined via the application of image classification techniques which correlate antenna patterns in a reduced vector pattern space in terms of their overall global features.
Operationally active hangers are not well suited for making wholebody RCS measurements for aircraft signature diagnostics. While it is much more feasible to make localized regional or zone measurements in a hanger, the utility of such data for determining overall signature growth has significant limitations. The most obvious limitation is not having all the information necessary to re-assemble the wholebody signature. In this paper we present some discussion and experimental results which explore the limiting factors associated with estimating an entire aircraft signature from localized regional (zone) measurements. An example will be shown where zonal measurement data is inserted into a reference image and then reconstructed to form two-dimensional frequency vs aspect angle RCS. It is shown that a precise coherent alignment of the zone image with the reference wholebody image is not necessary and that only a coarse incoherent alignment is needed if only RCS statistics are desired. This is an important finding which leads to conclusion that it is logistically feasible to make zonal measurements and reconstruct a wholebody RCS estimate for impact analysis.
The maintenance, test, and repair workload for the Air Force's AN/MSQ-118 satellite ground-based receiving communication system has been transferred from the closing McClellan Air Force Repair Facility in Sacramento, California to Tobyhanna Army Depot located in Tobyhanna, Pennsylvania. The workload requires the support of four maintenance shops and two planar near-field ranges. The shops are the antenna repair, power supply repair, low-noise amplifier (LNA) repair, and radome repair shops. The near-field ranges are a 4' x 4' planar scanner used for antenna diagnostics and an 8' x 8' planar scanner used for certification of the repaired antenna-under-test (AUT). This paper will bring the AMTA community up to date on the status of the new Tobyhanna Antenna Repair Facility, focusing on the techniques and methods used to quantify the alignment and performance characteristics of the planar near-field antenna measurement system used for certification. With the relocation complete, test data obtained at both locations will be analyzed and compared to show differences between the baseline measurements taken at McClellan Air Force Base versus those taken at Tobyhanna Army Depot.
In-situ pattern measurement of JHU/APL's 60-foot parabolic reflector antenna (S-band), using a low-earth orbit satellite as the source is described. The signal strength and X and Y tracking error voltages are measured as the antenna dish sweeps a matrix of points around the position of the moving satellite. The swept region is approximately ±0.30° from the antenna's boresight. This technique was evaluated during April 1998. This measurement was used to baseline the current performance of the ground station before the feed underwent significant modifications. Before the new feed assembly was installed, the position of the current feed was translated to the new feed assembly. Once installed the performance of the reflector was verified. Misalignment of the feed broadens the main beam and increases the sidelobes. More importantly, the inclusion of new components inside the feed also has the potential to introduce phase errors onto the tracking signals. These phase errors will be translated by the auto-track electronics into pointing errors causing the antenna system to inaccurately follow a target. This paper describes the measurement of the reflector antenna pattern and tracking pattern before the new assembly was installed. Results of pattern measurements with the new assembly will be presented at the conference
Concise mathematical relations have been derived for Planar Near-Field measurements that quantify the effects of x, y and z-position errors on antenna parameters such as gain, sidelobe level, pointing, and cross polarization. Because of the complexity of the theory, similar relations for spherical near-field measurements have not been developed. The requirements for the spherical coordinate system are generally defined in terms of the alignment parameters such as orthogonality and intersection of axes, q-zero, x zero and y-zero rather than individual errors in q , f and r. Mechanical, optical and electrical techniques have been developed to achieve these alignments. This paper will report on the development of methods to estimate the antenna parameter errors that will result from spherical alignment errors for typical antennas.
DATE is a portable, rapid assembled, planar near field measurement system for ERIEYE Airborne Early Warning System. DATE shall be used both as a production range at Ericsson Microwave Systems (EMW) and as a maintenance equipment delivered with the ERIEYE AEW System. Up to now ERIEYE has been measured and phase aligned at EMW's large nearfield range. The active antenna is interfaced through a Beam Steering Computer (BSC) and hardware interface. The disadvantages with this approach is a slow communication speed and reduced Built In Test. Since the large nearfield range is designed to meet the requirements from many different antenna types the transport, mounting, alignment and range error analysis are very time and personnel consuming. The DATE-scope is to provide a portable planar near field test system that's custom-made for ERIEYE. The time from stored system to completed measurement shall be very short and performed by a "non antenna test engineer". This is done by: • Incorporate the BSC as a radar-mode. • Use the radar receiver and transmitter for RF measurement. • Reduce alignment time and complexity by a common alignment system for antenna and scanner. Scanner alignment for very high position accuracy. • Automatic Advanced Data Processing: Transformation from near field to far field to excitation to new T/R-module setting-up-table in one step.
Probe correction is required to accurately determine the far-field pattern of an antenna from near-field measurements. At Raytheon Primary Standards Laboratory (PSL) in El Segundo, CA, data acquisition hardware, instrument control software, and a mechanical positioning system have been developed and used with an HP Network Analyzer/Receiver system to perform these measurements. Using a three antenna technique, the on-axis and polarization parameters of a linearly (or circularly) polarized probe are calibrated. The relative far-field pattern of the probe is then measured utilizing the two nominal, orthogonal polarizations of the source antenna. All measurements are stepped in frequency and use a time domain gating technique. The probe and the source antenna are optically aligned to the interface and unique, kinematic designed interface flanges allow repeatable mounting of the antennas to the test station.
The backtransformation in (planar) Near Field processing is often claimed to be a very powerful tool for antenna diagnostics. Less known is a kind of defocusing effect which is introduced by the processing. Selecting the visible space in the Far-Field domain has a similar effect as a bandfilter in the frequency domain of an electric signal. In that analogous case it is better known that after the transform to the time domain, one has to deal with sin(x)/x behavior, limiting the resolution. The mathematics and convolution effects of both the onedimensional time-frequency transform as the two dimensional Near-Field Far-Field transform will be explained. Some measurement procedures are proposed, including S/N requirements. It turns out that the back transformation technique has some nasty properties which limit the use for alignment purposes. Some alternatives are discussed.
Calibration standards for radar systems are being developed cooperatively by NIST and DoD scientists. Our goals are to develop standard procedures for polarimetric radar calibrations and to improve the uncertainty in the estimation of system parameters. Dihedrals are excellent polarimetric calibration artifacts, because (1) the consistency between dihedral scattering data and the mathematical model of scattering can be easily verified, and (2) symmetry properties of the dihedral data provide powerful diagnostics to reveal system problems. We apply Fourier analysis to polarimetric data from dihedrals over a full rotation about the line of sight to reduce the effects of noise and clutter, misalignment, and other unwanted signals. An extension of the analysis to satisfy nonlinear model constraints allows us to monitor data quality and to further improve the calibration. We obtain the system parameters from the Fourier coefficients of the data in a simple manner. We illustrate these concepts using polarimetric radar cross section calibration data obtained as part of a national interlaboratory comparison program.
Nearfield Systems, Inc. (NSI) has delivered the world's largest vertical near-field measurement system. With a 30m by 16m scan area and a frequency range of 1GHz to 50GHz, the system consists of a robotic scanner, laser optical position correction, computer and microwave subsystems. The scanner and microwave equipment are installed in an anechoic chamber 40m in length by 24m in width by 25m in height. The robotic scanner controls the probe positioning for the 33m by 16m vertical scanner using X, Y, Z and polarization axes. The optical measurement package precisely determines the X and Y axes position, alignment errors along the X and Y axes, and Z-planarity over the XY scan plane.
During the design of spacecraft antennas a well defined geometrical configuration of antenna components is supposed. Also the requirements for the accuracy of the antenna integration normally will be given. The antenna alignment processes have to ensure, that the designed configuration with the required accuracy can be met. Additionally the antenna pointing has to be determined with respect to the RF measurement facility. In this paper the concepts are treated, how to determine the actual and the designed orientation and location of the components of the space antennas during subsystem and system level integration and tests. This includes also the definition of needed references for the antenna components, the creation and application of coordinates or orientation matrices at manufacturing or integration level, the used coordinate systems and the attainable accuracy for different methods. For the evaluation of the RF pattern performance, the correlation between the spacecraft coordinate system and the facility coordinate system has to be known. Basic principles of this pointing alignment and an error analysis of the measurement accuracy will be explained. The presented concepts are based on the experience at DSS' test facilities with various antenna types and agreed with different antenna manufacturers and customers.
ORBIT/FR designed and manufactured a plane wave scanner of unprecedented accuracy. It was delivered to Intespace in Toulouse, France, to verify the compact range quiet zone performance of the compact range system installed by Dornier Satellitensysteme GmbH. The design is of the plane polar type. The linear axis has an accurate travel range of 5.5 meters with additional acceleration and deceleration ranges. The polar axis has a travel range of over 180 degrees, so that a full circular plan of 5.5 meters in diameter can be evaluated. The mechanical overall planarity is better than ± 80 micrometers peak to peak. This is equivalent to ± 3.8° phase at 40 GHz. Special attention was given to the design of the RF cable track. A maximum phase variation equal to the mechanical accuracy was specified. However, no phase variation was noticed due to cable movements, even at 40 GHz. A new application for this scanner was to verify the actual boresight of the plane wave in both normal and so-called scanned boresight applications (compact range feed moved out of the focal point). For this purpose, the scanner was equipped with an optical mirror cube. Overall system alignment accuracies of 0.01° were typically achieved.
The DREO-DFL Antenna Research Lab (DDARLing), contains far-field and planar near-field antenna measurement ranges. Measurements can be made on both ranges from 1.0 to 62.5 GHz. In the early implementation stages of our antenna measurement ranges, most of our energy was absorbed in mastering the mechanics of the positioners and the intracies of the operation of the software, and addressing component failures. To make useful measurements, it is necessary to minimize system errors. Early experience and frustration has led us to the development of an ordered series of standardized procedures that are aimed at careful set-up, calibration, and operation of the ranges. Within these procedures, attention is paid to the identification and minimization of errors due to alignment, equipment calibration, linearity, leakage, multipath, and drift. Following a brief description of the two ranges in the DDARLing facility, the paper provides details of one of these procedures.
The mechanical rotator must be correctly aligned and the probe placed in the proper location when performing spherical near-field measurements. This alignment is usually accomplished using optical instruments such as theodolites and autocollimators and ideally should be done with the antenna under test mounted on the rotator. In some cases it may be impractical to place the alignment mirrors on the AUT or optical instruments may not be available. In these and other cases, it is desirable to check alignment with electrical measurements on the actual AUT and probe. Such tests have recently been developed and verified. Appropriate comparison and analysis of two near-field measurements that should be identical or have a known difference yields precise measures of some rotator and probe alignment errors. While these tests are independent of the AUT pattern, judicious choice or placement of the antenna can increase the sensitivity of the test. Typical measurements will be presented using analysis recently included in NSI software.
Scientific-Atlanta has recently begun work on a large 55 ft.(W) x 45 ft.(H) compact range reflector. The reflector is a Model 5738 with a 45 ft. focal length and a 38 ft. diameter by 38 ft. long cylindrical quiet zone. Due to the large size of the reflector, it is necessary to form the surface as several large, independent sections and assemble and align the reflector at the installation site. The 5738 reflector is shown in Figure 1 with the 38 ft. quiet zone superimposed. Figu re 1. Front View of 5738 Reflector Showing Sections The independent and predictable behavior of large sections proves to be very beneficial for performing an electrical alignment of the reflector based on field probe phase data. This paper discusses the required alignment tolerances and analytic tools developed to predict the effects on quite zone performance due to alignment errors in the sections of the reflector.
The DTU-ESA Spherical Near Field Antenna Test Facility in Lyngby, Denmark, which is operated in a cooperation between the Danish Technical University (DTU) and the European Space Agency (ESA), has for an ex tensive period of time been used for calibration of Standard Gain Horns (SGHs). A calibration of a SGH is performed as a spherical scanning of its near field with a subsequent near-field to far-field (NF-FF) transformation. Next, the peak directivity is determined and the gain is found by subtracting the loss from the directivity. The loss of the SGH is determined theoretically. During a recent investigation of errors in the measurement setup, we discovered that the alignment of the antenna positioner can have an extreme influence on the measurement accuracy. Using a numerical model for a SGH we will in this paper investigate the influence of some mechanical and electrical errors. Some of the results are verified using measurements. An alternative mounting of the SGH on the positioner which makes the measurements less sensitive to alignment errors is discussed.
For frequencies above 30 GHz, RCS reference target method is, in general, more accurate than scanning the field by a probe. Application of mechanically calibrated targets with a surface accuracy of 0.01 mm means that the phase distribution can be reconstructed accurately within approximately 1.2 degrees across the entire test zone at 100 GHz. Furthermore, since the same result can be obtained for both azimuth and elevation patterns, all data is available for the characterization of the entire test zone. In fact, due to the fact that the reference target has a well known radar cross-section, important indication of errors in positioning can be obtained directly from angular data as well. In the first place the data can be used in order to recognize the first order effects (+/- 5 degrees in all directions). Applying this data, defocussing of the system reflector or transverse and longitudinal CATR feed alignment can be recognized directly. Furthermore, mutual coupling can be measured and all other unwanted stray radiation incident from larger angles can be recognized and localized directly (using timedomain transformation techniques). Inmost cases even a limited rotation of +/- 25 degrees in azimuth and +/- 10 degrees in elevation will provide sufficient data for analysis of the range characteristics. Finally, it will be shown that sufficient accuracy can be realized for frequencies above 100 GHz with this method.