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Compact range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signature. This paper investigates strategies for improving compact range measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the range source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational finite-difference time-domain (FDTD) model at VHF/UHF frequencies to determine baseline performance and develop trade rules for more advanced designs. Physical optics (PO) models were used to analyze the final design at the frequencies of interest.
Stray signals/scattering suppression techniques will be deployed to enhance measurements quality of a combined compact antenna test range (CATR) and spherical near-field (SNF) measurement facility. Spherical mode filtering and softgating techniques will be the focus of this paper. Using soft-gating the mutual effects between the CATR and SNF facilities will be shown and mitigated. The use of SNF decomposition to enhance the far-field measurements will be also shown. This contributes to a reduction of the costs arising from the need of absorbers to shield both facilities and cover the antenna's support structure.
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.
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.
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.
Large dual reflector compact ranges are typically designed for antenna and payload testing of spacecraft antennas and payload units. Astrium's Compensated Compact Ranges have two major advantages for such measurements: (a) A very small cross-polarization (< -40 dB over the entire test zone) for frequencies = 3 GHz due to the compensating reflector design, (b) A scanning capability of the test zone due to the short effective focal length of the reflector system. The first item is a necessary condition for precise spacecraft antenna measurements at which the cross-polar performance is an important requirement and was subject to multiple publications in the past. The second one, the scanning capability, is an additional feature that was addressed in the past, but has not been analyzed in detail so far. This paper addresses practical implementations, achieved performance figures of the latest installations and inherent limitations by the utilization of the scanned quiet zones at a CCR test facility for antenna and payload testing.
As a novel concept for field probes, RCS measurements on long rigid objects rotated within a small angular range about the broadside condition are reported. The rotation was maintained either in a horizontal (H) plane or in a vertical (V) plane containing the center of the quiet-zone (QZ). Processing the RCS data by DFT yields a spectrum which is recognized as the field distribution along that object. This spectrum compares extremely well to traditional field-probes taken earlier by translating a sphere across the QZ in H- or V-direction. Preliminary results at several S-band frequencies are presented and discussed.
The Electronic Proving Ground's Antenna Test Facility at Fort Huachuca, Arizona has some of the most interesting testing structures in the world. These structures include a wooden Arc measurements system with a 23 m radius, a 30 m tower, and a compact range with an 18 m quiet zone. All of these structures are outdoors and support testing from UHF to mm frequencies on antenna systems mounted on large land and air vehicles. This paper describes the ranges supported by these structures (some of which were built in the late 1960’s) and the efforts made to keep these ranges current. This paper also describes an economical approach to arc range design which moves the arc instead of the vehicles. This paper discusses plans to build one of these systems outdoors at EPG within a limited budget.
A spherical array designed for hemispherical coverage of satellite communications at S-band that is approximated by hexagonal and pentagonal planar panels. Ball Aerospace built a segment of a 10m diameter spherical array that has one center pentagonal panel and five surrounding hexagonal panels. This paper describes our efforts at testing one large hexagonal panel in a compact range.
Normally, field power density is inversely proportional to distance from a radiating antenna. In a compact range, however, the reflector focuses the radiated field onto the feed. This dramatically increases the power density – similar to the sun through a magnifying glass. Naturally, if the power density gets high enough, it could set the feed area absorber on fire. In order to determine the focusing effects on the feed horn and surrounding absorber, a series of transmit tests were conducted to measure feed absorber heating with an IR camera. This paper describes the test set up, the test results, and provides an analysis of the test results with suggestions for increasing power handling at the feed.
This paper describes test methods and challenges for performing radio frequency (RF) characterization of a phased array antenna with element-level digital beamforming using planar near-field (PNF) and compact range technologies. The characterization of a digital array requires the synchronization of measurement equipment including positioner controllers, transmitters, and receivers. All hardware and software must remain synchronized with the array clock to achieve accurate amplitude and phase samples and ensure a coherent phase front. This synchronization is achieved through handshake triggers and communication protocols that are managed through external software. The acquisition of element-level data over large PNF scans presents unique challenges in data and post-processing that precipitate the need for optimization of array architecture as well as design of processing software. Advantages of the digital array architecture include being able to generate multiple receive beams from a single near-field scan for each frequency and the ability to compare multiple calibration methods efficiently using off-array processing.
In the first part of the contribution the simulation setup of compensated compact ranges is described. The Multilevel Fast Multipole Method (MLFMM) can be efficiently employed for frequencies up to C-band. In the second part of the paper the field distribution is investigated for horizontal and vertical feed antenna excitation. Diffraction effects of edge serrations and their influence on the plane wave quality are outlined. Apart from the theoretical point of view it is discussed how to deal with low frequency effects under practical considerations, e.g. moving the DUT positioner toward the main reflector might be limited. Thus the measurement performance has to be evaluated w.r.t. typical test range conditions.
The Enhanced Antenna Subsystem (EAS) is a 12 beam, receive only antenna which uses a combination of switched elements and phase delay to accomplish independent beam steering. The upper portion of the domeshaped antenna is populated with 45 circularly polarized antenna elements in an icosahedron pattern along with 15 additional circularly polarized elements along the cylindrical skirt extension. The antenna was tested in our 35’ by 35’ by 65’ compact range. Pattern testing was accomplished by mounting the antenna to a roll positioner atop a high load tower, which was then mounted to an azimuth turntable. The range has a 20’ by 20’ reflector providing an 8’ quiet zone. Using a switching network, we simultaneously characterized 11 statically pointed beams while tracking the range source antenna with the 12th beam. Post processing of the data was performed to separate the beam data and calibrate out losses through the switching network.
This paper presents a hybrid analysis algorithm, which is used at Radiation Group (UPM) to carry out the design of a conformal serrated-edge reflector for the mm-Wave compact range UPM facility. Main features of this algorithm involve its capability of handling conformal serrated rim parabolic reflectors, accuracy and computational efficiency.
A fully-polarimetric compact radar range operating at 240 GHz has been developed for obtaining Ku-band RCS measurements on 1:16th scale model targets. The transceiver consists of dual fast-switching, stepped, CW, X-band synthesizers driving dual X24 transmit multiplier chains and dual X24 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 45 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 27” FWHM quiet zone. In this paper a description of this 240 GHz compact range is provided along with an ISAR measurement example.
Very often far field conditions are violated at high frequencies RCS measurements and in real life scenarios. People go to great lengths to carry out these measurements in the far field. They make large investments to build suitable compact ranges, or long outdoor ranges. Others make extensive efforts to correct the near field measurements to the far field values. This paper suggests that those elaborate measures are superfluous, as far as the total RCS is concerned. Although near field measurements clip the high peaks, they broaden their shoulders compensating for the loss. Simulations and actual measurements show that the accumulative distribution of RCS values in the near field is equal or slightly higher than the distribution of these values in the far field, until one looks for very high 90th percentiles. Thus, for detection and survivability estimates the near field measurements provide a close upper bound.
In the last decades radar imaging techniques have been widely studied. Electromagnetic imaging is a very promising technique for many practical application domains (medical, surveillance, localization …). As an example, many RCS imaging systems have been developed for compact range indoor RCS measurement layouts. In this paper, a preliminary comparison of near field RCS images from Multiple Input Multiple Output (MIMO) arrays and monostatic radar is presented. The main objective of this study is to make use of efficient radar imaging algorithms, which were originally conceived for SAR systems, with MIMO arrays (ex. back projection) in order to develop real-time imaging applications based on MIMO array systems. The study was conducted with a one-dimensional MIMO array composed of 14 transmitting and receiving antennas. The goal of the optimization is to obtain radar images as similar as possible to those from monostatic radar. This paper presents the experimental layout, the imaging algorithms and the experimental results. As a conclusion, the imaging capabilities of MIMO arrays are discussed.
Compact ranges are well suited to perform accurate indoor RCS measurements. These facilities are limited at the lower end of their bandwidth by the size of the parabolic reflector. Therefore, when RCS characterizations are required in the UHF band, RCS measurement facilities usually operate large horns or phased array antennas in a near field measurement layout. However, these calibrated near field measurements cannot directly be compared to the plane wave RCS characteristics of the target. One way to compare simulation and measurement results is to take the near field radiation pattern of the antenna into account. This paper first presents the design of a phased array antenna developed for indoor UHF RCS measurements. Then a model of this antenna is derived and a simulation of the experimental layout is performed. In parallel, near field RCS measurements of a canonical target were performed with this phased array antenna in an anechoic chamber. As a conclusion, a comparison between simulation and experimental results on this particular canonical target is discussed.
The design of a specialized reflector antenna set that supports dual polarization, dual beam widths, and an integrated wideband monopulse tracking capability in the X-band range is described in this paper. The reflector antenna code available at The Ohio State University has been used as the design tool. The design of such an antenna has posed several challenges in the feed and reflector assemblies. The requirement for an integrated wideband monopulse has resulted in a feed array that contains 5 rectangular feed elements with a center-to-center spacing of 1" and a diamond configuration. The 5 feed design has been selected to enable a shared feed array and reflector surface for both transmit and receive beams that eliminates the need for a high-power wideband receiver protector in the radar system. The center feed element is used for transmit waveform and the 4 outer elements are used as receive elements only. Each feed element operates with horizontally and vertically polarized waveforms, requiring a total of 8 waveguide input ports. In this paper, the challenges of the dual beam widths, dual polarized, integrated RCS and tracking antenna are delineated and the tradeoffs among several design configurations are shown. The final design is selected based on the performance predictions using The Ohio State University Reflector Antenna Code. The performance of the antenna has been validated at the OSU compact range for pattern and gain. Both the design and measurement data are presented in this paper.
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 [6].