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RCS data from 8 to 18 GHz on an ensemble of aluminum spheres (dia. 14", 8", 6". and 3.22x) and stainless steel ball bearings (dia. 1.25", 1.0", and 0.75"), as supported by strings in the 9-77 Range, have been collected. For inter-range comparison, the same spheres as supported separately by strings and by a foam tower have been measured in the Millimeter Wave Range (MMWR). By taking selected dual calibration pairs, the uncertainty analyses on the three sets of data show general consistency between the two Ranges, as well as between the two methods of support. In addition, the results allow us to sort out the good spheres for calibration from the bad ones.
There is a trend within the RCS community to use squatty cylinders in place of spheres for calibration. A higher degree of accuracy can be achieved; however, cylinder calibrations require much more precision in the alignment procedures. This effort is doubled when the dual calibration target is also a cylinder. The dual calibration test article could be a sphere thus reducing calibration efforts as long as good correlation exists between theory and measurement sphere data. A series of measurements were collected at the NASA Langley Research Center Compact Range Pilot Facility to study measurement errors of spheres atop foam columns to determine their feasibility for dual calibration use.
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.
Rapid characterization and pre-qualification measurements are becoming more and more important for the ever-growing number of small antennas, mobile phones and other wireless terminals. There is a need driven by the wireless industries for a smart test set-up with reduced dimensions and capable of measuring radiating devices. Satimo has developed a compact, mobile and cost-effective test station called StarLab which is able to perform rapid 3D measurements of the pattern radiated by wireless devices. The StarLab equipment is derived from Satimo’s StarGate systems which are now well established spherical near field test ranges. StarLab uses a circular probe array to allow for real time full elevation cuts and volumetric 3D radiation pattern measurement within a few minutes. It is operating between 400MHz and 6GHz and can be configured for passive measurements and also cable less-active measurements. This paper describes in detail the multi-probe antenna test station and its different configurations for passive and active measurements. The accuracies for gain and power measurements are also presented as well as considerations on the total radiated power measured by the equipment. Additionally, calibration issues are discussed. Finally, measurements performed with the StarLab test station at Satimo are shown and illustrate the capabilities of the system. The measurement results are validated by comparison to the results obtained in other test ranges.
This paper describes the development, testing and evaluation of a new, automated system for calibration and AUT alignment of a planar near-field scanner that allows the calibration system to remain in place during AUT measurement and which can be used to support AUT alignment to the scan plane. During scanner calibration, probe aperture position measurements are made using a tracking laser interferometer, a fixture that positions the interferometer retro reflector at a precise location relative to the probe aperture and a probe roll axis that maintains the proper orientation between the retro reflector and the interferometer as the probe position is moved. Aperture scan path information is used to construct a best-fit scan plane and to define a Cartesian, scanner-based coordinate system. Scan path data is then used to build a probe position error map for each of the three Cartesian coordinates as a function of the nominal position in the scan plane. These error maps can be used to implement software-based corrections (K-corrections) or they may be used for active Z-axis correction during measurements. By using a set of tooling points on the antenna mount, an AUT coordinate system is measured with the interferometer. The system then directs an operator through a set of AUT adjustments that align the AUT with the planar near-field scanner to a desired accuracy. This paper describes the implementation and testing of the system on an actual planar scanner and AUT test environment, showing the improvement in effective scanner planarity.
Accurate near-field calibration of a large 60 ft. diameter reflector can be accomplished with a minimal sampling technique. Near-field amplitude and phase is collected as the reflector scans across a receiving calibration tower. The near-field data is then transformed to a far-field pattern using a Fourier transform technique. Information on far-field EIRP, directivity, pointing, axial ratio and tilt, as well as encoder timing is obtained with accuracies comparable to anechoic chamber measurement techniques. The system was analyzed for sampling and multipath effects, as well as the effects of phase and amplitude stability. A spherical wave expansion technique was compared to a straight-forward summation technique for the Fourier transform.
We present a first-order analysis of the RCS errors resulting from non-uniform ground-bounce illumination in mobile, ground-to-ground, diagnostic RCS measurements of aircraft. For the case of a non-planar ground surface, these errors are a function of both aspect angle and position on the target. We quantify the errors in terms of their impact on the sector mean RCS as a function of position on the target. For typical targets, we show that the mean RCS error increases significantly for points displaced (either horizontally or vertically) from the calibration point. Conversely, the sector mean RCS is relatively insensitive to small-scale variations in the height of the ground, even though the errors at a single frequency and aspect angle can be quite large.
The National RCS Test Facility (NRTF) has measured radar cross section (RCS) of fixed wing aircraft for many years. In order to expand our testing options at the NRTF Mainsite test facility, the NRTF has developed a rotorcraft measurement capability. The design is compatible for use with our 50-foot pylon, but unlike existing rotators, allows for RCS measurement of test articles that require significant forward and aft target pitches. Target mounting and positioning was not the only challenge. Our new capability required the control and collection of rotor blade position information, in addition to the control and collection of traditional target azimuth and elevation data. Modification of our existing acquisition software and command and control systems was also required. In order to maintain the integrity of the NRTF’s calibration processes and enable the use of existing calibration devices, hardware was constructed to enable mounting of these devices to the spindle system. Other important considerations that influenced the design and implementation of the spindle mount capability include cost effective mounting/dismounting of test articles (to include the targets and calibration devices) safety of the test articles and personnel, and the effective determination of backgrounds.
In general, the RF test setups of antenna test facilities are designed and optimized for antenna pattern and gain measurements. However, the operation of test facilities, especially the here considered 'Double Reflector Compact Ranges', can be extended, so that they can also be used for RCS testing. A simple and very practical expansion of the RF antenna test setup - while maintaining the real-time capability - can be achieved with the aid of a hardware gating system. With this type of setup, RCS measurements have successfully been performed in the Compensated Compact Ranges of EADS Astrium. The applied gating system was the high resolution Hard- gating System HG2000 of EADS Astrium, developed together with the Munich Univ. of App. Sciences. Within this paper, the applied facility and the gating system will be described firstly. Subsequently, the modified test setup and the test results obtained by calibration measurements will be shown. They will give an indication of the achievable resolution for the extended test system w.r.t. object size detection and resulting amplitude dynamic range.
Satimo’s STARGATE probe array systems are now well established as an efficient tool for testing radiated performances of wireless devices and antennas. Since 1998, about forty STARGATE measurement systems have been successfully installed worldwide. Recently, a range of new applications have also demonstrated the suitability of probe arrays for large radiating structures and directive antennas. These new generation of measurement set-ups present innovative aspects regarding their rapidity, dynamic range, and accuracy. This paper will describe several novel antenna testing concepts based on probe arrays that cover automotive, aerospace, and military applications and a wide range of frequencies. The basic difference between traditional approaches using single probe and the STARGATE approach using an array of probes will be explained along with probe array calibration procedures. An error analysis budget using the conventional NIST error terms will be presented including the specific terms related to the use of probe arrays. Also a discussion will be made on some of the key technical challenges to making large probe arrays including such issues as dynamic range, mechanical tolerances, and data truncation effects.
New results are presented on using small spheres mounted on a foam tower for calibration. Subtraction of the foam tower response is found to be necessary and sufficient for the dual-calibration method to work.
A calibration uncertainty analysis was conducted for the Air Force Research Laboratory’s (AFRL) Advanced Compact Range (ACR) in 2000. This analysis was a key component of the Radar Cross Section (RCS) ISO-25 (ANSI-Z-540) Range Certification Demonstration Project. In this analysis many of the uncertainty components were argued to be small or negligible. These arguments were accepted as being reasonable based on engineering experience. Since 2000 the ACR radar has been replaced with an Aeroflex Lintek Elan radar system. A new measurement uncertainty analysis was conducted for the ACR using the Elan radar and for a general (non-calibration) target. We present results comparing the previous results to the current analysis results.
We have been developing an uncertainty analysis of RCS calibrations and measurements in the 2 – 18 GHz range at the Etcheron Valley RCS outdoor ground-bounce facility. In this study we report on the results of the uncertainty analysis primarily at 11.3 GHz, but results at some other frequencies are also discussed. We plan to address all components of uncertainty, and present here in some detail the procedures used to determine the uncertainties due to nonplanar illumination, drift, noise-background and nonlinearity. We use a measurement-based approach to obtain upper-bound estimates for the component uncertainties, which are combined using root-sumsquares (RSS) to obtain the overall uncertainty. The uncertainties at any frequency can be determined using these measurement procedures.
Probe calibration is a prerequisite for performing high accuracy near-field antenna measurements. One convenient technique that has been used with confidence for years consists of using two auxiliary antennas in conjunction with the probe-to-be-calibrated. Inherent to this technique is a calibration of all three antennas. So far the technique has mostly been applied to measure polarization and gain characteristics. It is demonstrated how the technique can be extended to also measure an antenna’s phase-versus-frequency characteristic.
The microwave characterisation of the electromagnetic parameters of lossy materials is an essential part of the work of the BAE SYSTEMS Advanced Technology Centre, Stealth Materials Group at Towcester (UK). The electromagnetic parameters of lossy materials change rapidly with frequency below 5GHz, therefore for stealth applications it is vitally important to be able to characterise materials at these frequencies. This paper describes a unique quasi-optical free space focused beam system for the measurement of microwave electromagnetic material parameters. The system employs two spherical reflectors which are illuminated from the side by gaussian beam forming antennas. The frequency range of 1.7GHz to 5.8GHz is covered in three bands with three pairs of corrugated feed antennas. An advantage of this system is that a parallel beam is formed between the reflectors whose beam waist diameter (or illumination area) is essentially the same across each frequency band. The measurements from the system are taken using a vector network analyser under computer control. The parallel beam enables a “Through, Reflect, Line” calibration technique to be used. After calibration the sample under test is placed in the beam (mid way between the reflectors) and the four microwave ‘S’ parameters are recorded automatically in complex form. The permittivity, permeability or lumped admittance ( if the sample is very thin
A Technique is presented that allows for broadband nondestructive material electrical parameter measurements. Electrical parameters of a large number of materials are not readily available over extremely broad bandwidths (multiple octaves as an example). This information is required for accurate modeling of microwave circuits and antenna(s). These parameters consist of complex permittivity and complex permeability that result in loss due to the types and thickness of materials to be used. A Method is required that allows for fast, accurate and low cost measurements of the materials under test. The technique of using dual coaxial probes provides a solution that can be applied to numerous materials including thin films. It takes advantage of the full frequency extent of the network analyzer. This measurement uses dual coaxial probes, as compared to the implementation of cavity resonators, coaxial lines, waveguides and free space measurements, and performs the measurement in a 2-port calibration procedure. The resultant analytical solution is a transcendental equation with complex arguments. The Coaxial probes are described and can be easily made with available components where the only limitation is the valid component frequency bandwidth. Several material examples show the expected accuracy versus frequency range of this measurement technique.
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.
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 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.