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We present the results of developing a MATLABbased Near-field Antenna measurements toolbox. The purpose of this package is two-fold. First, it functions as a training tool, to help the user understand the near-field measurement process. Second, it can also function as an analysis aid, providing insight into the effect of errors on the measurement process. Results obtained from using the beta version of the toolbox are presented and the toolbox will be available as a download from the website listed in the paper, to solicit feedback from the measurement community.
A technique is presented for determining the insertion phase of array elements directly from time domain measurements. It is shown that the Inverse Discrete Fourier Transform (IDFT) commonly used in swept frequency time delay measurements may yield unreliable phase results. A compensation to the IDFT is proposed which allows the phase of an array element to be accurately estimated from time domain data without gating and without taking a second DFT. The technique is applied to determine the insertion attenuation and phase of the elements in a linear L-band phased array. Compared to conventional array calibrations, the removal of extraneous range reflections implicit to the time domain technique resulted in a significant improvement in measurement accuracy.
The restriction of ? 2D2 R = is a commonly employed criterion for the minimum required separation between the range antenna and the Antenna Under Test (AUT) in a Far-Field (FF) antenna test range. However, this criterion, which is suitable for most common and simple cases, may not be adequate for more specialized test applications. Direction-finding (DF) interferometer antenna array testing is one such example. In a DF interferometer antenna array the phase difference between any two antennas serves as an Angle-Of-Arrival (AOA) discriminator for the radiation impinging on the array. At the system level, the array must be tested in order to calibrate its AOA discrimination function and to evaluate its accuracy, which, in many cases is done using a FF test range. In this paper, interferometer array FF testing is analyzed and an expression is developed for estimating the required separation between the range antenna and the array under test, in order to satisfy certain angle discrimination accuracy requirements. The results are compared with the common FF criterion and with restrictions imposed by other considerations.
In order to diagnosis array antennas we implemented the back projection technique in our bi-polar near field laboratory. Using capabilities of microwave holography we investigated actual distribution of excitation coefficient values in a variety of antenna arrays operating at 5 GHz and around 10 GHz. We investigated phase and amplitude alignment of the linear MC-8 phased array with eight elements operated in the band centered at 5000 MHz. The reconstructed phase distribution images reveal phase distributions consistent with the design values. A major technical impairment is that the resolution at the element level can not be easily assured and it is related to the element spacing.
MW HF phased antenna array, capable of very wideband frequency and scan coverage, is constructed as part of the High Frequency Active Auroral Research Program (HAARP). Operation of the HAARP Developmental Prototype (DP), consisting of 96 dipoles, demonstrated importance controlling even mode excitation originated by electrical asymmetry of the phased active array. For this purpose antenna matching units (AMU) are using hybrids with resistive termination on its even mode ports. Accurate predictions of the array ERP and safe operation of the extended to full 360 dipoles array require a trusted model. This paper presents an application of the measurement and analytic array characterization used for the active array performance prediction and optimization. Mixed mode sparameters of sampled dipoles were measured before multiport mixed-mode S-matrices were constructed using learned array symmetry properties. Created mixed-mode Smatrix model allows for estimation of the ERP, prediction of dipoles power distributions, the AMU and even mode termination impedance optimization for better efficiency.
The installation and operation of large horizontal UHF phased array antennas in arctic regions is challenged by severe environmental conditions. It can be shown that large and moderate snowfall can impact the operation of exposed dipole array elements and reduce aperture efficiency. Reflection coefficients measured at the antenna terminal of an embedded array element can vary drastically as a function of snow depth. In this paper we will describe several measurements that show embedded array element impedance variations versus snow height. Measured results will be presented for an array operating from 400 MHz to 470 MHz.
Ideal phased array antennas offer advantages for communication systems, such as wide-angle scanning and multibeam operation, which can be utilized in certain NASA applications. However, physically realizable, electronically steered, phased array antennas introduce additional system performance parameters, which must be included in the evaluation of the system. The NASA Glenn Research Center (GRC) is currently conducting research to identify these parameters and to develop the tools necessary to measure them. One of these tools is a testbed where phased array antennas may be operated in an environment that simulates their use. This paper describes the development of the testbed and its use in characterizing a particular K-Band, phased array antenna.
A Wideband Optically Multiplexed Beamformer Architecture (WOMBAt) was developed and characterized at the Crane Naval Surface Warfare Center Active Array Measurement Test Bed (AAMTB) facility. The project includes development and integration of the true-time delay (TTD) WOMBAt photonic beamformer with the Active Array Measurement Test Vehicle (AAMTV). The AAMTV is a 64-channel transmit-receive (TR) module based phased array beamformer that is integrated with the AAMTB facility 12’x9’ planar near-field scanner. The AAMTV provides phase trimming and a small amount of delay using electrical components while the WOMBAt provides longer delays using commercial-off-the-shelf (COTS) optical components typically manufactured for the telecommunication industry. By integrating the WOMBAt with the AAMTV, a highly flexible test environment was achieved that includes system calibration, multi-frequency scanning, and antenna pattern analysis. Phase I receive tests for this system were previously described and presented to AMTA[1] in 2002. This paper will describe the results of reconfiguring the AAMTV into a transmit architecture for Phase II. WOMBAt successfully demonstrated wideband TTD in both receive and transmit configurations at angles greater than the system goal of ±65º while exceeding all other system level performance goals. System level performance included a beam squint of less than 1.1º for receive and 0.5º for transmit, a worse case amplitude variation of 2.4 dB receive and 1.6 dB transmit and differential delays of less than 3.5 picoseconds.
The Intelsat-IX spacecraft carries a C- and Ku-Band payload. It provides coverages from five different orbital locations over Atlantic (AOR) and Indian (IOR) ocean regions. The feed arrays for the C-band multifeed offset parabolic reflector antennas were designed, manufactured and tested by EADS Astrium GmbH in Munich, Germany. Design drivers for the antenna subsystem were the high power requirement for the transmit antenna and stringent isolation specification for both transmit and receive antennas. The final designs feature as many as 145 feed horns and up to ten switches. Due to the complexity of the beam forming network and the large number of SCRIMP (Short Circular Ring loaded Horn with Minimized Cross-Polarization) horns at every feed array a special test philosophy was introduced in order to detect any malfunction of the array at an early stage of the antenna assembly and integration. This paper will present details of the applied test sequence starting at the initial beam forming network measurements and the intermediate near-field feed testing under extreme environmental conditions up to the final antenna testing in a compact range at unit and at spacecraft level. The used inhouse data evaluation software platform allows the evaluation of any measurement at any stage of the testing sequence independent of the actual applied losses and /or design error allocations.
The existing planar near-field antenna test range at Alcatel Space (ASP) in Toulouse has recently been enlarged and the frequency bandwidth increased to 18.5 GHz to allow for the testing of large fully integrated space flight antennas. This upgraded test range, including a specific reconfigurable reflector antenna support tool, will be described. The range assessment method, carried out with a Ku-band single reflector antenna, will be outlined: error budgets obtained with the NIST 18-term method as well as absolute gain measurement and inter-comparisons with compact antenna test range (CATR) measurements will be presented. Typical applications for L-Band and C-Band antennas will be presented with error budgets. Comparisons with simulated data will further demonstrate the range performance.
The recent launch and successful orbiting of the EO-1 Satellite has provided an opportunity to validate the performance of a newly developed X-Band transmitonly phased array aboard the satellite. This paper will compare results of planar near-field testing before and after spacecraft installation as well as on-orbit pattern characterization. The transmit-only array is used as a high data rate antenna for relaying scientific data from the satellite to earth stations. The antenna contains distributed solid-state amplifiers behind each antenna element that cannot be monitored except for radiation pattern measurements. A unique portable planar near-field scanner allows both radiation pattern measurements and also diagnostics of array aperture distribution before and after environmental testing over the ground-integration and pre-launch testing of the satellite. The antenna beam scanning software was confirmed from actual pattern measurements of the scanned beam positions during the spacecraft assembly testing. The scanned radiation patterns on-orbit were compared to the near-field patterns made before launch to confirm the antenna performance. The near-field measurement scanner has provided a versatile testing method for satellite high gain data-link antennas.
This paper deals with different aspects of the on-ground antenna pattern characterization of the MIRAS radiometer for ESA’s SMOS mission. Various technical challenges of the project are briefly described. Special attention is given to the effect of multiple reflections between the antenna under test and the measurement probe. A series of antenna measurements of the MIRAS radiometer antennas is now on-going at the DTU-ESA Facility. The main objectives of these are to investigate the accuracy of the forthcoming antenna characterization, to find solutions to the already known problems, to identify new possible difficulties, and to establish an optimal measurement strategy, which should allow for the tight error requirements and minimize the overall time of the measurement campaign.
As known, spacecraft antenna design is an everdemanding task, due to the tight requirements on performance and the small available space. Moreover, the trend of installing array antennas onboard makes this task even more complex. For this reason, every antenna design must be verified against its installation constraints, in term of pattern distortion, due to interaction with spacecraft structure, inter-antenna coupling with nearby antennas, which can be remarkable due to the reduced available space, and generation and propagation of passive inter-modulation products (PIM), that can seriously affect the performance of transponders. For the above reasons, a Design Framework has been developed in the frame of an ESA contract. In this activity, European universities, industry and research centers cooperated in order to integrate within a single environment different prediction codes providing the required modeling capabilities. The system is able to guide the user from the antenna design phases through antenna installation simulation. The framework also allows for storage and management of experimental data in the more common formats or in user-defined ones, making able the designer to validate its numerical models with measured data obtained from intermediate breadboards. A validation activity is in progress and comparisons between simulation and measurements are reported, together with main characteristics of the design system. The system has been applied, among others, in the design and compatibility analysis, of Galileo antennas.
Bistatic radar cross sections of targets are computed from field measurements on a cylindrical scan surface placed in the near field of the target. The measurements are carried out in a radio anechoic chamber with an incident plane-wave field generated by a compact-range reflector. The accuracy of the computed target far field is significantly improved by applying asymptotic edge-correction techniques that compensate for the effect of truncation at the top and bottom edges of the scan cylinder. The measured field on the scan cylinder is a “total” near field that includes the incident field, the field of the support structure, and the scattered field of the target. The background subtraction method determines an approximation for the scattered near field on the scan cylinder from two measurements of total near fields. The far fields of metallic sphere and rod targets are computed from experimental near-field data and the results are verified with reference solutions.
The National RCS Test Facility (NRTF) has designed, fabricated, and implemented an efficient and robust calibration procedure and test body applicable to pylon based monostatic RCS measurements. Our unique calibration test body provides physical separation between the calibration device and pylon allowing the pylon to be outside the range gate of the calibration device. This separation reduces the calibration device uncertainty due to target support contamination and interaction. Spectral analysis and feature extraction of rotational dihedral/dipole data allows further rejection of background noise and clutter that possess different angular dependencies from those of the dihedral/dipole. Due to the significant reduction in the achievable crosspolarization isolation that occurs with a small degree of positioning error in dihedral/dipole roll angle, a data driven search algorithm has been developed to select the two dihedral/dipole angles used by the polarimetric distortion compensation algorithm.
We introduce a new calibration standard geometry for use in a static RCS measurement system that can simultaneously offer multiple “exact” RCS values based on a simple azimuth rotation of the object. Called the “cam,” the new calibration device eliminates the problem of frequency nulls exhibited by other resonantsized cal devices by shifting the nulls through azimuthal rotation. Furthermore, the “cam” facilitates the use of dual-calibration RCS measurements without the need to mount a second cal standard. The “cam” is practical to fabricate and deploy; it is conducting, composed of flat and constant-radius singly-curved surfaces, and is compatible with standard pylon rotator mounts. High-accuracy computational results from moment-method modeling are presented to show the efficacy of the new standard.
Coherent radar cross section measurements on a target moving along the line-of-sight in free space will trace a circle centered on the origin of the complex (I,Q) plane. The presence of additional complex signals (such as background, clutter, target-mount interactions, etc.), which do not depend on target position, will translate the origin of the circle to some complex point (I0,Q0). This type of phase-dependent I-Q data has been successfully analyzed. However, the presence of outliers can introduce significant errors in the determination of the radius and center of the IQ circle. Hence, we implement a combination of a robust and efficient Least-Median Square (LMS) and an Orthogonal Distance Regression (ODR) algorithm is used (1) to eliminate or to reduce the influence of outliers, and then (2) to separate the target and background signals. This technique is especially useful at sub-wavelength translations at VHF, where spectral techniques are not applicable since only a limited arc of data is available. We analyze data obtained as an Arrow III target moves relative to its supporting pylon. To demonstrate the effectiveness of the technique, we introduce rf interference signals into S band data and show that the uncontaminated parameters can be recovered with acceptable uncertainties.
Over the past few years, range certification activities have become more commonplace, as industry, government and academia have embraced the process and acted to implement documented procedures at their facilities. There is now a significant amount of documentation laying out the process, as well as templates to assist ranges in developing their range books. To date, however, there have been fewer examples of useful tools to assist the ranges in better understanding how the process will affect their specific range. The authors have developed a first generation MATLAB toolbox designed to provide ranges a “what-if” capability to see the impact of specific range errors on the range’s operations. Included within the toolbox are several types of additive and multiplicative errors, as well as means of modeling various aspects of radar operation.
A single tapered resistive strip (R-Card) has been used in the past in several applications related to antenna designs and ground bounce reduction for far-field ranges. Several antenna designs use single tapered R-Card to significantly reduce the diffracted fields from the antenna to achieve low side lobe performance and also maintain stable phase center location across wide frequency bandwidth. Single layer R-Card fences have also been successfully designed and used to reduce the ground bounce stray signal in far field ranges. Recently, a multilayer tapered R-Card concept has been investigated and implemented in two different applications for interaction reduction due to performance requirements. One of the applications is to use multilayer R-Card fences to reduce the groundbounce effect between two antennas for GPS applications. The second application is to embed the multilayer R-Card with the Styrofoam target support column used in RCS measurements to reduce the interaction between the target-under-test and the metallic azimuth rotator underneath the Styrofoam column. In both applications, the multilayer R-Card concept, with different resistance distributions and proper spacing, has been designed and evaluated such that it behaves as an absorber to reduce the interference/interaction between two antennas or two scattering objects. The design and evaluation of this new multilayer R-Card concept will be presented in this paper.
System Planning Corporation (SPC) is pleased to announce our new instrumentation radar measurement system denoted the Cheetah radar line. This radar system is based around the new Agilent PNA series of network analyzers. The PNA operates from 0.1 to 67 GHz and is utilized for making gated CW or CW RCS and Antenna measurements. The PNA has a built in synthesizer that allows the unit to be used without costly external synthesizers and external mixers. The PNA also has four identical receiving channels, two signal and two reference, that permit simultaneous co and cross pol measurements to be made. PNA IF bandwidth is selectable from 1 Hz to 40 kHz to optimize measurement sensitivity, dynamic range and speed. Using the segmented sweep feature of the PNA a single frequency sweep can be broken into segments, to further optimize the sensitivity, dynamic range, and speed. Each segment can have its own start and stop frequency, frequency step size, IF BW and power level. SPC has developed the high speed RF gating, low noise RF preamplifiers and high speed digital timing system, which allow maximum sensitivity, full up gated CW or CW radar measurements using the PNA. SPC has coupled the system to the CompuQuest 1541 RCS and Antenna Data Acquisition and Data Analysis Processing Software. This exciting new product line offers reduced cost and improved performance over current network analyzer based systems using the HP 8530, 8510, etc. Performance improvements are in the reduced noise figure, sensitivity, dynamic range and measurement speed. Measurement speeds are increased by at least a magnitude of order over the older systems and in some cases a couple of orders of magnitude.