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Numerical Study of the Effects of Absorber Permittivity Variations on Quiet Zone Illumination of Tapered Chambers
Tapered chambers use the reflections from the surfaces adjacent to the range antenna to illuminate the quiet zone (QZ). Polyurethane substrate is the preferred and most widely used radio frequency (RF) absorber in these chambers, due to its ability to be cut into complex shapes to conform to the tapered sections. Unfortunately, this type of absorber always presents slight differences in permittivity related to the manufacturing process. To analyze the effects of the permittivity of the lossy foam on the QZ illumination in a tapered chamber, a series of numerical experiments using a full wave analysis technique are executed. The results are mainly obtained for frequencies under 1 GHz. The upper frequency of the simulation is limited by the electrical size of the problem and by the available information on the material permittivity. However, frequencies below 1 GHz is where the tapered chambers are superior to other methods for indoor antenna measurements. Magnitude and phase are recorded over a 1.82m diameter spherical QZ to show the effects of the different absorber on the illumination. Results show that a variation on the absorber around the range antenna will deviate the illumination and skew the amplitude taper across the QZ. The amplitude distribution peak can be shifted by as much as 3.5 degrees from boresight. The effect on the phase taper is smaller with a negligible change in phase.
Analysis and Characterization of Tapered Chambers
Tapered Chambers are best suited for antenna pattern measurements at low frequencies. The advantage of such chambers over rectangular shaped chambers would be achieving a desired performance level in terms of field uniformity and ripple at the quiet zone due to the low reflectivity of the chamber. To achieve such performance using a rectangular shaped chamber could lead to design of larger rooms and associated significant cost. Hence, this paper tries to analyze the characteristics of the tapered chambers using the novel Fourier analysis characterization method. The Fourier analysis method was applied on the transverse and longitudinal planar scan data at the quiet zone of a tapered chamber. The analysis yield the performance of the chamber at different frequencies depicting the plane wave behavior at the low frequencies and breakpoint of the plane wave behavior with increase in frequency. It also shows the images or reflection hotspots formed at the throat of the tapered section at the higher frequencies. In addition, the longitudinal scan analysis portray the reflections from the back wall of the chamber. In conclusion, the known concepts and ideologies of the tapered chamber design are reexamined from a different perspective based on the analysis results.
Antenna Measurements from UHF to V-Band in AFRL's Newly Commissioned OneRY Indoor Range
Experimental measurement plays a key role for technology maturation in an R&D environment. In this paper we highlight the versatility of a new compact range at the Air Force Research Laboratory (AFRL), Sensors Directorate. In its first year of operation, the OneRY Range supported a wide variety of projects ranging from electrically small antennas to 20’ structures, spanning frequencies of 400 MHz to 45 GHz, and involving applications covering land, airborne, and space-based platforms. Here we present measured results from three different antenna development efforts for the Air Force. The first effort involves a UHF meta-material inspired antenna developed for an airborne application. In addition to successfully demonstrating relatively low frequency capability for a compact range, this effort met the challenge to measure antenna patterns from a physically large target. Results from OneRY are compared to those collected from a tapered chamber. Next we show experimental measurement of digital beam forming (DBF) in a large conformal phased array antenna operating at L and S bands. The DBF experimental testing is part of a follow-on effort to an Advance Technology Demonstration conformal array supporting satellite tracking, telemetry and command (TT&C). Finally, we present results from a “quick look” investigation into the operability of a COTS antenna system matched to a third party radome. The project supports airborne satellite communications at K, Ka, and Q bands. Performance of a high frequency extension (18-50 GHz) to the compact range is examined to include an inter-range comparison to planar near-field measurements. A description of the OneRY Indoor Range is also provided.
“Defects” of Specular Patches in Elongated Anechoic Chambers
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.
Extension of Tapered Chamber Quiet Zone with Large RF Lens
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.
Side Wall Diffraction & Optimal Back Wall Design in Far-Field Antenna Measurement Chambers at VHF/UHF
Anechoic chambers utilized for far-field antenna measurements at VHF/UHF frequencies typically comprise rectangular and tapered designs. The primary purpose of conventional far-field chambers is to illuminate a test zone surrounding the Antenna Under Test (AUT) with an electric field that is as uniform as possible, while multiple reflections from the side wall absorber assemblies are kept to a minimum. The cross section dimensions of far field chambers at VHF/UHF frequencies can be electrically small, often as little as 3.. In this paper the side wall reflections at VHF/UHF bands are studied in more details for elongated rectangular and tapered chambers. In particular, the reflectivity is evaluated in rectangular chambers as a function of electrical dimensions of the chamber cross – section and of the ratio W (width of the chamber) or H (height of the chamber) to L (length – separation between antennas) for values ranging from 0.5 to 2. The methods of reflectivity improvement are presented and compared. In particular, the conventional chamber design is compared with a “Two Level GTD” approach [4,5,7] and the latter one shows significant reflectivity improvement in the test zone, even at longer source antenna AUT separations. The side wall reflections are examined in tapered chambers as well. The back wall reflection mechanism, which assumes multiple incident waves – direct from the source antenna and reflected from the side walls, floor and ceiling, is offered and confirmed by the simulation, which, in turn, yields an optimized back wall chamber design (see also ).
Back Wall Design Trade â€“ Offs in High Performance VHF/UHF Chambers
The back wall is an important element in a high performance tapered or compact range anechoic chamber operating at VHF/UHF frequencies, as by design it is intended to absorb the non-intercepted portion of the incident plane wave containing the majority of the power transmitted by the chamber illuminator. Back wall reflections may interfere with the direct illumination signal and thus influence the test zone performance. Consequently, in order to ensure that the overall test zone reflectivity specification is met, the reflectivity produced by the back wall should be better than the reflectivity specified for the test zone. The conventional approach used to achieve good reflectivity is to apply high performance, high quality absorbing materials to the back wall. Further improvement of up to 10 dB can be achieved if a Chebyshev absorber layout is implemented [1, 2]. This layout consists of high performance absorbing pyramids of different heights, and assumes that the performance does not depend on a metallic backing plate. This approach is expensive, and presents technical challenges due to the complexity involved in the design and manufacturing of the absorbing material. In addition, installation and maintenance is an issue for such large absorbers. In this paper an alternative approach is presented which is based on an implementation of a shaped back wall as, for example, suggested in [3-5], and use of lighter, lower grade absorbing materials whose performance essentially depends on reflections from the metallic backing wall. This type of design can be optimized at the lowest operating frequency, if the back wall and absorber front face reflections cancel each other. Different back wall shapes are considered for a tapered chamber configuration, and the test zone reflectivity produced by a flat, inverted “open book” and a pyramidal back wall are evaluated and compared at VHF frequencies using a 3D EM transient solver .
An Integrated UWB Dual Polarized Tapered Chamber Feed Design Examply
Wave-launching in tapered chambers is often done by placing a commercially available antenna in the feed section. This approach has its own drawbacks: First, the physical sizes of these commercial antennas are often too big and cause the actual radiation center to be significantly away from the desirable apex point, resulting in poor measurement performance. Second, these antennas may need to be rotated when taking dual-polarization measurements or they may even need to be replaced completely when taking measurements at a different frequency band for which the existing antenna is not operational anymore. This antenna positioning in turn introduces another place for the uncertainty in the measurements. Previously, a novel integrated wave-launcher mechanism was presented by The Ohio State University-Electroscience Laboratory (OSU/ESL) researchers to overcome the problems stated above. In this work, a new integrated chamber feed has been designed employing new design ideas to address the issues encountered in this previous effort, such as transmitted power attenuation caused by waveguide cut-off at lower frequencies.
Introduction to the New MIT Lincoln Laboratory Suite of Ranges
A new antenna and RCS measurements facility consisting of four anechoic chambers has recently been constructed at MIT Lincoln Laboratory. The facility was designed with a rapid prototyping focus. The four chambers include a tapered chamber covering the 225 MHz to 18 GHz band, a millimeter wave rectangular chamber covering 4 to 100 GHz, a large rectangular anechoic chamber covering 150 MHz to 20 GHz, and a large compact range covering 400 MHz to 100 GHz. The compact range will be highlighted.
Update on a Novel Dual-Polarized Tapered Chamber Feed Design
A UWB dual linear polarized feed design for taper chambers was implemented and tested. The low frequency limit of a typical taper chamber was investigated. An improved design that includes a quad- ridge feeding structure allows for operating at lower frequencies was developed.
Design and Construction of a Production Antenna Test Cell
In order to accommodate the high volume of RF testing required for a specific large production antenna build, Ball Aerospace designed and built a miniature antenna test cell. The test cell is capable of performing VSWR measurements and antenna patterns, namely principal planes and conics, per the test requirements of the program. A significant effort was made to streamline the manufacturing process of the antennas and minimize the test time in order to reduce costs and meet production goals. The test cell features an integrated laptop PC, barcode scanner, and requires a HP8753E network analyzer. Human factors and process flow were important drivers in the chamber’s design. Specific test parameters for the antennas reside in a database referenced by a unique bar-code serial number attached to the back of each antenna. The operator is not required to have any a priori knowledge of the antenna or its performance parameters. The operation involves scrolling though a set of prompts from the computer. For this chamber, custom mechanical drawings, motor control systems, and software was designed and engineered to provide maximum efficiency on the production floor. The chamber, measuring only 6’ x 6 ‘ x 8 ‘, has provided comparable results to an on-site 75 foot tapered chamber. This approach is expected to be adopted by additional antenna programs internally in order to off-load capacity from large tapered antenna chambers.
UWB Dual Linear Polarized Feed Design for Tapered Chamber
New taper chamber feed section was created for numerical analysis. To launch the undisturbed electromagnetic wave into the test zone, newly designed dual polarized aperture-matched blade mode bowtie (ABB) antenna was designed and implemented at the vertex of the feed section of the tapered chamber. For the accurate calculation, wall type absorber samples are obtained and measured. These values are included for realistic configurations. From the simulated time domain result, field distributions at the aperture of the feed sections are investigated. Determination of the usable spaces for different frequencies is discussed. Also, cross-talk levels are presented since the feed antenna designed for dual polarization.
Shipboard EMI Reduction with Low Sidelobe Modifications
Undesirable antenna to antenna coupling has caused EMI problems between the WSC-6 SATCOM system and various systems in many shipboard installations. Long term solutions are currently being explored to resolve this EMI problem, which include adaptive interference cancellers and redesign of the WSC-6 feed and subreflector. However, these solutions are expensive and require several years to develop. An intermediate solution using RAM shrouds around the main reflector and subreflector edges of the WSC- 6 antenna has been proposed. The RAM shrouds were designed to reduce the spillover and diffraction of the antenna while having minimal impact on the antenna performances. A lightweight RAM was chosen to minimize the weight increase of the antenna. A prototype unit with the proposed modifications has been fabricated, assembled and tested in a tapered anechoic chamber, a near-field range, and a compact range. Significant reductions in the WSC-6 antenna sidelobes and backlobe have been verified via these measurements. Highlights of these modifications are described. Measured data (near field, compact range, tapered chamber, and shipboard) are presented.
Numerical Analysis of a Novel Tapered Chamber Feed Antenna Design
Tapered chambers have long been used for far-field antenna and RCS measurements. Conventional taper chambers used commercial antennas such as horns or log-period dipoles as wave launchers. One problem of this approach is the movement of the phase center associated with the antenna design. The positioning of the antenna inside the chamber is also critical. Undesired target-zone amplitude and phase distortion are caused by the scattering from the absorber walls. A novel feed antenna design for a tapered chamber is proposed here to provide broadband and dual polarization capabilities. This design integrates the absorber and the conducting walls behind the absorbers into to ensure a stationary phase center over a wider frequency range. In such a design, the dielectric constant of the absorber is utilized to maintain a clean phase front and a single incident wave at high frequencies. The conductivity of the absorber is also utilized to shape the field distribution at low frequencies. As a result, a wider frequency range can achievable for a given chamber size. One trade-off of this design is its reduced efficiency could be associated with the absorber absorption. Some simulation results from a 3-D FDTD model of a prototype design will be presented.
Cellular Band Far Field and Cylindrical Near Field Tapered Anechoic Chamber, A
A novel, combined far-field and cylindrical near-field tapered anechoic chamber was designed for RACAL Antennas (UK). Advanced ElectroMagnetics Inc. (AEMI) and ORBIT/FR-Europe collaborated in the design and the facility was completed in April 2000. The far-field tapered chamber performance was verified by Shielding Integrity Services. The tapered chamber far field facility performance after construction is compared with the original design predictions at several cellular band frequencies. Near-field measurements, in the rectangular section, compare well with outdoor measurements. There is discussion of the installation of the shielded facility and the absorbers intended for engineers interested in the cellular antenna test and measu rement arena.
Enhanced Tapered Chamber Design, An
The tapered chamber has been used for more than 35 years for mainly lower-frequency antenna measurements. The basic design of the tapered chamber has not changed significantly since its inception. Tapered chambers provide better quiet zone fields by placing the feed antenna's phase center at the vertex of the tapered walls, virtually eliminating reflections from the side walls. Recent innovations that better chamber versatility include an ultra-wideband SBH feed antenna [1,2], a less visible rotating support structure for the AUT and a new Chebyshev-based absorber treatment . Utilizing these new features, a tapered chamber has been designed to have a large bandwidth, yet have an overall structure that is small enough to allow portability. This paper describes a chamber that operates from 400MHz to 40GHz and has an overall length less than 30' long. Structure, components, and field results are presented in this paper.
An Ultra-wide bandwidth, tapered chamber feed
The tapered chamber was originally developed about 30 years ago to provide better quiet zone fields by eliminating the reflected fields from the side walls. This concept works well if the feed antenna is mounted at or near the vertex of the tapered section. Unfortunately, there has not been a feed specifically developed for this application; as a result, range operators have been forced to use sub-optimal feed antennas. This paper describes a new tapered chamber feed that is specifically designed to optimize the total system so that the originally intended performance can be achieved. This feed has been designed, built and tested. It covers the frequency band from 100 MHz to 2 GHz and has been optimized to provide the largest quiet zone possible. The description and capability of this new feed is presented in this paper.
Acceptance of the Sanders Merrimack 23 compact range for RCS measurements
In 1993, we presented the newly completed compact range and tapered chamber facility . As part of this presentation, the issue of “range certification” was presented. This paper will discuss the work that we have done with the compact range for radar cross section (RCS) measurement acceptance. For customer acceptance, we had to “prove” that the compact range made acceptable measurements for the fixtures and apertures involved. Schedule and funding did not permit the full exploitation of the uncertainty analysis of the chambers, not was it felt to be necessary . The determination of our range capabilities and accuracy was based on system parameters and target measurements. Targets that were calculable either in closed form solutions (spheres) or by numerical methods (cylinders and rods) were used. Finally, range to range comparisons with the Rye Canyon Facility  of a standard target was used. The range to range comparison proved especially difficult due to customer exceptions, feed differences, and target mounting. This paper will discuss the “success” criteria applied, the procedures used, and the results. The paper will close with a discuss of RCS standards and the range certification process.
High-polarization-purity feeds for anechoic chamber, compact, and near field test ranges
With the recent use of dual-polarized transmission and reception on communications links, the capability to perform accurate polarization measurements is an important requirement of test-range systems. Satellite antennas are commonly measured in the clean, protected environment of compact and near-field ranges, and a circularly polarized feed/field probe is a primary factor in establishing their polarization properties. The feeds also provide excellent source-horn systems for tapered anechoic chambers, where their circular symmetry and decoupling of the fields from the absorber walls improve the often troublesome polarization characteristics of tapered chambers. Circularly polarized feeds are generally composed of four primary waveguide components: the orthomode transducer, quarter-wave polarizer, scalar ring horn, and circular waveguide step transformer. Linearly polarized feeds omit the quarter-wave polarizer. This paper discusses the design and performance of high-polarization-purity source feeds for evaluating the polarization properties of antennas under test. Circularly polarized feeds have been constructed which operate over 10- to 20-percent bandwidths from 1.5 to 70 GHz. Gain values are generally in the area of 12 to 18 dBi, with cross-polarization isolation in excess of 40 dB. Representative measured data are presented.
Lockheed Sanders, Inc., antenna measurement facility.
Lockheed Sanders, Inc., has constructed a state-of-the-art electromagnetic measurement system. Cost considerations dictated the use of existing facilities and space, We took advantage of the lessons learned from the Lockheed Advanced Development Company's (LADC) Rye Canyon, California Facility . Lockheed Sanders, Inc. now has a complete indoor measurement capability from VHF to MMW. Lockheed Sanders, Inc. needed a facility capable of making measurements over a broad range of frequencies. The system consists of a tapered chamber and a compact range. The system consists of a tapered chamber and a compact range. The tapered chamber has a measurement area of 28' x 28' x 34'. This range is capable of antenna and RCS measurements from .1 to 2 GHz. The compact range is designed for 2 to 40 GHz. Using a Scientific Atlanta, Inc. reflector scaled from the Rye Canyon reflector, a 6' x 6' quiet zone is possible. Feeds consist of a feed cluster aligned for phase and limiting parallax and horn cross-talk. Both chambers use the Flam and Russell 959 measurement system. This paper will discuss the chambers and their operation. The paper will close with a demonstration with measurements on standard, complex targets.
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