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Full polarimetric scattering measurements are increasingly being required for radar cross-section (RCS) tests. Conventional co-and cross-polarization calibrations fail to take into account the small amount of antenna cross-polarization that will be present for any practical antenna. In contrast, full polarimetric calibrations take into account and compensate for the cross-polarization the calibration process. We present a full polarimetric calibration procedure and a simulation-based performance study quantifying how well the procedure improves measurement accuracy over conventional independent channel calibration.
The results of theoretical calculations or measurements for antennas are generally given in terms of the vector components of the :radiated electric field as a function of direction or position. Both the vector components and the direction parameters must be defined with respect to a coordinate system fixed to the antenna. Along the principal planes there is no ambiguity about the terms such as vertical or horizontal component, but off the principal planes the definition of directions and vector components depends on how the spherical coordinate system is defined. This paper will define four different spherical coordinates that are commonly used in measurements and calculations, and propose a terminology that is useful to distinguish between them, and define the mathematical transformations between them. These concepts are essential when the results of different measurements or calculations are compared or when an antenna's orientation is changed. Both mathematical and graphical representations will be presented.
The use of dual polarization in meteorological radars offers significant advantages over single polarization. Recently a standard single-polarization Cuband radar was upgraded to operate in dual-polarization mode. The antenna has a 4.2m diameter parabolic reflector with a prime-focus feed. A spherical Fresnel-zone holographic technique was used to obtain the radiation pattern for the upgraded antenna. The sidelobes were higher than predicted and so the data was analyzed to identify the relative contributions of shadowing from the feed crook and surface errors in the dish. This paper describes practical considerations in the measurement of this antenna and the analysis of the results.
Three common methods of measuring circularly antennas on a far-zone range are: using a spinning linear source antenna (SPIN-LIN), measuring the magnitude and with a linearly polarized source antenna in two orthogonal positions (MAG-PHS), and using a circularly polarized source antenna (CIRC-SRC). The MAG-PHS and CIRC-SRC methods are also used in a near-field or com pact range. The SPIN-LIN method is useful because an accur te measurement of the axial ratio and gain can be made without the need to measure phase. The MAG-PHS method is the most general method and can also completely characterize the polarization of the test antenna. The CIRC-SRC method is the simplest and least time-consuming measurement if the antenna response to only one polarization is needed. The choice of measurement method is dictated by schedule, accuracy requirements, and budget. An analysis is presented that provides errors in the measured gain, relative gain pattern, and phase of the test antenna depending on the polarization characteristics of the source and test antennas. These results are useful for deciding which measurement method is the most appropriate to use for a particular job. These results are also useful when constructing more complete error budgets.
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.
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.
Calibration of monostatic radar cross section (RCS) has been studied extensively over many years, leading to many approaches, with varying degrees of success. To this day, there is still significant debate over how it should be done. It is almost a certainty, that if someone proposes a way to calibrate RCS data, someone else will come up with reasons as to why the "new" approach will not yield results that are "good enough." In the case of full scattering matrix RCS measurements, the lack of information concerning calibration techniques is even greater. The Air Force's Radar Target Scattering Facility (RATSCAT) at Holloman AFB, NM,has begun an effort to refine monostatic and bistatic cross polarization measurements at various radar bands. For the purposes of this paper, we have concentrated on our monostatic cross polarization developments. Such issues as calibration targets and techniques, system stability requirements, etc. will be discussed. During several programs we have attempted to collect sufficient data to do full scattering matrix corrections. In a previous paper, "Bistatic Cross-Polarization Calibration," our collected data had a high background which obscured much of the cross polarized return. The data presented here is from a program conducted at RATSCAT recently which utilized the Ka band. Because of the sensitivity of measurements at Ka to many effects, an error estimate was required. This paper presents this error estimation and some results of full scattering matrix correction of RCS data. This analysis is based upon "The Proposed Uncertainty Analysis for RCS Measurements", NISTIR 5019, by R. C. Wittmann, M. H. Francis, L. A. Muth and R. L. Lewis. This paper was aimed at principle pole measurements, e.g. HH and VV. The tabular data presented in the paper are from this paper with additions for errors associated with cross polarization and cross polarization correction.
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.
Offset parabolic reflector Compact Ranges are limited for cross polarization measurements in comparison to compensated dual reflector systems. This means that, in some cases, the crosspolar measurements at low levels show a significant content of the compact range reflector cross polar. An investigation has been carried out at INTA to reduce the crosspolarization measurement errors levels to those of a compensated dual reflector system by the application of vector deconvolution techniques. Results are shown of the validation of the algorithm in a far-field range where a crosspolar field is introduced by depointing the transmitter antenna.
Large Compact Ranges for test zone sizes of 6 meters or can be used for both payload or advanced antenna and RCS testing. In order to determine the range accuracy, test zone field evaluation is required. For physically large test zone dimensions, scanning of the test-zone fields is difficult and impractical in most situations. Furthermore, the accuracy of planar or plane-polar scanners is usually not sufficient for applications above 10 GHz. An alternative approach is the RCS reference target method where the test zone field is derived from the RCS measurement of a flat plate. Such a target can be manufactured as a single sheet aluminium honeycomb structure with rectangular or circular cross section. Reference targets with large dimensions and high surface accuracy are available. Consequently, test-zone fields can be accurately determined for test zone diameters up to about 10 meters and frequencies up to 100 GHz. In this paper the application of this method will be demonstrated at the Compact Payload Test Range (CPTR) at ESA/ESTEC. Large rectangular plate has been used for field determination within a test-zone of 5.5 meters. A 2 meter diameter circular flat plate has been used to map the residual cross-polarization level within the test zone. It will be shown that valuable information about range performance (amplitude, phase and cross-polarization) can be accurately retrieved from the RCS measurements
A slot spiral antenna and its associated feed are presented for conformal mounting on a variety of land, air, and sea vehicles. By exploiting the inherent broadband behavior good pattern coverage and polarization diversity of the spiral antenna, a conformal antenna which can be concurrently used for cellular, digital personal communications (PCS), global positioning (GPS) and intelligent vehicle highway systems (IVHS) as well as wireless LAN networks has been developed. A key requirement for achiev ing such broadband behavior (800-3000MHz) is the avail ability of a broadband planar feed and balun. Such a feed was proposed last year by the authors. However, addi tional design improvements were found to be necessary to achieve satisfactory pattern and gain performance. Among them were a broadband termination for the spiral arms and the suppression of cavity and waveguide modes. Both of these improvements played a critical role in achieving acceptable performance over the 800-3000 MHz bandwidth. After a general description of the slot spiral antenna and the above modifications, this paper presents a comparison of the performance before and after the modifications.
Andrew Corporation, founded in 1937 and headquartered in Orland Park, Illinois, has evolved into a worldwide supplier of communication products and systems. To develop a superior, high performance line of base station products for a very competitive marketplace, several new antenna measurement systems and upgrades to existing facilities were implemented. This engineering project developed an indoor test range facility incorporating design tool advantages from among Andrew Corporation's other antenna test facilities. This paper presents a 22-foot vertical by 5-foot diameter cylindrical near-field measurement system designed by Nearfield Systems Incorporated of Carson, California. This system is capable of measuring frequencies ranging from 800 MHz to 4 GHz, omnidirectional and panel type base station antennas up to twelve feet tall having horizontal, vertical or slant (+/- 45 degree) polarizations. Far-field patterns, near-field data and even individual element amplitude and phases are graphically displayed.
This paper presents a design of dual shaped reflector feed system suppressing cross polarized components for compact antenna test range(CATR). This system consists of a parabolic main reflector, two shaped reflectors and primary horn. As for co-polarization characteristics, these subreflectors are shaped to achieve a plane wave with uniform amplitude in a test zone. As for cross polarization characteristics, cross polarized components are eliminated in following way. An initial reflector system before shaping is satisfied with a condition of eliminating cross polarized components based on a beam mode expansion technique. This condition needs more than three quadratic reflectors, and frequency independent design can be derived. In this paper, the effect of higher order modes are considered. When shaping reflectors, however, additional cross polarized components are generated and the condition of eliminating cross polarized components is not satisfied. In this paper, a correction method of the cross polarization is also presented. A design result shows that the system has a test zone of 2.5m diameter and in test zone, lower ±0.5dB amplitude ripple, ±4.5° phase ripple and lower -44dB are achieved.
In this paper we will compare different techniques that can be used to measure the performance of automobile antennas. The use of indoor scale-model and outdoor full-scale range measurements will be discussed. These rangetype techniques characterize the engineering parameters of the antenna using signals with well defined polarizations and angles of arrival. These techniques are important in the initial stages of the antenna development. In the final stages of automobile antenna development, it is important to know how well the antenna will perform in the "real-world". We have developed mobile measurement techniques that use commercial off-the-air signals to characterize the performance of the automobile antenna in the real-world environment. We will describe three different systems that were developed to measure the performance of AM/FM, cellular, and GPS antennas.
Calibration of monostatic radar cross section (RCS) has been studied extensively over many years, leading to many approaches, with varying degrees of success. To this day, there is still significant debate over how it should be done. In the case of bistatic RCS measurements, the lack of information concerning calibration techniques is even greater. This paper will present the results of a preliminary investigation into calibration techniques and their suitability for use in the correction of cross-polarization errors when data is collected in a bistatic configuration. Such issues as calibration targets and techniques, system stability requirements, etc. will be discussed. Results will be presented for data collected in the C and X bands on potential calibration targets. Recommendations for future efforts will also be presented.
The RATSCAT radar cross section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and customer desires for certified RCS data. This paper discusses the low frequency characterization of the RATSCAT VHF/UHF Measurement System (RVUMS). The characterization was conducted on a portable pit with a 30' foam column at the RAMS site. System noise, clutter, backgrounds and generic target measurements are presented and discussed. Potential error sources are examined. The use of background subtraction and full polarimetric calibration are presented. Potential errors, which can occur from using certain cross-pol calibration techniques, are discussed. The phase relationship between each polarization components of the scattering matrix and cross-pol validation techniques are considered.
State-of-the-art range design requires that the feed antenna possesses key features including ultra wide band operation, stable beamwidth, stable phase center and versatile polarization capability. Traditional ultra wide band antennas such as the dual-polarized quadridged horn has versatiles polarization ability; however, the radiation beamwidth, which is dictated by the ridge structure, is not constant. Current development of the R-card version of the Slotline Bowtie Hybrid (Rcard-SBH) antenna possesses all the required features except that it is limited to linear polarization. A novel dual-polarized antenna which can meet all these requirements is presented. The feeding structure is constructed using two pairs of coaxial lines with their outer conductors commonly grounded. Each pair is connected to an ultra wide band hybrid circuit (1-18 GHz) and forms a balun structure. The guiding structure is made of numerous radial wires that form an orthogonal bow-tie geometry. Note that with these wire structures, for each polarization, only two guiding plates are visible; while, the other side plates having wires orthogonal to the E-polarization are nearly invisible. By integrating the rolled edge concept into the guiding structure, for each polarization, this new dual-polarized antenna has similar performance as the conducting rolled edge SBH antenna developed earlier.
For indoor antenna measurements, compact ranges or near-field/far-field techniques are most frequently used. One of the major problems is the handling of physically large antennas. Compact ranges will in general provide test-zone sizes up to approximately 5 meters in diameter. Applying the planar NF/FF technique, even larger test-zone sizes can be realized for certain applications. On the other hand, requirement of real-time capability, for instance in production testing, will exclude NF/FF techniques. It has been shown previously that single-plane collimators have a pseudo real-time capability which makes these devices comparable to compact ranges. Furthermore, the physical test-zone sizes which can be realized when compared to compact ranges are approximately 2-3 times larger for the same size of the anechoic chamber. Finally, it will be shown that the accuracy in sidelobe level determination, gain and cross polarization is considerable higher than with other indoor techniques, even at frequencies below 1 GHz.
Multipactor breakdown is a possible payload failure for communication satellites. The multipactor phenomenon significantly increases noise levels interfering with signals relayed by a satellite; it can even damage or destroy RF components and transmission lines. In order to retire the risk of single point failure in a recent spacecraft C-band antenna suite, certain critical components were tested to determine the threshold of multipaction. The components tested were: waveguide isolators, a waveguide polarization switch, and a waveguide PIM filter. The need to test the components at higher power levels than had been attempted on previous multipaction tests required the development of a new test system. The system utilized ring resonant multiplication to develop pulsed power levels of +77 dBm. The multipaction global detection methods utilized a spectrum analyzer montoring the notched noise level, and peak power meters monitoring the arrier pulse shape for distortion. This paper briefly describes the test system developed.
Scientific-Atlanta has developed a new algorithm for obtaining high accuracy cross-polarization measurements from prime focus, single reflector, compact ranges. The algorithm reduced cross-polarization extraneous signals to levels that rival or exceed much more expensive dual reflector systems, but with the associated cost and simplicity of a single reflector system. This paper provides an overview of the new algorithm. It explains the limitations on conventional polarization measurements in single reflector systems and the methods for overcoming these limitations without error correction for some antennas. A method for determining if error correction is needed for a particular antenna is reviewed and the fundamentals of the error correction algorithm are explained. Preliminary test results are provided.