AMTA Paper Archive
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Characterization and modelling of conducting polymer composites and their exploitation in microwave absorbing materials
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
Scattering by a simplified ship deckhouse model
To gain greater insight into the design of surface ships with reduced radar cross-section characteristics, a structure resembling a ship deckhouse was physically modeled and measured. The structure was represented as a truncated pyramid. Four scaled pyramids were fabricated, all identical except for the radii of the four vertical (slanted) edges. The pyramids were measured at the University of Massachusetts, Lowell Research Foundation, submillimeter laser compact range. Measurements were made a scaled X-band using a laser-based system that operates at 585 GHz with the pyramids scaled at a ratio of 1:58.5. These shaper were measured at 0.75 degrees depression angles on a smooth metal ground plane at both HH and VV polarizations. The goal of this study was to determine if small changes in the radius of the curvature of the slanted edges could significantly affect the radar cross-section of the pyramid. In this paper the results of measurements of the pyramids will be presented. The data are compared with computer code predictions and the differences are discussed.
Cross-polarized pattern measurement on point-source compact ranges
Earlier measurement results are reviewed to understand the result that cross -polarized patterns agree well when compared between a point-source compact range and spherical near-field scanning. By taking account of the symmetry of the aperture distribution, one can see how the cross-polarized pattern can be affected only moderately by the classic polarization feature of an offset reflector geometry.
Anechoic chamber evaluation
This paper details the evaluation of a major aerospace company's tapered anechoic chamber. Using an NSI 3' x 3' near-field scanner and software, the chamber was evaluated at 11 frequencies and two polarizations. SAR imaging techniques were used to map the chamber reflections. A new addition to the software provided the ability to map the difference between the measured phase front and the theoretical spherical phase front; the software also derives the x,y,z phase centers of the source. Error estimates for all aspects of the evaluation will be discussed.
Radar absorbing material thermal characteristics
The Benefield Anechoic Facility, Edwards AFB, California contains a large anechoic chamber for avionic integration test and evaluation. Because of the large chamber size, operational tests can require high-power aircraft radar emissions. To define the range of energy safely accommodated by currently installed radar absorbing material (RAM), a detailed analysis was performed and the results presented. The incident radar energy generates a heat transfer to the RAM. The RAM boundaries dissipate heat through convection, conduction, and radiation. A finite-difference solution demonstrates the temperature distribution in the material varies with the angle and polarization of the incident electric field. Discussions include the use of the RAM thermal characteristic's pretest evaluation to improve operating capability determinations and to facilitate assessment of customer requirements.
Efficient antenna testing using current antenna test systems
Accurate, fast, and cost effective antenna test equipment is necessary to meet many programs measurement requirements and schedule and budget constraints. Testing time may be significantly reduced y measuring multiple channels of data simultaneously. Further time savings are realized via the electronic storage of data, which allows easy pattern overlays and changes in the page setup parameters. Electronic storage of data also allows the user to accurately ascertain test parameters. Test data for a dual band, multi-channel antenna measured wit the Scientific Atlanta 1590 Pattern Recorder and multi-channel 1795 Microwave Receiver is presented. This antenna has transmit and receive ports, multiple polarization capability, data and tracking channel outputs and multiple frequency bands. The substantial savings in testing costs are estimated.
High performance medium gain antenna applications in the compensated compact range
The Compensated Compact Range (CCR) has been proven to be a high performance test facility for payload and large satellite antenna measurements. To efficiently measure complete antenna farms with various types of antennas on the spacecraft in the same test campaign led to the growing demand for testing e.g. Global Horn antennas on the spacecraft in the CCR. As a matter of fact, medium gain antennas feature a small aperture simultaneously requiring larger test angles. Therefor, main interferer like "direct leakage" between the CCR feed and the antenna under test have to be quantified and their impact on the measurement accuracy have to be reconsidered. The presented paper will investigate theoretically the feasibility of testing high performance widebeam antennas in the Top-Fed-Cassegrain double reflector system. Qualified measurement results of INTELSAT Global Horn Antennas featuring medium gain and extreme crosspolarization purity will be presented.
Polarization extraction of circularly polarized antennas
A technique is presented for obtaining the radiation patterns and the antenna gain of elliptically polarized antennas from two vector measurements of the far-field. The two measurements correspond to different polarizations which can be obtained by rotating one of the antennas around its boresight axis. The discussion emphasizes a particularly interesting case, for which accurate radiation patterns and gain of the antenna under test (AUT) can be obtained without prior knowledge of the polarization of the second antenna. The radiation pattern of a nearly circularly polarized (CP) antenna is conveniently represented by the CP co-polarized and cross-polarized components. The axial ratio and any other quantities commonly used to specify the antenna polarization can also be obtained since the pair of initial vector measurements completely characterize the polarization of the AUT. The technique is illustrated by measurements of a CP patch antenna.
Instrumentation upgrade for ultra-high speed data acquisition in the DASA compensated compact range
Deutsche Aerospace is developing and testing high performance communications antennas for the INTELSAT program. A large number of antenna measurements must be performed, for two polarizations, multiple frequencies and multiple beams. To measure all parameters in a single rotation of the antenna, a highspeed instrumentation system is required. The instrumentation was upgraded using the latest technology in receivers, sources and control systems. Commercially available components were used for all components. The resulting system can perform a complex antenna measurement consisting of over four million data points within only two hours.
Automated production test facility for a MMW radar system
Martin Marietta has developed a new, automated facility for high-volume production testing of the Longbow millimeter wave missile. Two dedicated far field anechoic chambers were designed, both automated to support component test and analysis in the production environment. One standard far field chamber is used to perform the complete characterization of the antenna and rac1orne; it allows very accurate measurements of power sidelobes, monopulse errors, and cross polarization isolation. The completed radar missile sensor group is evaluated in the second far field chamber, which can reach higher-level parameters of the antenna, transceiver, and gimbal. This paper describes chamber and test station capabilities; time reduction benefits; and the novel, new assembly technique which allows for future portability of these chambers with limited downtime.
Design and testing of an adaptive array for analog cellular
This paper describes an adaptive array that was designed to improve the carrier-to-interference ratio (C/I) delivered to base station radios by 6 dB in U.S. 800 MHz analog cellular networks. The C/I performance of this kind of system is difficult to verify, because it cannot be characterized in terms of traditional antenna specifications such as beamwidth and directivity. This paper describes a simple C/I measurement strategy in which the antenna under test and a collocated reference antenna are placed into simultaneous operation in an actual cellular network. Relative C/I performance can then be deduced from a statistical analysis of the antenna outputs. This method is particularly well-suited to software radio based systems, because no special test equipment is required to gather the necessary data.
Enhancement of efficiency and accuracy of near-field measurement
This paper examines the possibility of increasing the speed of Near-Field measurement of an Antenna, by reducing the number of measurement points and by determining the degree of truncation permissible while maintaining a prescribed degree of precision of the reconstructed far-field. The Near-Field of a planar radiating array is analysed in depth. A formulation and a procedure to correct the spectral domain of the field are established. It is shown that correction in the spectral domain can improve the accuracy of the Far-Field while using the same amount of Near-Field data. The technique has a good potential to be applied to Near Field data of large radiating Antennas leading to new information about the accuracy and speed of measurement achievable.
Frequency dependent scattering effects on Fourier domain imaging of ultra-wideband data
Forming radar images from large fractional bandwidth data can often lead to unusual artifacts or resolutions degraded from "expected" theoretical point-target values. The frequency dependencies of typical scatter ing mechanisms, such as diffractions, surface waves and speculars, can be significant over processing apertures when data are collected using large fractional bandwidth measurement systems. For example, it is well known that resonant scatterers exhibit blurring in the downrange direction of an image. Other scattering mechanisms have linear or quadratic amplitude dependencies which can also alter the impulse response from that of an ideal point scatterer. This paper will first provide a brief description of the frequency dependencies of various scattering mechanisms. The paper will then describe the corresponding effects seen in the impulse response, primarily in the range profile domain. Impulse response plots will be compared for data with large and small fractional bandwidths. Lastly, the effects of frequency dependent scattering on the impulse response will be shown using images generated from data collected in indoor compact ranges.
Polarimetric calibration of reciprocal-antenna radars
We discuss how RCS target depolariza tion enhances cross-polarization contamination, and we present a graphical study of measurement error due to depolarization by an inclined dihedral reflector. Error correction requires complete polarimetric RCS measure ments. We present a simple polarimetric calibration scheme that is applicable to reciprocal antenna radars. This method uses a dihedral calibration target mounted on a rotator. Because the calibration standard can be ro tated, there is no need to mount and align multiple sepa rate standards, and clutter and noise may be rejected by averaging over rotation angle.
Triband radome measurement system: installation and testing results, A
In an earlier paper ("System Engineering for a Radome Test System," John R. Jones, et al, AMTA, October 1994) the system level design of a compact range enhancement for the testing of the Triband Radome was presented. This paper will discuss the installation and testing of the radome measurement system in the compact range. The purpose of the radome measurement system is to determine (within close tolerances) boresight shift, transmission loss, antenna pattern changes and polarization effects caused by the radome. Unique features include novel coordinate transformation and correction by means of a laser autocollimator and data reduction algorithms. Also featured is the tracking subsystem which consists of a specially designed two-axis track pedestal, an autotrack controller, and three five-horn compact range feed arrays operating at X, K, and Q-bands. The performance of the triband radome measurement system in the compact range setting will be presented.
Investigation of circular Archimedean spiral antenna for automobile applications
Due to the revolution in communication technology very sophisticated communicative and navigational tools are becoming a part of automobile electronics. These different applications need antennas that operate at various frequencies and with different polarization requirements. One such antenna is a cavity-backed Circular Archimedean Spiral Microstrip Antenna (CASMA). This pa per will compare radiation pattern measurements of a CASMA with pattern predictions using a hybrid FEM /M oM/GT D technique. The measurements were done at NAS A-Langley Research Center's Low Frequency Antenna Chamber. The predicted and measured patterns are presented and are shown to exhibit a reasonable degree of agreement.
Development and measurement of a frequency selective surface highway stripe
A frequency selective surface has been developed for use as a part of an automatic highway system. The FSS is attached as a stripe along the edge or center of the lane, and is designed to a strong retro-reflective echo for the design frequency, polarization, and elevation angle of the forward-looking radar installed on an automobile. The stripe provides directional information for automated steering, as well as other coded information such as lane number, and exit advance warning. This paper reports on initial development and testing of a prototype FSS highway stripe. The stripe was designed for an operating frequency of 10.5 GHz, and was built and tested using a prototype autonomous vehicle. Both FSS stripe performance, and performance of the vehicle will be reported.
A Novel dual-polarized, ultrawide bandwidth horn antenna
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.
Single-plane collimators for measurements on large antennas
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.
C-band multipactor breakdown using a ring resonator and peak power meters
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.
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