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Polarization
Full Test-Zone Field Evaluation Using Large RCS Targets
S.C. Van Someren Greve,J. Lemanczyk, J. Reddy, L.G.T. van de Coevering, V.J. Vokurka, November 1998
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
Experimental Issues in Ultra Near-Field Measurements
D. Smith,M. Morgan, M. Parent, S. Browning, S. Love, November 1999
In this paper we discuss the experimental issues encountered in the measurement of the local electromagnetic fields in the reactive region of a scattering or radiating body using the NRL Near-Field Facility. Our investigations require high-resolution measurements, reaching spatial resolutions of small fractions of wavelength, high polarization sensitivity, and broad bandwidth. We present techniques currently successfully being used in these investigations. The complexity of the reactive zone fields imposes difficult requirements on probe designs. Currently, the NRL probes include coaxial and coplanar configurations. We will discuss their properties and characterization. Design trade-offs to reach spatial resolutions of one-tenth of a wavelength, bandwidths of several giga-hertz and limitations on polarization sensitivity will be addressed. We will correlate these observations with the results of the back-propagation algorithms being developed at the Naval Postgraduate School.
Satellite Payload Parameter Measurements in a Compensated Compact Antenna Test Range
J. Habersack,H. Kress, H-J. Steiner, W. Lindemer, November 1999
Modern Satellite Antennas and Payloads are characterized by a lot of physical parameters like e.g. Radiation Pattern, Gain, EIRP, Flux Density, Gff and PIM, whereas the available time frame for measurements is getting shorter and shorter. The DSS Compensated Compact Range (CCR) allows a time efficient measurement of all payload parameters with high accuracy under controlled environmental conditions. The CCR consists of two doubly curved reflectors, which prevent inherent cross-polarization and create a very high constant amplitude and phase distribution in the quiet zone with a very good scanning performance. Most of the payload parameters can be measured directly or have to be calculated from a set of measurement values. For the G/T measurement of active antennas a new method for the noise power measurement was established. This paper describes the principle test set-ups with the corresponding measurement techniques to improve the measurement accuracy. Error budgets will be presented for pattern and gain measurement.
Performance Evaluation of the Automated Field Probe System (AFPS)
M.C. Brinkmann,G.R. Whitley, T.L. Lane, November 1999
The Georgia Tech Research Institute (GTRI) under contract to the U.S. Air Force 46 Test Group, National Radar Cross Section Test Facility (NRTF) at Holloman AFB, NM, has designed and developed an Automated Field Probe System (AFPS). The AFPS operates as a one-way probe for evaluation of the electromagnetic field at the test zone and provides a mobile capability to rapidly, smoothly, and accurately probe the field at the various test-areas. The AFPS provides the ability to probe over an area as large as 40-ft x 40-ft all under computer control from the radar(s) while sweeping over 1-18 GHz and 34-36 GHz for both H and V polarization. The RF, phase reference, and control signals from the radar are transmitted to the AFPS over a microwave fiber optic link. This paper will describe the design and performance of the AFPS. Quick-look data products will be included in the presentation.
Design and Testing Techniques for Automotive Conformal Diversity Antennas
W. Villarroel,E.K. Walton, November 1999
The automobile antenna industry is facing two rapidly growing trends: (1) the incorporation of effective, low cost, AM/FM conformal antenna designs and (2) the antenna capability to handle diversity FM radio receivers. The development of techniques for testing automotive conformal diversity antennas' performance becomes necessary to evaluate and compare them. Testing techniques to obtain the antenna Input Impedance (Zin), Standing Wave Ratio (SWR) and Mismatch Loss (MML) as well as the azimuth gain patterns and the combined diversity signal (maximum of the diversity signals) are described. Experimental results for the Annular Slot Windshield Diversity Antenna using polarization diversity are shown. It is demonstrated that the Annular Slot Windshield Diversity Antenna can be used effectively to reduce multipath fading.
Evaluation of Scattering Level of TT&C Antennas with Geometrical Scale Modeling Technique
J.Y. Lee, November 1999
Omnidirectional antennas are typically used as Tracking, Telemetry and Command (TT&C) antennas for satellites. However, the omnidirectional patterns of TT&C antennas located on satellite structures are susceptible to substantial scattering and polarization mismatch loss, especially at the initial mission stage. Consequently, it is very important to properly evaluate the extent of these effects for each of the initial mission configurations. In this paper, measurement techniques to achieve proper evaluation of scattering level and polarization mismatch loss for TT&C antennas of NASA's Tracking and Data Relay Satellite (TDRS) are presented. The paper encompasses a test approach, a test procedure and test results. Application of these test techniques is essential to the TDRS TT&C antenna qualification program.
Low Cross-Polarized Compact Range Feeds
J.A. Fordham,J.H. Cook, November 1999
Compact antenna test ranges intended for low cross­ polarization antenna measurements require the use of feeds with polarization ratios typically greater than 40 dB across the included angle of the quiet zone as well as across the frequency band of interest. The design for a series of circular corrugated aperture feeds to meet these requirements is presented. The feeds are based on a circular waveguide OMT covering a full waveguide frequency band with interchangeable corrugated apertures to cover three sub-bands. In order to validate the design of this series of scalar feeds, high accuracy cross-polarization data was collected. The primary limiting factor in the measurement of the polarization ratio was the finite polarization ratio of the source antennas. A technique for correcting for the polarization ratio of the source is presented along with measured data on the feeds. The technique begins with the accurate characterization of the linear polarization ratio of the standard gain horns using a three antenna technique, followed by pattern measurements of the feeds, and ends with the removal of the polarization error due to the source antenna from the measured data. Measured data on these feeds is presented before and after data correction along with data predicted using the CHAMP® moment method software.
High Accuracy Cross-Polarization Measurements Using a Single Reflector Compact Range
C.A. Rose,J.H. Cook, November 1999
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.
TUD-ESA Standard Gain Horn Facility, The
J. Lemanczyk,J. Reddy, J.E. Hansen, N.E. Jensen, O. Breinbjerg, November 1999
The European Space Agency (ESA) began serious investigations into the implementation and exploitation of near field antenna testing techniques already in the early 1970s where all three near field measurement geometries were considered (1). Spherical near field scanning was selected by ESA as being the most promising alternative to even larger conventional outdoor ranges. In the meantime, work was underway at the Technical University of Denmark (TUD) on spherical wave theory and its application to near field antenna measurements (2,3). As work began under ESA contract to demonstrate the technique, the most important aspect, the transformation algorithm and software was developed allowing dual polarized probe pattern and polarization corrected spherical near field measurements to be implemented (4).
524 GHz Polarimetric Compact Range for Scale Model RCS Measurements, A
M.J. Coulombe,G. Szatkowski, J. Waldman, T. Horgan, W. Nixon, November 1999
A fully-polarimetric compact range operating at 524 GHz has been developed for obtaining Ka-band RCS measurements on 1:16th scale model targets. The transceiver consists of a fast switching, stepped, C W , X-band synthesizer driving dual X 4 8 transmitmultiplier chains and dual X 4 8 local oscillator multiplier chains. Software range-gating is used to reject unwanted spurious responses in the compact range. A motorized target positioning system allows for fully automated sequencing of calibration and target measurements over a desired set of target aspect and depression angles. A flat disk and a dihedral at two seam orientations are used for both polarization and R C S calibration. Cross-polarization rejection ratios of better than 45 d B are routinely achieved. The compact range reflector consists of a 1.5m diameter aluminum reflector fed from the side to produce a 0. 5 m diameter quiet zone. Targets are measured in free-space or on a variety of ground planes designed to model most typical grou nd surfaces. A description of this 524 GHz compact range along with 30 ISA R measurement examples are presented in this paper.
Precision Polarization Measurements of Circular Polarized Global Horn Satellite Antennas
J.D. Hazelwood,E. Carpenter, G.H. Schennum, R.W. Gruner, November 1999
This paper will describe the measurement series performed on Global Horn flight antennas to be used on the Intelsat IX satellite series. The work was performed by MEMCO under contract to Space Systems I Loral. The Global Horn antenna system provides highly isolated RHCP and LHCP beams that cover the earth disc, as viewed from synchronous orbit. The corrugated wall horn is designed to maximize the gain at the edge of earth coverage angle, which in this case is defined as plus or minus 9.8 degrees from the beam peak. The horn has near perfect E-and H-plane amplitude and phase equality to achieve low off-axis cross-polarization (-55dB) across the earth disc. A well-matched orthomode transducer (OMT) and low axial ratio polarizer complete the antenna assembly. The paper will describe the anechoic chamber measurement series and techniques used to measure the circularly polarized cross-polarization isolation values in the -50dB (A.R.=0.05 dB) to -60dB (A.R.=0.02dB) region. Bench measurements of the polarizer, which has a measured axial ratio less than 0.02dB, will also be presented. Directive gain measurements of the flight antennas will also be presented and discussed. The techniques presented in this paper are also used by MEMCO to design and measure circularly and linearly polarized probes and source antennas used in Nearfield Scanners and Compact Antenna ranges.
Application of the NIST 18 Term Error Model to Cylindrical Near-Field Antenna Measurements
A.C. Newell,D. Lee, November 2000
This paper describes error analysis and measurement techniques that have been developed specifically for cylindrical near-field measurements. A combination of analysis and computer simulation is used to show the comparison between planar and cylindrical probe correction. Error estimates are derived for both the pattern and probe polarization terms. The analysis is also extended to estimate the effect of position errors. The cylindrical measurement geometry is very useful for evaluating the effect of room scattering from very wide angles since scans can cover 360 degrees in azimuth. Using a broad beam AUT and scanning over a large y-range provides almost full spherical coverage. Comparison with planar measurements with similar accuracy is presented.
Dielectrically-Loaded Horns for Use as Antenna Measurement Range Illuminators
K.J. Greene,C. Granet, November 2000
The design of hybrid-mode dielectrically-loaded horns [1][2] for antenna test range illumination is described. These horns have a wide operating bandwidth of 5:1 or greater and were designed to replace conventional corrugated- or smooth-walled illumination horns that, typically, have a bandwidth of 2:1or less. Dielectrically-loaded horns have the radiation characteristics desirable for test range illumination: principal plane pattern symmetry, reasonably low cross-polarization and low sidelobes, low reflection coefficient and relatively constant beamwidth. At CSIRO we have developed software and manufacturing techniques to design and make these horns accurately. Measured results, that show close agreement with predicted values, are presented for a horn made for the frequency range, 7 to 40 GHz.
Raytheon 23' x 22' 50GHz Pulse System
T. Speicher,A. Puzella, J.K. Mulcahey, November 2000
Nearfield Systems, Inc. in Carson, California delivered a vertical 23' by 22' (7.0m x 6.7m) near-field test range to Raytheon Electronic Systems in Sudbury, Massachusetts. This planar and cylindrical measurement system is capable of characterizing antennas of various physical sizes in continuous wave or in pulse mode from 800MHz to 50GHz. The near-field measurements are computer controlled and capable of multiple frequency, multiple beam and multiple polarization in AUT transmit or receive modes. The precision robotic system uses a Data Acquisition Controller running NSI software to provide four-axes for probe positioning and three-axes for antenna positioning. The RF subsystem is based on the HP 8530A microwave receiver, HP 83630B RF source, HP 83621A LO source and HP 85309A LO/IF Distribution Unit. The test range was evaluated using the NIST 18- term error analysis on a 45GHz 54" diameter left-hand circular polarized reflector antenna.
Ground Bounce Reduction Using a Tapered Resistive Sheet Fence
Y. Kim,E.K. Walton, November 2000
One problem in a RCS ground bounce range is that the direct signal can be interfered with by the ground reflected signal. The undesired ground bounce signal will cause errors in the RCS measurement. This paper presents a study of ground bounce reduction using a tapered and stepped resistive sheet fence. In order to show that the proposed R-card fence technique can reduce the ground reflected signal significantly, both experimental and theoretical studies are performed. The resistance of the R-card varies based on a Kaiser-Bessel taper function. The experimentall results with and without the R-card fence show that the ground reflected signal can be attenuated by about 20dB. Both vertical and horizontal polarization cases are considered. This paper also the results of a simulation using NEC-BSC (Numerical Electromagnetic Code - Basic Scattering Code, developed at The Ohio State University ElectroScience Laboratory). Comparison of the results between measurements and simulations will be shown in this paper.
NFR Cross Polarized Pattern Errors Using a Linear Probe to Measure a Circularly Polarized Antenna
W.G. Scott,R.E. Wilson, November 2000
For greatest efficiency and accuracy in measuring patterns of a circularly polarized antenna on a planar near field range (NFR), a recommended procedure is to use a fast switched, dual circularly polarized probe. With such equipment one obtains complete pattern and polarization data from a single scan of the antenna aperture. For our task of measuring high gain shaped beam apertures, measurement efficiency is further improved by using a moderately high gain (about 12 dBi) probe that has been accurately calibrated for patterns, polarization, and gain over the test frequency band. Such a probe allows scan data point spacing to be typically at least one wavelength, thus keeping scan time minimized with acceptably small aliasing (data spacing) error. The measured near field amplitude and phase data is transformed via computer to produce the angular spectrum that is further processed to remove the effect of the probe patterns, i.e. probe correction. The final output is a set of (principal and cross) circular­ polarized far field patterns. However on one occasion, due to fast breaking changes in requirements, we were unable to obtain a calibrated circular polarized probe in the available time. For this test we used an available calibrated 12 dBi fast-switched dual linear-polarized probe with software capable of processing principal and cross circular-polarized far field patterns. As anticipated, we found from preliminary tests that the predicted low cross-polarized shaped beam pattern was not achieved when using the calibrated fast Ku band probe switch. Further tests showed the problem to be due to small errors in calibration of the probe switch. This paper will discuss test and analysis details of this problem and methods of solution.
1.56 THz Spot Scanning Radar Range for Fully Polarimetric W-Band Scale Model Measurements
G.B. A. DeMartinis,J. Waldman, M. Coulombe, T.M. Goyette, W. Nixon, November 2000
A radar transceiver operating at 1.56 THz has recently been developed to obtain coherent, fully polarimetric W-band (98 GHz) RCS images of 1:16 scale model targets. The associated optical system operates by a scanning a small focused beam of swept­ frequency radiation across a scale model to resolve individual scattering centers and obtain the scaled RCS values for the centers. Output from a tunable microwave source (10 - 17 GHz) is mixed with narrow band submillimeter-wave radiation in a Schottky diode mixer to produce the chirped transmit signal. Two high-frequency Schottky diode mixers are used for reception of the V-pol and H-pol receive states, with a fourth mixer providing a system phase reference. The full 2x2 polarization scattering matrix (PSM) for each resolved center is obtained following off-line data processing. Measurement examples of five simple calibration objects and a tank are presented.
How to Specify an RF System for Antenna Measurements
D.S. Fooshe,M. Schultz, November 2000
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.
Small, Broadband, Dual-Polarized, Phased Array Aperture Implemented Using Flare Notch Elements, A
A. Torres,A. MacFarland, P. Beyerle, W. Mohuchy, November 2000
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.
Broadband Polarization Selectable Feed for Compact Range Applications, A
C.W. Sirles, November 2000
Many aircraft radome structures are designed to operate simultaneously over multiple RF bands and incident polarizations. Critical parameters must be measured over the electrical apertures of the radome and across each operating band. Automated measurement techniques are required to efficiently collect the large volume of test data required. A modular broadband feed assembly has been developed to allow the simultaneous collection of multi-band, multi-polarization data on a compact range without the need to mechanically change feeds. The feed assembly utilizes a sinuous antenna as the radiating element and is capable of operation from 2-18 GHz with electronically selectable polarization states. Feed design criteria as they relate to compact range antenna and radome measurements are discussed. Of primary importance are reflector illumination pattern, linear polarization cross-polarization level, and circular polarization axial ratio. Polarization switching requirements for a specific test application are defined and the physical implementation of the integrated feed assembly is described. Measured feed and quiet zone performance data is presented for this application. The polarization switching configuration can be readily modified to support other applications.


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