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.)
A new spherical near field facility has been recently implemented at the Electromagnetic Propagation Area of INTA. The facility makes use of an existing big anechoic chamber (12 x 12 x 12 m.) and the near field/fair field transformation software developed by TICRA. This range has been calibrated by measuring an offset reflector antenna and comparing the results with those obtained in previous measurements of this antenna in other European testing facilities of different types. An experimental study has been carried out to check the dependence of the transformation software on the scanning parameters and different misalignments have been produced in order to determine the impact of the mechanical deformations on the accuracy of the system.
As the accuracy of antenna range instrumentation improves, multipath on the range is becoming the key limitation in antenna metrology. A fundamental requirement to improving range performance is the accurate and repeatable characterization of scattering on a range. A promising technique for range characterization is the planewave spectral (PWS) range probe. Earlier papers have demonstrated the ability of the PWS probe to locate multiple scattering centers on a range. Of equal importance to the user is the ability to correctly assess the magnitude of the scattering centers. This paper presents the problem of spectral peak broadening due to phase curvature from localized scatterers. Methods for improving the accuracy of scattering center estimation are presented along with numerical studies of the performance of these methods.
Antenna measurement accuracy is often not addressed in a rigorous manner. A methodology for projecting antenna measurement accuracy is described together with some of the error components that limit measurement accuracy. Antenna measurement accuracy is approached through an error budget projection, which requires the first and second order statistics of the individual error sources. Typical error sources are described along with methods of obtaining the statistics required for the error budget.
This paper describes two signal processing techniques that have been used to overcome specific problems in a Lockheed Aeronautical Systems Corporation (LASC) indoor compact RCS measurement range. Both techniques are post processing techniques used to enhance the accuracy of RCS vs. Aspect measurements. These two techniques can speed up measurement time, increase measurement accuracy, and increase target sizes on a compact range.
The construction of a large reflecting surface is invariably a compromise between the technical requirements and what is economically achievable. During the past three years, the compact range team at Harris has learned a great deal about this process. While aligning and testing the Harris Model 1630/1640 Compact Ranges, we have gone through a long learning experience. This paper presents some of the results of that experience.
A new measurement technique that is used to measure the polarization properties of dual port, circularly polarized antennas is described. A three antenna technique is used, and high accuracy results are obtained for all three antennas without assuming ideal or identical properties. This technique eliminates the need for a rotating linear antenna, reduces the setup time when gain measurements are also performed, and reduces errors for antennas with low axial ratios.
In the last few years, the interest in millimeter wave systems, like radars, seekers and radiometers has increased rapidly. Though the size of narrow-beamwidth antennas in the 60-200 GHz range is limited to some 20 inches, an accurate far-field antenna test range would need to be very long. The achievement of precision antenna pattern measurements with a 70' or even longer transmission length requires the use of some power that is hardly available and expensive. A cost-effective and more accurate solution is to use a lab-sized compact range that presents several advantages over the classical so-called far-field anechoic chamber: - Small anechoic enclosure (2.5 x 1.2 x 1.2 meters) meaning low cost structure and very low investissement in absorbing material. No special air-conditioning is needed. This enclosure can be installed in the antenna laboratory or office. Due to the small size of the test range and antennas under test, installation, handling and operation are very easy. For spaceborne applications, where clean environment is requested, a small chamber is easier to keep free of dust than a large one. - The compact range is of the single, front fed, paraboloid reflector type, with serrated edges. The size and shape of the reflector and serrations have been determined by scaling a large compact range of ESD design, with several units of different size in operation. The focal length of 0.8 meter only accounts in the transmission path losses and the standard very low power millimeterwave signal generators are usable to perform precision measurements. The largest dimension of the reflector is 1 meter and this small size allows the use of an accurate machining process, leading to a very high surface accuracy at a reasonable cost. The aluminum alloy foundry used for the reflector is highly temperature stable. - Feeds are standard products, available from several millimeter wave components manufacturers. They are corrugated horns, with low sidelobes, constant and broad beamwidth over the full waveguide band and symmetrical patterns in E and H planes. - The compact range reflector, feeds and test positioner are installed on a single granite slab for mechanical and thermal stability, to avoid defocusing of the compact range. - A micro-positioner or a precision X Y phase probe can be installed at the center of the quiet zone. Due to their small size, these devices can be very accurate and stable. Due to the compactness of this test range, all the test instrumentation can be installed under the rigid floor of the enclosure and the length of the lossy RF (waveguide) connections never exceeds 1 meter.
In an earlier paper the virtual vertex compact range reflector was introduced and data from a specific design was reported. This paper describes the extension of the vertical vertex serrated edge concept to other reflectors that serve a wider range of application. Two new 12 ft focal length reflectors have been built that possess 3 ft and 6 ft diameter symmetric test zones. We describe the electromagnetic considerations and the mechanical design approach that has been used for these reflectors. We demonstrate the performance with field probe data showing the excellent surface accuracy of these units.
Measured component RCS results are frequently dominated by the test body and target mounting structures. This paper will present a measurement technique that will improve measurement accuracy using a less complex and expensive test body. The design of the test body and measurement geometry allows isolation in both range and cross range from the static return of the room and mounting structure. This is accomplished by first creating an ISAR image of the target and test body, gating the image in two dimensions, then transforming back into the frequency and spatial angle domains to determine the scattering levels of the target by itself. Details of this technique, covering both its advantages and limitations, will be discussed. Data will be presented to verify the approach and illustrate the level of performance attainable using this technique.
This paper examines the primary differences between gated-c.w. and pulsed-i.f. instrumentation radar systems. Following a brief explanation of the fundamental theory behind each radar type, a performance trade study is presented. The impact of i.f. bandwidth on the operation and performance of the radar is presented by first briefly describing the major similarities and differences between the two radar types and the resulting impacts on performance. Differences in the gate performance, sensitivity, dynamic range, speed, and accuracy are summarized. To show the performance advantages and shortfalls of each radar type, benchmark test scenarios are presented. The resulting summary can be used as a guide in determining the optimal radar type for a specific range geometry and measurement requirement.
Precise and complete measurements of advanced electromagnetic systems demand dramatically higher data acquisition speeds than those commonly attainable. Specific challenges include requirements for wideband measurements with arbitrarily spaced frequency steps. These types of measurements are often encountered in characterizing EW/ECM systems, radars, communications systems, and in performing antenna and RCS measurements. The Scientific-Atlanta Model 1795 Microwave Receiver offers capabilities directly applicable to solving measurement problems posed by highly frequency agile systems. These problems include: 1) timing constraints 2) data throughput 3) RF interfacing 4) maintaining high accuracy A technique is discussed which shows the application of the Model 1795 Microwave Receiver in its high frequency agility mode of operation. Measurement examples are presented showing the advantages gained compared to previous methods and instrumentation configurations.
Large arrays require large separations between the transmit antenna and the antenna under test (AUT) to measure pattern parameters in the far field. For the subject AUT, a range of 6 miles with a spurious signal level of -58 dB was necessary to obtain the required accuracy. Measurements have been performed on a significantly shorter range without serious degradation. The antenna was focused for the angle of electronic scan and the resulting pattern measured. The theoretical far field patterns were compared with the calculated focused patterns for the short range. The maximum sidelobe error of 1/2 dB occurred at 60 degrees scan. There was no noticeable degradation in beamwidth, gain, or foresight at any scan angle. A 6-mile range would have produced a 2-dB sidelobe error. The measured range reflection level was -50 dB. The transmit dish with sidelobes of 22 dB was replaced with an array that had 40 dB sidelobes. This change reduced the reflections to below the required -58 dB. The antenna was focused using a range calibration technique and the measurements substantiated the theory.
The purpose of this paper is to discuss the accuracy requirement of a generic measurement system for in-flight antenna pattern evaluations. Elements of the measurement technique will be described. An attempt is made to distinguish the measurement requirement for a narrow beam radar antenna in contrast to that for broad beam communication antennas. Major elements of the measurement technique discussed include the flight path geometry, the multipath propagation problem, and the measurement errors. Instrumentation requirements consist of the ground segment, the receive and the tracking subsystems, and the airborne equipment, the radar components and the navigation and attitude sensors. Considering the in-flight antenna pattern testing as a generalized antenna range measurement problem, various sources of measurement errors are identified. An error budget assumption is made on each error component to estimate the overall expected accuracy of the in-flight antenna pattern measurement.
Ray Proof has recently completed the construction of a shielded anechoic chamber in the Air Force Anechoic Facility at Edwards Air Force Base in California. Measuring 250 feet by 264 feet x 70 feet high, it is believed to be the largest anechoic chamber in the world. The facility will be used for EW testing of full-scale aircraft such as the B-1 B and B2 and will be operated for the Air Force by Rockwell International, the prime contractor for the project. This paper discusses parameters, statistics, and design features. The shielding was designed and quality controlled during construction in order to meet the NSA 65-6 specification, modified to extend to 18GHz. Layout of pyramidal anechoic material, varying from 12 inches to 24 inches in thickness with 36 inch around lighting fixtures, was designed to meet a return loss specification of 72 dB at 500 MHz, and up to better than 100 dB in the 3-18 GHz region. The chamber features a sliding pocket door 200 feet long and 66 feet high. To meet the stringent NSA 65-6 requirement, a threefold inflatable-bladder/ fingerstock seal was used around the door. The other feature of the chamber is an 80 foot turntable with a separately shielded control room suspended beneath. The table can rotate a 250,000 pound load through plus-or-minus 190 degrees, positioning to an accuracy of plus-or-minus 0.1 degree. A number of innovative procedures such as locating a portable factory to manufacture the absorber near the construction site enabled Ray Proof to complete and test the chamber ahead of schedule.
An RCS measurement error model, calibration procedure and correction algorithm are discussed. A distinction between frequency response reflections and range-target reflections is made. Special emphasis is placed on the selection of the gate span with time gating used with the calibration and test target measurements. Mathematical simulations and actual measurements illustrate the discussion. It is concluded that frequency response related reflections must and range-target reflections must not be included in the gate for the frequency response calibration measurement.
For transforming a Fresnel region pattern to a far-field pattern, we present here two methods, the "discrete beam sampling" method (DBSM) and the "displaced beam" method (DBM), which allow an accurate characterization for both linear as well as circular antenna apertures. Both methods assume a simple Fourier transform relationship between the aperture field distribution and the far-field of the antenna. The Fresnel region field is then essentially perturbed by an aperture quadratic phase error assumed to exist because of the finite distance at which the field pattern is characterized. Numerical simulation and its results are presented to show the accuracy of the reconstructed far-field data. Finally, an error analysis is performed to show the sensitivity of the above two methods.
Implementing an antenna test range has traditionally been viewed as a major and costly undertaking, requiring significant long term facility planning, computer hardware interfacing, and software development. This paper describes a complete low cost, yet high accuracy portable near-field measurement system that was privately built for less than $2,000 and interfaced to a PC compatible computer. The design and operation of this system, including the scanner, microwave hardware, and computer system will be described. This system has since been extended into a commercial product capable of providing rapid and accurate measurements of small to medium size feeds and antennas within a small office or lab space at significantly lower cost than standard antenna test techniques. The system has demonstrated an equivalent sidelobe noise level of less than -50 dB, includes a probe corrected far-field transform and holographic back projections, and can output pattern cuts, contour plots, 3D plots, and grey scale images of antenna performance.
The National Institute of Standards and Technology (NIST) has developed a multi-axis controller and software data acquisition system that has improved probe position accuracies in near-field scanning. This extends the usefulness of the NIST planar near-field scanner to higher frequencies. This system integrates programmable power supplies into an existing planar measurement system with new software that controls the power supplies and the data acquisition. It provides the higher positioning accuracy required for millimeter wave measurements at a reasonable cost. This system uses the NIST planar near-field scanner's existing DC motors, computer and laser. The programmable power supplies are connected to the motors, with a separate power supply for each motor'a armature and a common power supply for each of the motor's field windings. This allows for concurrent movement in each axis and eliminates delays in switching between axes. Directional control, motor protection, and special software features are implemented by logic control.
There is often a need for a laboratory to make quick, inexpensive, and accurate measurements on individual absorber samples. Different types and sizes of absorber need to be quickly analyzed at several frequencies to determine which type best maintains or improves the facility's RF characteristics. The National Institute of Standards and Technology has devised an improved version of the Doppler shift method to measure the scattering levels of different sizes and types of microwave absorber. This technique is useful as an inexpensive and simple method for measuring individual absorber pieces with good accuracy and sensitivity. The system does not require a large anechoic facility nor a sophisticated measurement system for gating out background scattering. Reflectivity levels on the order of -80 dB can be measured and relative changes of 1 dB can be detected. Sample results for absorber with and without fire retardant salts and different sizes are presented.
The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited. This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence. Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.