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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].
ESA’s Compact Antenna Test Range at ESTEC has been relocated which has given the chance to improve the alignment of the reflectors. Based on measure-ments of the reflector surfaces the best-fit positions and orientations of the reflectors have been deter-mined. It turned out that the choice of parameters to describe the reflectors and their position had impor-tant impact on the optimization process: The parame-ters shall – as far as possible – be orthogonal in the sense that a change in one parameter must not influ-ence the final value of the other parameters.
Compact test ranges are worldwide used for real-time measurements of antenna and payload systems. The Compensated Compact Range CCR 75/60 and 120/100 of Astrium represent a standard for measurement of satellite antenna pattern and gain as well as payload parameter due to its extremely outstanding cross-polar behavior and excellent plane wave field quality in the test zone. The plane wave performance in the test zone of a compact test range is mainly dependent on the facilities reflector system and applied edge treatment as well as on the RF performance of the range feed. To provide efficient and economic testing and maintaining the needed measurement accuracy the existing standard set of high performance single linear feeds covering the frequency range from 1 - 40 GHz had been extended to operate simultaneously in dual linear polarization. In addition several customer specific range feeds had been developed and manufactured and validated. More detailed information and achieved test results for the new high performance range feeds will be presented.
ORBIT/FR has recently delivered several high performance millimeter wave antenna systems for use in compact range and direct illumination chambers. These systems utilize the latest Agilent Technologies millimeter wave modules used in conjunction with the PNA-X vector network analyzer. These systems offer the following features: . Integration of the transmit and receive modules at the AUT and source/feed antenna for a low loss implementation . Integral range reversibility for AUT transmit or receive capability . Full integration with a baseline 2-50 GHz system capability . Full and automatic harmonic selection for all measurement bands . Optimized RF equipment placement in the chamber The systems are integrated with the 959Spectrum Antenna Measurement Software to provide a fully automatic antenna measurement capability from 2-110 GHz. Patch panels were used to avoid any re-cabling form the microwave bands to millimeter wave bands. Greater than 70dB dynamic range was demonstrated at W band. Performance results are shown, and optimized layout considerations are discussed that demonstrate a design that results in high performance as well as operator convenience is setting up measurements over all the microwave and millimeter wave bands.
A broad band interferometer antenna was designed and manufactured by Saab Avitronics. Saab Aerotech has installed a test facility for calibration of the interferometer antenna. The main purpose of the facility is to measure the interferometric function of the antenna. The interferometric function of the antenna can be measured directly but this method puts very high demands on the test range performance. An alternative method where each element is centered on a short far-field range is evaluated and compared by measurement with a large compact range at Saab Microwave Systems. The paper also describes the design aspects when measuring broad band, broad beam interferometer elements together with the actual design of critical components such as positioners, RF-system and absorber treatment.
RF guided missile developers require flight simulation of their target engagements to develop their RF seeker. This usually involves the seeker mounted on a Flight Motion Simulator (FMS) as well as an RF target simulator that simulates the signature and motion of the target. Missile intercept engagements are unique in that they involve highly dynamic relative motion in a short period of time. This puts demanding requirements on the RF target simulator to adequately present the desired phase slope, amplitude, and polarization to the seeker antenna and electronics under test. This paper describes a newly installed RF Target Simulator that addresses these requirements in a unique fashion. The design utilizes a compact range reflector, dynamically rotated in two axes as commanded by the flight simulation computer, to produce the desired changing phase slope and an RF feed network dynamically controlled to produce the desired changing polarization and amplitude. Physical optics analysis establishes an accurate correlation between reflector physical rotation and resulting angle-of-arrival of the wave front in the quiet zone. The RF Target Simulator is self contained in a two-man portable anechoic chamber that can be disengaged from the FMS and rolled to and from the FMS as needed. Measurements are presented showing the performance of the RF Target Simulator.
A compact antenna test range has been analysed for stray signals. The analysis is based on GTD ray trac-ing, i.e. obeying the reflection law in the chamber walls and assuming straight edges of reflectors and walls. Comparisons to an RCS as well as a time-domain measurement of the quiet-zone performance show good agreements with respect to identification of the ray paths of the stray signals. Rough estimates of the power loss at reflections and diffractions show acceptable agreements with the measured levels.
This paper presents the initial field probe characterization results for an RF scattering compact range using a high precision calibration sphere. This approach uses an Inverted Stewart Platform to position the ultra-sphere through the target quite zone. The Inverted Stewert Platform and optical target tracking system provide a fast and efficient for performing a volumetric incident illumination field characterization of the compact range quite zone using a backscatter RF measurement. The Inverted Stewert Platform system uses six small diameter strings attached to the ultra-sphere to provide the ultra-sphere positioning over the entire quiet zone of the compact range. The inverted Stewart platform also offers increased stability of the target by damping out the torsional pendulum motion typically encountered in conventional string support systems. This presentation will discuss an in-house development of the sphere field probe and discuss advantages and disadvantages of the ultra-sphere volumetric field probe.
Measurement of radome beam deflection and/or Boresight shift in a compact range generally requires a complicated set of positioner axes. One set of axes usually moves the radome about its system antenna while the system antenna remains aligned close to the range axis. Another set of axes is normally required to scan the system antenna through its main beam (or track the monopulse null) in each plane so the beam pointing angle can be determined. The fidelity required for the beam pointing angle, combined with the limited space inside the radome, usually make this antenna positioner difficult and expensive to build. With a far-field range, a common approach to the measurement of beam deflection or Boresight shift uses a down-range X-Y scanner under the range antenna. By translating the range antenna, the incident field's angle of arrival is changed slightly. Because the X-Y position errors are approximately divided by the range length to yield errors in angle of arrival, the fidelity required of the X-Y scanner is not nearly as difficult to achieve as that of a gimbal positioner for the system antenna. This paper discusses a compact-range positioner geometry that approximates the simplicity of the down-range-scanner approach commonly used on far-field radome ranges. The compact-range feed is mounted on a small X-Y scanner so that the feed aperture moves in a plane containing the reflector's focal point. Translation in this 'focal plane' has an effect very similar to the X-Y translation on a far-field range, altering the direction of arrival of the incident plane wave. Measured and modeled data are both presented.
A large single reflector corner fed rolled edge compact range system, featuring an elliptical cylinder 12’ (H) x 16’ (W) x 16’ (L) quiet zone has been recently installed in a large anechoic chamber [1]. The Compact Range System parameters, such as reflector surface tolerance of better than 0.001” over the Quiet Zone section of the reflector and superior Quiet Zone field performance at frequencies down to 1.0 GHz were verified and validated. As a part of further studies of potential advantages delivered by the compact range system, the study of the compact range application to Antenna and RCS measurements at VHF/UHF frequencies was initiated. Though the reflector surface tolerance is not an issue at the VHF/UHF bands, successful compact range operation at these frequencies would be a significant expansion of the capabilities of the existing compact range system. In order to evaluate the system performance at VHF/UHF frequencies a number of challenging technical issues had to be resolved and performed. They include: Compact Range Quiet Zone Performance Analysis at the VHF/UHF bands Choice of a concept for a broadband feed suitable for the application and installation within the existing feed carousel Feed Design and Performance Validation Feed Installation in the existing feed carousel Quiet Zone Field Probing and Performance Verification All these issues were addressed in the development of a suitable low frequency feed, and are described in more detail below.
Orbit/FR has installed a new compact range for antenna measurements at ACE Antenna Corp. The measurement facility covers a frequency range from 0.8 to 40GHz with a Quiet Zone size of 3 m diameter x 3 m length. The design of the compact range is similar to the one already installed by Orbit/FR at Ericsson (Sweden) with some improvements in the mechanical design and in the system parameters. An intensive simulation of the reflector serrations had allowed for finding its optimal profile, thus improving the quiet zone parameters at entire frequency range, especially at low frequencies, at which a number of base-station and mobile antennas are expected for testing by ACE Antenna Corp. A new design of a feed positioner and a baffle house added more convenience for the compact range alignment and operation. The system was installed and qualified in March 2008. The field probing has been performed within the entire operating frequency range, which then allows for evaluation of the antenna measurement accuracy. A system description as well as results of simulation and excerpt of the qualification data is presented in the paper.