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 4 foot by 4 foot near field planar scanner is used to evaluate the performance of a SA5751 compact range in CSIST. Using the far field patterns integrated from the scanned aperture fields, the coming directions of the clutters in the chamber can be determined. Often the clutter level is less than the side lobe level of the far field pattern, the scanned field is multiplied by a certain weighting function before integration to pop out the clutter signal. However the weighting method would broaden the main beam and hence clutters coming close along the reflected wave of the reflector are still can not be seen (sic). In this article, a method called main beam suppression, subtracting a constant filed (sic) on the scanned aperture, is introduced to solve this kind of problem and the result shows it serves well for finding those clutters hidden by the main beam and the side lobes nearer to it.
The proposed synthesis method allows the calculation of the diffraction figure in the focal plane of the compact range, starting from a field distribution in linear polarization over a plane in the Fresnel zone. Applying this method (in only one dimension) to the ideal near field of a FFOC compact range, a linear array is synthesized which can be extrapolated to a planar array feeder design; providing excellent features in the quite zone.
As part of an effort to provide improved measurement services at frequencies above 30 GHz, scientists at the National Institute of Standards and Technology (NIST) have completed development of a swept frequency gain measurement service for the 33-50 GHz band. This service gives gain values with an accuracy of ± 0.3 dB. In this paper we discuss an example measurement and the associated errors.
Microwave holography has proven to be a powerful technique for various evaluations, diagnostics, and RF performance improvements for large reflector antennas. The technique utilizes the Fourier Transform relation between the complex far-field radiation pattern of an antenna and the complex aperture field distribution. Resulting aperture phase and amplitude distribution data can be used to precisely characterize various crucial performance parameters, including panel alignment, subreflector position, antenna aperture illumination, directivity at various frequencies, and gravity deformation effects. The methodology of the data processing presented in this paper was developed at JPL and has been successfully applied to the NASA/JPL Deep Space Network (DSN) 34m beam waveguide antennas. The performance improvement of the antenna was verified by efficiency measurements and additional holographic measurements. The antenna performance was improved at all operating frequencies of the antenna (wide bandwidth improvement) by reducing the main reflector “mechanical surface” rms error to 0.43 mm. At Ka-band (32-GHz) the estimated improvement is 4.1 dB, resulting in aperture efficiency of 52%.
The measurement of cryogenic antennas poses unique logistical problems since the antenna under test must be embedding in the cooling chamber. In this paper, a method of measuring the performance o cryogenic microstrip antennas using the closed cycle gas-cooled refrigerator in a far field range is described. Antenna patterns showing the performance of gold and superconducting Ka-band microstrip antennas at various temperatures are presented.
Antenna amplitude and phase pattern measurements on combat ships and other large ships have typically relied on traditional methods which include circling a fixed buoy in the far field, tracking a shore-based transmitter with an optical device, or circling the subject ship with a smaller boat outfitted with a transmitter. These techniques required the measurement of many independent variables using less than precise methods to compute antenna patterns relative to the ship’s structure. Using the global positioning system to precisely locate the ship relative to the transmitter site location and combining this with the ship’s heading, antenna measurements can be accurately and quickly obtained. This paper will describe the traditional fixed buoy and optical follower techniques and contrast these against the more accurate and faster GPS antenna measurement technique.
Some proposed attitude control systems will require sub-millimeter position accuracy and GPS signals. Toward this end, two antenna parameters must be determined and optimized. These two parameters are phase center location and phase center stability. The phase center location is defined as the point whose ideal spherical phase front has the minimum RMS difference between itself and the measured phase data. Phase center stability is the effective movement of a GPS antenna’s phase center caused by the deviation of the radiated phase front from an ideal spherical phase front. The RMS difference between the ideal phase and the measured phase is a good measure of phase center stability. This paper describes a method of determining the phase center location of a wide-beam GPS antenna. Once the phase center is determined, the phase center stability throughout the coverage region (usually a hemisphere) is characterized. Finally, some sources of error are identified. Methods of minimizing the effects of these error sources are addressed.
Advanced airborne radar antennas will consist of ultra low sidelobe arrays of thousands of T/R modules and radiating elements. The detrimental effects of the aircraft structure on the antenna performance becomes increasingly important for ultra low sidelobe antennas will require large aperture, high fidelity antenna test facilities. In this paper, the major errors associated with measurement of an ultra low sidelobe antenna on the far field range are isolated and demonstrated by computer simulation. Data from measurements of a T/R module array on a scale model aircraft is provided to demonstrate typical sircraft effects on antenna performance.
This paper will show that it is possible to make bistatic measurements in a compact range environments using near field scanning. A test scanner is designed and operated. Criteria for the accuracy of positioning and repositioning are presented. Algorithms for the transformation of the raw data into bistatic far field calibrated RCS are presented. Examples will be presented where comparisons with theoretical bistatic sphere data are shown. Bistatc pedestal interaction terms will be demonstrated.
Radar Cross Section (RCS) measurements are typically made at linear polarizations (usually horizontal and vertical) and the transmit and receive polarizations are the same (co-polarized). In addition, however, it is sometimes desirable to measure the cross-polarized RCS of a target (i.e., transmit horizontal, receive vertical or vice-versa). A complete set of both co-and cross-polarized RCS of a target is called a scattering matrix. This paper describes the algorithm used for calibrating a scattering matrix measurement in the McDonnell Douglas Technologies Inc. (MDTI), Radar Measurement Center (RMC). Verification data collected at Ka band on various targets is included to validate the algorithm and implementing computer code.
The radar cross section (RCS) of a target depends on nature environment as well as many physical variables. The objective of a compact range is to exclude environmental effects on RCS measurements of a target. It is also true for time gated RCS measurements as well. RCS obtained in above manners is more suitable for a space borne than for a ground based target. The contribution from surrounding environment is an inseparable part of RCS for a ship, truck, bridge, and building. We need a suitable method to characterize RCS of a ground based target and its dependence on the environment. The uncontrollable natural change makes environmentally dependent RCS results difficult to compare for a ground based target measured at different time instants. A way to reduce the uncertainties induced from changes is to exhaust all possible RCS measurements before the change. A measurement of this kind is referred to as a concurrent RCS measurement, which in a sense is equivalent to take an optical picture of a rapidly changing object with a strobe light. The step frequency radar located at Chesapeake Bay Detachment of Naval Research Laboratory is such a radar, which is equivalent to at least 45 single frequency radars operating simultaneously from 2.0-18.0 Ghz. Last year, we briefly mentioned this radar in our presentation. We will make a detail discussion of this radar and its capability on concurrent RCS measurements.
This paper compare some of the features and capabilities of gated CW and pulse radars for RCS and imaging measurements. At the conceptual level, these two types of radars are very similar. The primary conceptual difference is that a pulse radar has a relatively high bandwidth receiver while a gated CW system has a relatively narrow bandwidth receiver. The measures of performance of an RCS and imaging system include sensitivity, measurement time, clutter rejection, dynamic range and accuracy. Other considerations such as inter-pulse modulation may be important in some cases. For some applications, typically where long ranges are involved, a pulse system has significant performance advantages. For many applications, the performance advantage of a pulse system is not significant, particularly when viewed in light of the large difference in cost. This is particularly true of Quality Assurance applications which are normally characterized by both short range and lower budgets. Typically, the price of a gated CW system is in the range of ¼ to ½ the price of a comparable pulse system. This paper discusses general similarities and differences in the fundamental operating characteristics of the two systems. Specific performance measures are discussed including system sensitivity, gate performance, clutter rejection, and measurement times. Other considerations such as pulse modulation are discussed. A summary of the various considerations is presented in order to give the reader an understanding of the applications for which a gated CW system is more appropriate.
A bistatic calibration technique for wide-band, full-polarimetric instrumentation radars is presented in this paper. First general bistatic measurement problems are discussed, as there are the coordinate systems, the definition of polarization and the bistatic scattering behavior of convenient calibration targets. In chapter two the new calibration approach is presented. The general mathematical and physical description of errors introduced in the bistatic system is based on the radiation transfer matrix. The calibration procedure is discussed for the application with a vector network analyzer based instrumentation radar. For verification purposes measurements were performed on several targets.
Computer codes for the computation of scattering are based on physical, mathematical, and numerical assumptions and approximations that impact the accuracy of the results in ways that are not obvious or quantifiable analytically. This paper stresses the usefulness of a concurrent measurement program to provide reliable RCS data for targets of special interest in establishing the range of validity of the various assumptions upon which a specific computer code is based. This in turn assists in developing “modelling guidelines” restricting the design of computer models for input to the code such that reasonable accurate results are likely to be obtained.
Two measurement systems for Radar Cross Section (RCS) measurements are described. One system employs propagation over a ground plane whereas the other system employs free space propagation in an anechoic chamber for target illumination. A comparison of measured data for different targets over a wide range of frequencies is presented. The measured data is also compared to RCS data computed using the Numerical Electromagnetics Code (NEC) computer program. The results may be useful for evaluating radar systems operating in the HF band of frequencies.
This paper considers the radar cross-section (RCS) simulation, measurement, and analysis of rotating structures found in today’s modern airframes. Addressed will be scattering characteristics from helicopter main and tail rotor systems; how these characteristics can be simulated, measured, and reduced to identify the individual scatterers withing the helicopter. The effect of radar system parameters on the scattered signal will also be discussed. Finally, actual RCS measurements from helicopters in flight wil be resented and analyzed using the above discussed techniques.
Edge and corner diffraction and non-planewave illumination both cause measured free space relativity data to deviate from the infinite sample/planewave result which is predicted when using the Transmission Line Methos (TLM) for planar surfaces. The amount by which each of the two factors perturbs the measured data depends on the measurement system used; compact ranges, near field focused antennas and far field antennas on an NRL arch are all susceptible to the effects of non-planewave illumination and perimeter diffraction. Perimeter diffraction is virtually eliminated in the case of a near field focused system or where the sample is semi-infinite; however, the truncated illumination inevitable yields additional angular planewave components. In a far field system, the quadratic phase variation at the sample surface is shown to cause significant errors in the depth of resonant nulls. A uniform illumination is required to accurately map the depth of resonant nulls, but the consequent perimeter diffraction causes errors in null position. Perimeter diffraction does not cause errors in the null depth providing the illumination in uniform.
Recently, the theory and computer programs on the Periodic Moment Method (PMM) for scattering from both singly and doubly periodic arrays of lossy dielectric bodies have been developed. The purpose is to design microwave wedge and pyramid absorber for low reflectivity so that one can improve measurements and/or reduce the size of the anechoic chamber. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated on the Cray Y-MP supercomputer at the Ohio Supercomputer Center. Through these studies, some wedge and pyramid absorber configurations have been designed, fabricated and tested in the OSU/ESL Anechoic Chamber. Very good agreement between calculations and measurements has been obtained. In the 1990 AMTA meeting, several wedge absorber designs and results for the TM case and normal incidence were presented. In this paper, the measured and calculated frequency responses of some experimental wedge designs, as well as an 8” and 18” commercial wedge and pyramid absorber panels will be reported for both TM and TE polarizations. Time domain responses will also be shown for both measurements and calculations.
A three-layer sandwich structure consisting of a plastic film-to-foam lamination is presented as a low-RCS alternative to structural foam. Structural foam with 1-2 lb/ft density is commonly used as a low-RCS material. However, its RCS per unit load per unit volume is not as low as that of a composite foam structure. Equations relating mechanical strength and RCS are simultaneously solved for maximum mechanical strength and minimum RCS in the limit of Rayleigh and resonance region material thicknesses. A result is that a three-layer foam sandwich beam can have superior mechanical strength compared with an identical all-foam beam and a reduced RCS. Specific results for an optimized sandwich with mechanical strength equal to that of a homogenous beam and minimum RCS are presented. Experimental data quantify mechanical strength and RCS for several foam-mylar sandwiches.
An alpha-beta-gamma (a-ß-?) tracking algorithm has been devised to improve the performance of a laser-references planarity control servo. GTRI is developing a field probe for the USAEPG Compact Range at Ft. Huachuca, Arizona. The probe scans a surface whose planarity is controlled by a servo. A reference plane is generated by sweeping a laser beam with a pentaprism. The beam is detected by a photodiode mounted with the probe. The servo nulls any error detected. The servo must correct dynamic errors in the presence of high frequency electronic noise and low frequency atmospheric scintillation. A control algorithm based on the alpha-beta-gamma tracker has been developed and tested by simulation. The algorithm and simulation results are presented.