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.)
Specular patches comprising pyramidal absorber components are frequently used in anechoic chambers to suppress potential DUT coupling with the side walls, floor and ceiling of the chamber. However, these specular patches also interact with the incident field radiated by the source antenna, compact range reflector, or tapered chamber feed illuminating the chamber. If the specular patch reflects the incident field in GO fashion, then the reflected field is incident on the absorptive back wall and is sufficiently attenuated there, so that there is no significant degradation of the field uniformity in the Quiet Zone due to the reflected field. If, however, the chamber is long, and the grazing angle of the incident field on the specular patches is relatively low, “non-specular” reflections incident on the Quiet Zone will perturb the field, and accordingly will degrade the field uniformity. If the chamber is operating at high frequencies (e.g., above several GHz) and the distance between the Quiet Zone and side walls is significant in terms of wavelengths, then the “non-specular” reflections will not impact the field uniformity to a noticeable extent, as they are attenuated in free space while propagating from the specular patches to the Quiet Zone. If the chamber is intended for operation at VHF/UHF frequencies, as is prevalent in tapered chambers, then the “non-specular” reflections may be the dominant factor affecting the Quiet Zone uniformity. In this paper the measured reflectivity in a tapered chamber with pyramidal specular patches is presented, illustrating a significant rise of the reflectivity over a portion of the VHF/UHF bands. Thorough investigation has shown the source of the degraded reflectivity to be the specular patch. This effect has been confirmed by simulation, and is analyzed by modeling the specular area as a periodic structure. Replacement of the specular patches by wedges has materially improved the reflectivity in the chamber, as will be shown by comparative reflectivity measurement results. For the application under consideration, the coupling between the DUT and sidewalls was below the specified minimum and, thus, advanced coupling suppression techniques were not required. For more stringent coupling requirements, the use of the ORBIT/FR patented “Two Level GTD” technology (see, for example, [1-4]) is a good choice to minimize reflectivity and DUT/sidewall coupling simultaneously.
Some antenna measurement applications require the precise positioning of an antenna along a prescribed path which may be realized by a combination of several, independent physical axes. Coordinated motion allows for emulation of a more complex and/or precise positioning system by utilizing axes which are mechanically less complex or precise and are correspondingly more easily realizable. An ideal coordinated motion system should 1) Allow for the description of coordinated paths as parametric mathematical functions and/or interpolated look-up tables 2) Support control variable parameters which affect the trajectory 3) Compute a feasible trajectory within given kinematical constraints 4) Generate measurement trigger signals along the trajectory 5) Minimize control-induced vibration 6) Compensate for multivariate positioning errors. This paper will describe a novel approach to virtual-axis coordinated motion which offers significant improvements over existing motion control systems. This advancement can be applied to many antenna measurement problems such as Helicoidal Near-Field Scanning and Radome Characterization.
The increased complexity of an active array's transmit beams by itself elevates the need for an interface between the array and the acquisition system. With the embedded receivers of a DBF, however, standard antenna testing of a DBF becomes nearly impossible without such an interface. MI Technologies has developed a reasonably general interface between its acquisition system and active arrays with digital beamformers. MI has produced minor variations of this interface for multiple customers, and these customers will each use the interface to test multiple types of DBF active arrays. This paper discusses the challenges, capabilities, and architecture of this interface.
Antenna measurement systems employ mechanical positioners to spatially orient antennas, vehicles, and a variety of other test articles. These mechanical devices exhibit native positioning accuracy in varying degrees based on their design and position feedback technology. Even the most precise positioning systems have insufficient native accuracy for some specific applications. As the limits of economical positioning accuracy are approached, a new error correction technique developed by MI Technologies satisfies these higher accuracy requirements without resorting to extreme measures in positioner design. The new technique allows real-time correction of repeatable positioning errors. This is accomplished by (1) performing a finely grained measurement of positioner accuracy, (2) creating a map of the errors in both spatial and spatial frequency domains, (3) separating the errors into their various components, and (4) applying correction filters to algorithmically perform error correction within the positioner control system. The technique may be used to achieve extreme positioning accuracy with positioners of high native accuracy. It may also be applied to conventional (synchro feedback) positioners to achieve impressive results with no modifications at all to the positioner. The following paper discusses the new error correction technique in detail.
The paper presents in-house developed antenna positioner capable to acquire radiation pattern in the full spherical format for wearable and textile antennas. The positioner features remarkable advantages and mitigates troublesome to measurement accuracy shadowing by the positioner structure. Furthermore, development methodology of the human phantom without heavy liquids is proposed. Since evaluation of wearable and textile antennas must put considerations to major variations in antenna performance during antenna operation, we have found an urgent need to define new engineering measure that will help in quick evaluation of such antennas.
The feasibility of using adaptive acquisition techniques to reduce the overall testing time in spherical near-field (SNF) antenna measurements is investigated. The adaptive approach is based on the premise that near-field to far-field (NF-FF) transformation time is small compared to data acquisition time, so that such computations can be done repeatedly while data is being acquired. This allows us to use the transformed FF data to continuously compute and monitor pre-defined decision functions (formed from the antenna specifications most important to the particular AUT) while data is being acquired. We do not proceed with a complete scan of the measurement sphere but effectively allow the probe to follow a directed path under control of an acquisition rule, so that the sampled NF datapoints constitute an acquisition map on the sphere (the geographical allusion being purposeful). SNF data acquisition can be terminated based on decision function values, allowing the smallest amount of data needed to ensure accurate determination of the AUT performance measures. We demonstrate the approach using actual NF data for several decision functions and acquisition rules.
Streaming SDR (software defined radio) communication receivers are now high performance, common place and cost effective. Yet these receivers are not easily used for antenna measurements for a variety of reasons including their inability to accept phase reference and measurement trigger information. A new technique called Time Space Coherence Interferometry (TSCI) solves this problem in a simple and elegant manner. TSCI combines the concepts of temporal phase coherence with spatial division multiple access (SDMA) to directly encode phase and spatial information into a single continuous receiver data stream. The stream can be recorded for later analysis or efficiently decoded in real time producing conventional spatially sampled S21 amplitude and phase measurements. Additional data including antenna pulse timing, dynamics and signal quality metrics can be extracted from the TSCI data stream. Several representative TSCI systems are described.
In order to determine the permittivity of homogeneous dielectrics used in the production of microwave absorbing materials, it is necessary and sufficient to know /measure the complex reflectivity of thick dielectric bricks utilizing the materials [1]. If the permeability of the material is to be sought then, in addition, the knowledge of the complex transmission coefficient is necessary to determine unknown parameters. The coaxial lines like the one described in [2,3], as well as NRL arch systems, are widely used in the industry to characterize the performance ( reflectivity) of absorbing materials over a wide frequency range. In these systems, the conventional S11 or S21 methods of parameter extraction from the absorptive samples are utilized in conjunction with reference measurements from a metallic surface. The difference between the two measurements characterizes the reflectivity of the samples. In this paper, extension of the S11 and S21 methods to measure the permittivity and permeability of absorbing materials in conventional coaxial line and on NRL arches is described. The technique is based on measurements of thick absorptive bricks followed by signal processing with time - gating being utilized. The application is very useful for rapid dielectric property measurements of the raw materials prior to impregnation or full absorbing materials after impregnation prior to cutting the material into pyramidal or other shapes.
A low profile UWB VHF antenna with a maximum diameter of 60.96cm and height of 5.08cm is presented. The top of this antenna is made of a wide conducting plate top which is shorted to ground at one end and a 50-ohm coaxial feed at the other end. Specially selected and shaped ferrite bars are strategically placed between the top plate and ground plane for achieving good antenna performance for wide angle coverage in the upper hemisphere from 30 to 300 MHz. Minimal amount of ferrite was used to keep the antenna’s weight at below 9.07 kg. Simulated and measured results will be discussed.
The improved calibration model proposed in this paper is based on the traditional capacitance model which suffers from errors caused by the assumption that the capacitance is independent of frequency and the permittivity of the ambient medium under test. By analyzing the near-zone field of the coaxial opening, we introduce the new near-field capacitance to account for the dependency on the external permittivity. Simulation results show that the calibration error is significant reduced for low and moderate loss medium. And the calibration of the unknown coefficients simply requires the premeasurement of three known material including air, which provides convenience for the real field measurement. Measurement results obtained by a novel wideband in-situ coaxial probe are included to prove the accuracy improvement improved calibration model. by using this
The effect of the anatomical variation of the head on the ear-to-ear communication at 2.45 GHz has been investigated. Several anatomical characteristics of the head, such as the dimensions and the position of the ears, have been recorded for a group of 25 test persons. Active Packet Error Rate (PER) measurements have been made by the use of digital Hearing Instruments (HI) as small wireless platforms in both indoor and outdoor environments. Two fundamentally different antenna con.gurations are compared. It is found that there is an effect of the distances over-the-top, around-the-front and aroundthe-back on the PER, due to constructive and destructive interference between surface waves that propagate along the different paths. The effect is different for the two different antenna types.
APPROACH TO VHF GAIN MEASUREMENTS OVER SEAWATER D:\Documents and Settings\david.tonn\My Documents\Fishers Island\aerial.JPGpool3 Timothy Bolton, Dr. Stephen M. Davis, Paul Medeiros, Paul M. Mileski, D:\Documents and Settings\timothy.bolton\Desktop\Fishers Island Measurement Procedures\VHF Gain\VHF Whipcone Measurements\Pictures3\DSC00037.JPGGain at two hgtsDr. David A. Tonn, Isaac A. Wheeler D:\Documents and Settings\timothy.bolton\Desktop\Fishers Island Measurement Procedures\VHF Gain\VHF Whipcone Measurements\Pictures3\DSC00002.JPGD:\Documents and Settings\timothy.bolton\Desktop\Fishers Island Measurement Procedures\VHF Gain\VHF Whipcone Measurements\Pictures3\DSC00025.JPG-1 0 1 2 3 4 5 6 7 30 35 40 45 50 55 60 Realized Gain (dBi) Frequency (MHz) Specified Gain Measured Gain G = -106.1 + 10.594*F - 0.38865*F^2 + 6.2584E-3*F^3 - 3.6863E-5*F^4 -1 0
Tapered chambers are particularly suitable for antenna measurement at low frequencies and can provide quiet zones of up to 1.4m in a 12m range. A tapered chamber can also be used for measurement of antennas at high frequency. However, with increasing frequency, the quiet zone size reduces rapidly. For example, at a 12m distance from the feed to the turn-table, the quiet zone at 8GHz is reduced to 45cm. One possible solution to extend the quiet zone at high frequency is to use a large dielectric lens to improve the phase distribution of the field. A lightweight, broadband 2m lens was developed by Matsing Pte Ltd for this purpose. The parameters of the lens were specially customized for the tapered chamber built by ETS-Lindgren for the National University of Singapore in 2010. The lens has a focal length of 10m and weighs just 35kg. The performance of the tapered chamber with the RF lens is presented.
This paper describes the measurement campaigns carried out at P-band (435 MHz) for selection of optimum on-ground verification approach for a large deployable reflector antenna (LDA). The feed array of the LDA was measured in several configurations with spherical, cylindrical, and planar near-field techniques at near-field facilities in Denmark and in the Netherlands. The measured results for the feed array were then used in calculation of the radiation pattern and gain of the entire LDA. The primary goals for the campaigns were to obtain realistic measurement uncertainty estimates and to investigate possible problems related to characterization of the feed array at P-band. The measurement results obtained in the campaigns are compared and discussed.
A wideband scalable dual-polarized probe designed by the Electromagnetic Systems group at the Technical University of Denmark is presented. The design was scaled and two probes were manufactured for the frequency bands 1-3 GHz and 0.4-1.2 GHz. The results of the acceptance tests of the 0.4-1.2 GHz probe are briefly discussed. Since these probes represent so-called higher-order antennas, applicability of the recently developed higher-order probe correction technique [3] for these probes was investigated. Extensive tests were carried out for two representative antennas under test using the manufactured probes; the results of these tests are presented and discussed in details.
In 2008 and 2011 we proposed at AMTA a method of radar imagery based on Fast Fourier Transform. We demonstrated then that this method was faced with interpolation problematic (which could be almost solved) and that the obtained radar image had a poor resolution imposed by the low bandwidth used during the measurement. We saw then that because of this resolution issue, it was impossible to distinguish two scatterers when they were too close. This article presents methods allowing to improve the resolution of radar image without increasing the bandwidth of measurement but using 1D super-resolution techniques adapted to 2D or 3D measurements. More precisely, this article proposes first to explain the principle of three 1D super-resolution methods: the Capon method, the MUSIC (Multiple Signal Classification) method and the ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) method. This principle understood, these methods will be then adapted to 2D or 3D measurements in order to be used to calculate high resolution radar images. Eventually, a true target will be used in order to compare the different radar images obtained from the different super-resolution techniques.
Radar Cross Section (RCS) and Antenna measurement ranges share many common features and are often used for both purposes. Calibration of these dual-purpose ranges is typically done using the substitution method for both RCS and antenna testing, but with separate RCS and antenna standards. RCS standards are typically based on a geometric shape having a well known theoretical value – and corresponding small uncertainty. By contrast, antenna standards typically must be “calibrated” in a separate antenna calibration system to be used as a gain standard, often yielding higher uncertainties. This paper presents an efficient method for transferring an RCS measurement calibration to an antenna measurement range configuration, allowing a range to be used for both purposes with a single calibration. Insight into the best ways to re-configure the instrumentation between RCS and antenna testing is included. Validation measurements from a compact range are included along with an uncertainty analysis of the method.
Many modern antennas are incorporating LNAs into the aperture to maximize system receive performance. G/T (Gain over Temperature) quantifies the performance of these antenna systems. Historically, G/T measurements needed knowledge of absolute effective temperature of multiple noise sources, which is not practical in an anechoic chamber. A Y-factor method is presented which uses a reference antenna system with a known G/T to determine the G/T of the Antenna Under Test (AUT). This paper will review G/T, describe the measurement process, cover calibration of the reference antenna system and discuss error sources and their mitigation.
The speed of spherical near-field scanning is increased significantly when measurements are not restricted to standard measurement locations, i.e., the locations that are equidistant in theta and in phi. Measurement positions can be chosen so that mechanical positioners perform scans with a continuous motion; this will decrease the time it takes to acquire data for near-field measurements. The issue then becomes transforming the data acquired with non-uniform spacing. This paper describes the development of a spherical near-field to far-field transform that can efficiently process data acquired on a non-uniform grid.
A dielectric rod feed with a special radiation pattern of a tabletop form used for the compact range reflector is developed and analyzed. Application of this feed increases the size of the compact range quiet zone generated by the reflector. The feed consists of the dielectric rod made of polystyrene; the rod is inserted into the circular waveguide with a corrugated flange. The waveguide is excited by the H11-mode. The rod is covered by the textolite biconical bushing and has a fluoroplastic insert in the vicinity of the bushing. Mathematical modeling was used to obtain the parameters of the feed for the optimal tabletop form of the radiation pattern. The problem of the electromagnetic radiation was solved for metal-dielectric bodies of rotation by method of integral equations with further solving of the problem of the synthesis for feed parameters. The dielectric rod feed was fabricated for the X-frequency range. Feed amplitude and phase patterns were measured in the frequency range 8.2-12.5 GHz. Presented results of mathematical modeling and measurements for X-range radiation patterns correlate well. It is shown that this feed increases by 20-25% the quiet zone of the compact range with reflector in the form of nonsymmetrical cutting of the paraboloid of revolution 5.0 . 4.5 m in size in the frequency range 8.5-10.0 GHz as compared to a conical horn feed.