AMTA Paper Archive
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Ground Plane Simulation and Spherical Near-Field Scanning for Telematic Antenna Testing
This paper presents the results of a laboratory simulation of an outdoor telematic antenna test site that employs spherical near-field scanning to determine the far fields of telematic antennas mounted on vehicles.
MIMO Channel Capacity versus Mutual Coupling in Multi-Element Antenna Systems
In this paper the influence of mutual coupling on the capacity of a multiple-input multiple-output (MIMO) antenna system is demonstrated. No direct relation between the envelope correlation and the actual location and orientation of the antennas is found. Even though being essential for high MIMO capacity, configurations with the lowest envelope correlations are not necessarily the most suitable for a MIMO system. A certain bandwidth is required as well. Three planar inverted F-antennas (PIFA) located on the same 40 mm 100 mm ground plane. The antennas that haves a resonant frequency of 1.8 GHz yields envelope correlations ?e below 0.4.
A Novel Structure for Accurate Measurement of Antennas Mounted on Ground Planes
A novel structure for accurate measurements of antennas mounted on an infinite ground plane has been designed and built. The structure is eight feet in diameter and can be used to measure antennas as big as fourteen inches at the base at frequencies as low as 1 GHz. The structure is defined by blending a planar surface with an elliptical surface such that near the antenna under test the surface resembles a planar surface and then it slowly rolls back to minimize any diffractions due to discontinuities in the surface. Patterns of a few antennas mounted on the structure are presented and compared with the expected patterns of the antennas mounted on an infinite ground plane.
Near-Field Based Technique for Array Feeding Network Characterization
A technique to identify failures in an antenna array using corporate feeding is presented. This technique combines near-field imaging close to the radiating elements and the printed transmission lines forming the feeding network. Planar near-field probing is done with a loop and a miniature dipole probe less than 0.1 free-space wavelength above the antenna under test. Two cases are considered, one where the feed lines would be separated from the radiating elements by a metal ground plane and another where the lines and the elements are on the same metal layer. In the first case, precise diagnostic based on extraction of vectorial forward and reverse wave complex coefficients on each line segment is possible. In the second case, it is not possible to extract these coefficients. However, defect localization is still possible by relying on symmetries present in the near-field maps of selected line segments.
Quantitative Measurements of Imbalance in Vertically-Polarized Broadband Dipoles Situated Over a Conducting Ground
We present quantitative measurements of the imbalance induced in a broadband, wire-cage biconical antenna situated over a conducting ground plane and fed via a coaxial transmission line. The antenna and feed structure taken together are represented as a 2-port, 3-terminal network which, in turn is represented using a Ð equivalent circuit. A new measurement technique which requires no balancing network for determining the equivalent network component values is presented. The complex, frequencydependent elements of the equivalent network are derived from measured data and presented, clearly showing that the imbalance tendancy is strongest in the vicinity of the series resonances of the effective common mode circuit. Thus, it can be concluded that by avoiding feed arrangements which cause series resonances in the common mode circuit within the operating frequency range of the antenna, balance can be maintained without undue requirements on the balancing network.
Helendale Measurement Facility Uncertainty Analysis
This paper reviews the Helendale Measurement Facility (HMF) ground plane range uncertainty analysis and associated data collection. Range uncertainty analysis is a requirement for ISO-25/ANSI-Z-540 range certification and is a priority one section in the Helendale Range Book. Targets used for the analysis were two sets of right circular “squat” calibration cylinders. These cylinders are the dual calibration cylinders for HMF. Calibration measurement uncertainties are established statistically from a large number of repeated measurements at S, C, X, and Ku bands. Each measurement was taken at two target support locations down range. The field data collected included monostatic scattering from two calibration cylinders, backgrounds with no target and support, and drift data for quality control. I and Q imbalance, frequency stability, range accuracy, linearity, and field uniformity at target locations were considered in the analysis. The uncertainty analysis is based on RSS addition of errors and assumes all errors are additive and that targets are not LO. The statistical approach used to perform the uncertainty analysis reported in this paper was developed cooperatively at AFRL and Mission Research Corporation.
524 GHz Polarimetric Compact Range for Scale Model RCS Measurements, A
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.
Mitigation of Target Illumination and Multipath Errors in Ground Plane RCS Measurements
The close proximity of the ground to the radar antenna and the target under test is often hard to avoid at an outdoor RCS measurement range. Ground reflection of energy from the antenna leads to target illumination errors, and target-ground interactions lead to multipath errors. By proper positioning of the antenna and target, ground reflections of the antenna illumination can be exploited to increase overall system sensitivity by concentrating more energy on the target; however, this is only effectivefor narrowband measurements over a limited target region . Reducing target-ground interactions by increasing the target height above the ground generally has limits due to mechanical restrictions on both the radar antennas and the target. This paper will present a model-based data post-processing technique to mitigate illumination errors and target-ground interactions in ground plane range RCS measurements. The algorithm is an extension of the network model multipath mitigation technique previously developed for indoor RCS measurement ranges [2,3,4]. The technique will be described and demonstrated using a numerical simulation of the RCS measurement of a canonical target over a ground plane.
Radar Cross Section (RCS) Range Enhancements at Eglin's Seeker Test & Evaluation Facility (STEF)
The Seeker Test and Evaluation Facility (STEF) located on Range C-52A at Eglin AFB FL. is used to perform high-resolution multispectral (EO-IR-RF-MMW) signature measurements of US and foreign ground vehicles primarily to support the Research, Development, Test and Evaluation (RDT&E) of smart weapons (seekers, sensors and Countermeasure techniques). In order to support two major DOD signature measurement programs in 1997 this facility required significant range upgrades and enhancements to realize reduced background levels, increase measurement accuracy and improve radar system reliability. These modifications include the addition of a 350'X 120' asphalt ground plane, a new secure target support facility, a redesigned low RCS shroud for the target turntable and a new core radar system (Lintek elan) and data acquisition/analysis capability for the existing radars Millimeter-Wave Instrumentation, High Resolution, Imaging Radar System - MIHRIRS). This paper describes the performance increase gained as a result of this effort and provides information on site characterization and radar instrumentation improvements as well as examples of measured RCS of typical ground vehicle signatures and ISAR imagery
Understanding open area test site performance
The site attenuation of a practical open area test site differs from that of an ideal (infinite ground plane) test site. For example when transmit and receive antennas are vertically polarised there is a significant ripple in the site attenuation as a function of frequency. This causes an uncertainty in the measured antenna factor. This paper describes a theoretical model that accurately predicts the site attenuation for any test site geometry. It is shown that it is insufficient to consider the conducting ground plane in isolation: the region outside the ground plane must be taken into account. The work is illustrated with measurements made on the National Physical Laboratory's (NPL) 30 m by 60 m test site. .
Quasi 3D imaging on a ground plane RCS range
A method is presented that gives a 3D ISAR image from a 2D measurement. An ordinary 2D image is created. An extra receive channel is used to give height information in every pixel. This channel gets its signal from two extra receive antennas with different elevation lobes. The antennas feed a hybrid which creates a difference signal that goes to the extra receive channel. Height information is derived like in an amplitude comparison monopulse radar. This way a height number is assigned to every pixel in the 2D image. Thus a 3D image is created. It is required that in a 2D resolution cell the reflexes come from only one height. If not, the height information given by the difference signal will be a weighted average of the heights of the reflexes. The method is applied to a ground plane RCS range. No measurements have yet been performed.
Calibration of bistatic RCS measurements
Calibration of monostatic radar cross section (RCS) measurements is a well-defined process that has been optimized through many years of theoretical investigation and experimental trial and error. On the other hand, calibration of bistatic RCS measurements is potentially a very difficult problem; the range of bistatic angles over which calibration must be achieved is essentially unlimited and devising a calibration target that will provide a calculable scattering solution over the required range of bistatic angles is difficult, particularly for cross-polarized measurements. GTRI has developed a solution for amplitude calibration of both co-polarized and cross-polarized bistatic RCS, as well as a bistatic phase-calibration procedure for coherent systems.
Method to quantify target-support interaction terms, A
Target support interaction terms often drive Radar Cross Section Measurement limitations. These limitations are when mask needed information, or render interpretation difficult. Although support improvement is desirable and studied, there is a fundamental problem. Perhaps we can create a support that is 10 dB better than existing supports. The technology producing that improvement will usually be applicable to targets. Result: The same ratios recur. Modern instrumentation Radar possesses many acquisition agility's. Processing power currently available permits handling huge volumes of data. This paper studies evaluation and/or elimination of interaction terms using these agility's. Interactions within the test article are often significant. Controlled of this method would select and retain, or remove the terms.
Phase-stationary high performance antenna test body, A
Modern low profile and conformal antennas are fre quently evaluated in the presence of a conducting surface. Antenna designers usually predict the an tenna scattering and radiation performance over an infinitely conducting ground plane. To bridge the gap between a (possibly curved) antenna host surface and the designer's infinite ground plane model, an antenna testbody is required. This testbody must possess a variety of demanding attributes, such as a very close approximation to an infinite ground plane, low testbody signature, ability to provide positional accuracy (in both azimuth and elevation), physical stability (for repeatability and background subtraction), just to name a few. The most widely regarded testbody has been the "almond" testbody [1, 2], which boasts a very low signature, and excellent fidelity when compared to an infinite ground plane. This paper addresses a new variation from the traditional "almond" testbody, in which a unique positioning design provides a phase-stationary antenna aperture center under rotation of both azimuth and elevation. This testbody will be used for a variety of antenna tests at Wright Laboratory's Radiation and Scattering Compact Antenna Laboratory (RASCAL).
R-card ground planes
There is a need for finite ground planes to test an tennas which are normally mounted on large struc tures. These ground planes are used to simulate a large structure such as an aircraft fuselage but are limited in size based on the available target zone di mensions. For example, TCAS antennas are tested on a 4' circular ground plane based on FAA require ments. Since the conducting ground plane creates significant diffraction errors which are not present in the intended application, these ground plane tests become difficult to interpret because one can not easily separate ground plane diffraction errors from antenna characteristics. A solution to this dilemma is to attach an R-Card (resistive sheet) to a con ductor (PEC) and form an R-Card ground plane. With a properly designed resistive profile, an R-Card ground plane can greatly reduce the edge diffrac tion errors. As a result, the desired antenna charac teristics without significant ground plane corruption terms can be obtained. This paper demonstrates this new concept through calculated and measured results. Also, a Genetic Algorithm (GA) to optimize the resistive profile is presented.
Band concatenation for higher resolution RCS imaging
Radar Cross Section (RCS) measurements are often performed in discrete frequency bands for a variety of reasons. Although some indoor ranges are capable of performing very wide-band measurements (with bandwidths up to or exceeding 9: 1), some are designed with very rigid illumination requirements on the coIIimating reflector(s) that can only be met over a narrow band. In addition, the bandwidth available on most outdoor ranges is limited by "ground plane" effects which make it impossible to maintain an adequate broadband field over the target. Often, RCS measurements are limited to half an octave at most. Since resolution in RCS imaging is directly proportional to bandwidth, there exists a need for concate nati ng several discrete bands of measurements into a single continuous band. This resulting band must be free of both amplitude and phase discontinuities that would affect the quality of the resultant image. This paper discusses the sources of discontinuities between measured bands on both indoor and outdoor ranges, and provides algorithms for removing them using linear filtering methods. Data is presented from an outdoor range illustrating the results on targets up to 70-feet in length.
Contrast of VHF RCS measurement challenges indoor/outdoor, A
This paper contrasts indoor and outdoor implementation of efforts during upgrades of VHR RCS measurement capabilities. Sites studied are two McDonnell Douglas Technologies Incorporated, Range Measurements Services facilities. Indoor. Radar Measurement Center (San Diego, CA) is a large compact range. Equipment-Harris Corporation Model 1630 Collimator System, Scientific Atlanta Model 2090 radar. Outdoor. Microwave test facility (Victorville, CA), large ground plane facility. Equipment-Steerable dipole feed dish, System Planning Corp, Mark III radar.
Use of an infrared camera system in the analysis of phased array boresight errors, The
The use of electronically scanned phased array antennas in demanding rolls such as satellite communications and radar systems has led to an increasing desire to analyze the sources of error present in the boresight alignment of such systems. Not the least among these errors are those introduced by thermal effects on the various components which comprise the array structure. In an effort to understand this mechanism, this paper will discuss a technique which uses an infrared camera system to analyze the beam deflection errors caused by the effects of temperature gradients present in the antenna system.
Substitution and 3-antenna measurements of an 8-element VHF ocean-buoy antenna
A description of antenna measurements performed on an ocean-buoy mounted antenna array is given. The array is designed to measure the E and H fields of a received wavefront at four different heights over the ocean. Four collocated electrically-small loop-dipole antenna pairs at 2 meter height spacings were integrated into a non-conducting buoy support structure. The frequency band was 50-250 MHz. Data was taken with both the Substitution (2-antenna) and 3 Antenna Measurement Methods for comparison purposes. The ground plane range that was used is described as well as the various range setups used to accumulate all of the required data.
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.
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