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In recent years the need for higher quality RCS calibrations has lead to several different calibration technique investigations, such as squat cylinders, bi-cones and hybrids of both. A desirable calibration technique requires: easy implementation, a known theoretical or calculable solution and minimal interaction. The sphere as a calibration target satisfies two of the three requirements. It has no alignment issues and can be easily calculated, but the sphere-holder interaction introduces several dB of error. To reduce this interaction error, a 3D string-reel support system has been developed and demonstrated that significantly improves sphere calibration accuracy. The string-reel sphere positioning system utilizes low dielectric and highly swept strings to achieve minimal calibration error. An additional benefit of this technique allows for field probing and quick quiet zone evaluations.
The paper deals with the Range Book review process, and describes the evaluation of the National RCS Test Facility (NRTF) RATSCAT Advanced Measurement System (RAMS) Central Measurement System (RCMS) Range Book against the criteria approved by the Range Commander’s Council Signatures Measurement Standards Group (RCC/SMSG). Three RCC/SMSG approved reviewers and one observer were charged with reviewing the processes and procedures documented in the RCMS Range Book against published criteria based on the ANSI-Z540 standard. The paper will concentrate on the processed used by the evaluators to perform their task, the training opportunities afforded the observer, the lessons learned by the evaluation team, and the benefits of the process to both the RCMS site and measurement community at large.
An internal research and development project at the Georgia Tech Research Institute (GTRI) focused on cohering multiple apertures into a single distributed aperture. Cohered distributed aperture antenna patterns were collected on the GTRI far-field range for a 1.5 GHz bandwidth at X-band frequencies. Both 1-way and 2-way antenna patterns were measured, with the 1-way antenna pattern measurement requiring coherence on receive only and the 2-way antenna pattern measurement requiring coherence on transmit and receive. The resulting data were compared with the ideal angular resolution and power-aperture gain product improvements from a perfectly cohered distributed aperture, and the results are presented. As measurement techniques were developed for collecting 1-way and 2-way antenna pattern data, sources of potential errors in measurement collection and aperture coherence were identified, with potential methods of error mitigation outlined.
In this paper the electrical displacement of phased array elements along the axis of a linear array, and in the direction normal to the array are examined. A closed-form solution is presented for determining the location of phased array elements from the first and second derivatives of the phase measured on a near-field antenna range. The technique is applied to swept CW measurement patterns of a 20-element, S-band array of open-ended waveguides. It is shown that the electrical location of edge elements differs significantly from the physical location in both x-dimension and z-dimension. The effects of wide array bandwidth on the phase center displacement are illustrated.
Phased arrays antennas are designed to control their radiation characteristics by accurately setting the phase and amplitude distribution of the elements. Inaccurate control of the phase and amplitude can significantly alter the radiation pattern of an array. In fact, the operating principle of scanning arrays of elements for applications such as target tracking or mobile satellite communications, where the requirements for low side lobes and high gain are of very high importance, is primarily based on precise control of the phase and amplitude of the elements. For these reasons, the complexity of antenna measurement system design for phased array antennas measurements involves high accuracy and precise time synchronization between all the components of the system. This paper presents a comprehensive solution for accurate and reliable measurement of very large phased array antennas at high frequencies. The presented solution addresses the following issues: • Accurate positioning of the RF sensor / probe. • High-speed multi – frequency data collection. • High-speed multi - port data collection. • Programmable and real-time TTL position event triggers. • Pulse measurement. • Multi beam measurement. • Synchronization with the radar computer.
Techniques for measurement of the phased-array antenna system include ambient temperature measurements in a compact antenna range, thermal vacuum testing, and spacecraft level testing. There have been two novel developments in the characterization of the phased-array system. The first is a “gain envelope” response, which is a measurement of the gain of the array at the intended scan angle as the array is electrically scanned in 1° increments. This response was produced through a combination of hardware and test software to synchronize the gain measurement with the mechanical and electrical scanning. The second is a phase steering verification test that utilizes couplers in each steered element in conjunction with previously measured element patterns to confirm that the antenna beam is steered properly. This method allows functional verification of the phased-array system while radiating into an RF absorber-lined hat during spacecraft-level tests.
Measurements of antenna array’s patterns are usually taken with feed networks, but for some digital beam forming arrays, feed networks are not included. To measure such arrays in traditional way, we must design a feed network first, which is too complex and inefficient when steering is required. A VNA can act as a multichannel receiver to form a digital beam forming array for testing. Today’s VNA may have many test ports (for example, 9-ports in Agilent E5091A multiport test set and 11-ports in Advantest R3986 multiport test set.) designed for multiport devices, which is very suitable for measuring small arrays without using feed networks. Another advantage of this method is that it can eliminate errors from imperfect feed networks. Automatic measurement program is required to calculate array patterns from S-parameters, which is easily developed using Labview or Matlab. Test results of a 3-elements adaptive anti-jam array with different jam DOA are demonstrated.
For enhancing the performance of existing near field antenna test facilities it is quite reasonable to use both conventional (the amplitude and phase measurements) and the phaseless measurements techniques during electrically scanning phased array antennas (PAA) testing. This simple yet critical approach helps to improve the quality of PAA alignment and testing reducing measurement errors and saving costs. In this way many difficulties related to precise phase measurements are overcome. Both simulation and measurement results will be presented to demonstrate the utility of such approach to PAA alignment and determination of its parameters. Comparison will be made between the PAA patterns for electrically scanned beams calculated using traditional near field - far field (NF/FF) transformations, the phaseless methods and the results obtained applying both measurement techniques.
This paper discusses the use of a laser-tracking device to provide position information in x, y, and z that can be used in position correction algorithms to correct for any displacement error in the actual measurement. Planar near-field measurements require taking amplitude and phase information at accurate and equal point spacing on a plane in front of the antenna under test. The required position accuracy on this plane has been determined to be approximately ./50. As frequencies increase higher, the accuracy in point spacing position on the planar grid becomes more difficult to achieve.
This paper presents a novel variable width time gating technique, which is applied to planar and cylindrical near-field data in impulse time-domain antenna near-field measurements. Due to the changing distance between the probe and the antenna under test (AUT) in planar and cylindrical scans, the conventional fixed time gating technique causes problems to remove multiple reflections from the desired AUT response. It further limits the application of time-domain measurement to planar and cylindrical scans. The new variable width time gating technique provides a flexible way to solve these problems. Test results for both planar and cylindrical near-field measurements are presented. The difference of far-field patterns between time-domain and frequency-domain near-field measurements is noticeable. We also show the effects on the far field patterns due to fixed and variable time gating windows. We further conclude that the time-domain technique also works for planar and cylindrical near-field measurements by using variable width time gating technique.
A new method to avoid the truncation error in antennas near-field measurements is presented. The truncation problem is solved by picking up the information that is lost due to the finite size of scanning area, in points of the space reachable by the measurement system. The points are chosen in order to obtain a stable reconstruction of the field falling outside the available scanning area. The method can be applied to any scanning geometry, including the planar and cylindrical ones, whenever the set-up allows to vary the distance between the antenna under test (AUT) and the probe during the scanning procedure. Application of the method to cylindrical near-field scanning is numerically investigated, assessing the effectiveness of the proposed technique.
Traditional antenna loading materials are dielectrics with a large dielectric constant. This results in a physical reduction, proportional to the square root of the ratio of the dielectric constant used for the miniaturized antenna and the dielectric constant used for the full-size antenna. Unfortunately, loading the antenna in this manner results in a rather sever reduction in efficiency as seen by a reduction in bandwidth. This paper suggests the use of a magneto-dielectric material as the substrate in order to increase the bandwidth. A magneto-dielectric material is has a non-trivial permittivity and permeability. It is often a composite mixture of a magnetic material and a dielectric material. The magnetic material is often an oxide, such as nickel zinc ferrite, which is most readily available as a sintered ceramic. Unfortunately, such a material is rather brittle, and so there is an interest in forming a composite of such materials with a flexible binder, such as urethane. This paper also begins the experimental evaluation for the use of these materials in patch antennas and discusses the next step in investigating further results.
Abstract Among the various methods to accelerate curing thermoset polymers, such as an epoxy, involves the use of radio frequency (RF) fields. In this, the polymer precursor materials are placed in a microwave reactor, high power RF fields are introduced, and the electrical (or for that matter, magnetic) loss mechanisms convert the RF power to heat and therefore inducing curing. One of the most challenging aspects of such curing methods lies in the fact that the materials being cured undergo chemical change during the process. This results in a time-dependent change in the electrical properties of the materials. It is therefore important to have accurate data on the material’s electrical properties as a function of both temperature and extent of cure. This paper describes an apparatus designed to facilitate such measurements.
A new rectangular anechoic chamber (20’L x 10’W x 9’7”H) has been established at California Polytechnic State University (Cal Poly) through donations and financial support from industry and Cal Poly departments and programs. The chamber was designed and constructed by three graduate students as part of their thesis studies to explore and further their understanding of chamber design and antenna measurements. The chamber project has included RF absorber characterization, overall chamber performance assessment, and software development for the coordination of a positioner with a vector network analyzer. This paper presents absorber characterization as a function of incidence angle and orientation to enable an overall chamber performance analysis. Test data at low incidence angles (< 30o) are compared to manufacturer performance curves at normal incidence. The mean response of the measured data indicates a correlation with manufacturer curves. Through ray tracing analysis, the ripple encountered in the test data is used to identify two effective reflection planes indicative of the foam geometry. The measured data are subsequently used to predict overall anechoic chamber performance to within 1dB for a majority of the actual scan data. Details of this analysis and comparisons to actual chamber performance are presented in a companion paper.
The use of electromagnetic scale models for evaluating the expected performance of a large test facility prior to its construction has been found to be useful in providing insight on how various absorber layouts effect the ultimate performance of the full scale test chamber. This report details the expected and measured performance of a series of absorber sets used in a 1/12 scaled model of a very large anechoic chamber to be used for both EMC and microwave measurements. Electromagnetic scaling of dielectric absorbers involves not just the geometry of the absorbers but also the amount of conductive carbon loading required to achieve a given reflectivity at the scaled frequencies. The goal is to scale performance over a very broad frequency range. It was found that absolute physical scaling is not always possible. Expected and measured performance of the scaled absorbers is detailed over the scaled frequency range of 360 MHz to 12 GHz. Selected measured chamber performance is included for Free Space VSWR, site Attenuation, and Field Uniformity to demonstrate the effectiveness of the scaling.
The focused-beam system can measure electromagnetic constitutive parameters of materials much more accurately than the classic free space arch system due to reduced scattering from the edge of the finite sample and its support structure. However, for a number of reasons, the system tends to perform poorly at frequencies below 4 GHz. In order to improve the system’s performance, we need to determine the causes of this degraded performance. One way to do this is by studying the field behavior of the system at the sample region. But instead of using Gaussian beam approximation for the exciting plane-wave and locating the antenna at focal lengths determined by geometric optics, we take advantage of recent advances in computational electromagnetic tools and high performance computing technology and find the field behavior near the sample by numerically solving the full Maxwell’s equations for the whole system. In this paper, we will present our approaches and our findings which lead to better understanding of the system performance.
A waveguide material measurement technique is developed for highly reflective or lossy materials. In order to extract the complex constitutive parameters from a material, experimental reflection and transmission scattering parameters are needed. In a traditional rectangular waveguide material measurement, the sample fills the entire waveguide cross-section, making it difficult to obtain a significant transmission scattering parameter with highly reflective or lossy materials. This paper demonstrates, through the use of a modal-analysis technique, how using a partially filled rectangular waveguide cross-section allows for better transmission responses to extract the complex constitutive parameters. Experimental results for acrylic and radar absorbing material are compared to stripline measurements to verify the modal-analysis technique.
Compact antenna test ranges such as the ESA/ESTEC CPTR are large facilities for the characterization of electrically and physically large antennas as well as end to end radiated payload testing. To achieve high accuracy measurements, time gating is used to filter out as many room effects as possible. The most common implementation of time gating is to perform a frequency sweep, Fourier transformation to the time domain followed by windowing, gating and back transformation to the frequency domain. All of this is at a time penalty. An alternative is to have a synchronised switching system to switch on and off the transmit power as well as switching on and off the receiver. Such a solution has been devised in a cooperative effort between EADS Astrium and the Munich University of Applied Sciences. The paper will present the capabilities of the Astrium HG2000 Hard Gate system (1) in the ESA/ESTEC CPTR, its implementation in the facility as well as presenting direct comparison of results obtained by the hard gate system with the conventional soft gate on both low gain and high gain antennas
This paper presents an analysis of the reflectivity performance of the anechoic chamber. Measurements indicating the performance of the chamber-installed foam absorbers (described in a companion paper) are used to complete this analysis. This is followed by a comparison of the analysis results to chamber measurements taken in accordance with the free-space VSWR procedure [1]. Agreement between the analysis results and worst-case VSWR test measurements is within 1dB for a majority of reflection angles. In addition to chamber performance predictions, this paper describes a method of identifying primary reflection paths through interferometer calculations that compare all single bounce reflection path lengths to the direct path length. The angular spacing between interferometer nulls is used to identify the primary reflection direction. This information can be used to improve the overall chamber reflectivity by identifying areas of significant reflections and enhancing absorber treatments in these areas.
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.