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Over the last few years, we have implemented several methodologies pertaining to uncertainty analysis of RF and Optical measurements. These methodologies are currently in use within the radar cross-section, electro-optic/infrared, and material measurement laboratories at the Air Force Research Laboratory. In this paper we discuss from a top level some of the approaches we have implemented, and identify some important issues one needs to address before beginning an uncertainty analysis. We illustrate one such approach as it applies to the estimation of radar cross-section uncertainty.
Some compact ranges use two orthogonal linear polarized feed horns for circular polarized antenna measurement. These two feed horns are symmetrically located along the vertical plane through the longitudinal axis (VPTLA). For accurate axial ratio measurement, the CP antenna under test (AUT) should also lie on the VPTLA. However, for some applications, the AUT has to be offset from the VPTLA during measurement. When this happens, rays from two feed horns reaching the AUT are out of phase. This extra phase error causes unwanted test error for the axial ratio measurement. This paper presents an analysis on the error cause, and provides a method to compute and correct the phase error, when the AUT is offset from the VPTLA. The method computes the extra phase difference from two feed horns to the AUT using geometrical optics method. This phase difference is then used to correct the tested data. This paper also shows a successful measurement example using this correction technique.
A first analysis of the equivalences between Multiple Input Multiple Output (MIMO) and physical/synthetic radar arrays is presented. The establishment of these equivalences is addressed to make use of efficient radar imaging algorithms, which were originally conceived for SAR systems, with MIMO arrays. The main advantage of MIMO arrays is that, with a reduced cost and complexity of the antenna feeding network, they offer imaging capabilities very close to those of SAR and physical radar arrays. This makes MIMO radar a very interesting option in real-time imaging applications (e.g., surveillance of small areas). The paper will present some numerical simulations using some reference scenarios where the imaging capabilities of MIMO arrays will be assessed. A comparative analysis with the well-known SAR and uniformly spaced radar arrays will be presented. Here the study is made with one-dimensional radar apertures, and subsequently will be extended to two-dimensional radar apertures. The analysis of the performance of the MIMO arrays is based on a Matlab simulation tool that is used to optimize the array topology and also to form the radar images of a synthetic scenario. The optimization technique is based on a genetic algorithm, using a fitness function measuring the degree of uniformness and uniqueness of the loci of the phase centers of the tx/rx pairs of the MIMO array. Results show that the found topologies show a performance close to uniformly spaced physical radar arrays.
Hardware-In-The-Loop chambers provide the chamber designer with many difficult obstacles to overcome in order to establish a high performance environment for the measurement of missile seeker systems. One of the most difficult challenges is to overcome the low performance of absorbing materials at low grazing angles. To solve this problem Tapered R-Card Fences have been used in conjunction with Chebyshev absorbers. Last year we reported on the ATK chamber built in Woodland Hills which showed preliminary test results well within the system requirements. This paper will make a direct comparison of chamber performance with and without Tapered R-Card Fences. The establishment of a sister chamber built in the ATK Alliant Techsystems Inc. ABL facility has provided us with the unique opportunity to test the chamber prior to the installation of the R-Cards and then to test it again with the installation of the R-Cards. This unique opportunity has allowed us a direct comparison of an advanced chamber deign with Chebyshev absorbers as would be utilized in a conventional chamber and the performance increase directly attributable to the introduction of the Tapered R-Cards in the anechoic chamber. The chamber evaluation is carried out utilizing The Ohio State developed TDOA measurement method utilizing their proprietary measurement and analysis software.
This paper describes joint studies of Wroclaw University of Technology and Denmark Technical University on optimizing placement and performance of low-profile antennas on small satellite, such as ESMO Moon orbiter. After comprehensive electromagnetic studies with use of numerical analysis, a spacecraft mockup modeling its conductive surfaces was developed. Two to four antennas were mounted and several placement configurations were investigated. For verification purpose of numerical analysis and formulating design guidelines to an actual Moon probe, precise measurements of combined radiation pattern were performed at the Near-Field Antenna Test Facility, Denmark Technical University.
Full-wave modeling and far-field measurements are utilized to study the effects of a microstrip feeder on the band rejection and the far-field purity of planar log-periodic antennas. Three different configurations are investigated. Specifically, band rejection by relevant teeth removal (near/far-field), integration of the band-stop filter (near-field only), and the combination of the two are studied. Far-field contamination effects due to a microstrip feed line, and coupling to the antenna radiator, are evaluated for both radiating and band rejection regions. Important guidelines regarding the position and distancing of the feed to the radiator, as well as the trade-offs between substrate and superstrate configurations are derived. Antennas are developed to have a VSWR better than 2.5:1 in the 2-4 GHz and 7-11 GHz bands, and band rejection centered at 6 GHz. It is clearly shown that log-periodic antennas can be readily designed to have arbitrary, even reconfigurable, band rejection regions where overall realized gain is notched for more than 20 dB. A computer aided analysis was performed using commercial finite element and method of moments software tools. The measurements were conducted at Lockheed Martin in Denver, Colorado.
An effort to ascertain the accuracy of the rectangular waveguide measurement technique for permittivity and permeability characterization of materials, has led to the development and application of a waveguide notch filter as a scattering parameter (S-parameter) reference standard. The S-parameters of this reference can be determined accurately using simulations that implement a full wave model of the waveguide measurement technique. The notch frequency response characteristic allows testing over the dynamic range of the measurement system. When fabricated in metal, the filter provides a predictable frequency response, has mechanical and temporal stability, and is reproducible using standard machining techniques. However, manufacturing errors introduce uncertainty in the measured S-parameters. Determining the sensitivity of S-parameter uncertainty as a function of manufacturing errors is important in assessing the appropriateness of the notch filter as a metallic standard for use throughout the material measurements community. This paper presents the characteristics of the filter, showing both calculated and measured S-parameter values, and provides an analysis that demonstrates the relationship between dimensional manufacturing tolerances and the resulting S-parameter uncertainty.
High frequency (up through X-band) magnetic materials are gaining in importance across a wide range of applications such as microwave components, electromagnetic shielding, and antenna substrates. Development of new magnetic materials and alloys requires convenient and accurate measurement methods with well-understood uncertainties. For this reason, a finite difference time domain (FDTD) model was developed of a shorted microstrip (single coil) permeameter, appropriate for measuring small samples or thin films. Simulating the response to various magnetic materials, this model was used to analyze the prevailing semi-empirical inversion methods and a new, more accurate inversion method was developed to correct deficiencies in existing techniques.
Decomposing a signal of unknown polarization into combinations of linear and circular components can be very confusing, especially when one wants to relate them to a known phase and amplitude reference. Many publications have addressed parts of this issue, some employing the square root of two and some not. There does not seem to be a substantial consensus on this is-sue, experts being somewhat evenly divided. The prob-lem relates to both mathematical analysis and meas-urement analysis, which must be in agreement when comparing measurement to theory. This paper presents an abbreviated mathematical analysis involving depolarization of a wave incident upon a radome, yielding the magnitude and phase of both resultant circular components. The result is com-pared to well-established published formulas. However, derivations of the same components from measure-ments may reach a different conclusion depending on the procedure used. The difference is a factor of the square root of two. These two conflicting results are compared and a resolution proposed.
Highlights of work at the Naval Research Laboratory in evanescent near-field electromagnetic holography (ENEH) will be presented. This work grew out of extensive experimental work in near-field acoustical holography at our laboratory that has been recognized formally by the Laboratory as one of the 75 most innovative technologies over the past 75 years. This new electromagnetic approach differs from the usual nearfield imaging in that it provides much better than halfwavelength resolution due to the inclusion of evanescent waves. Furthermore ”imaging” to a source surface provides a reconstruction of the surface currents, Poynting vector as well as the E and H field vectors. These quantities are derived from two measured holograms (phase and amplitude) of two polarizations of the electric and/or magnetic fields over a 2-D surface (the hologram). Experimental work in both low (100 Hz) and high frequencies (10GHz) is of interest, although we present here results of the latter along with the theory. Two approaches will be discussed for backtracking the measured fields: one that uses wave function expansions in plane, cylindrical or spherical geometries, highlighting the cylindrical geometry in this paper, and a second more general formulation that uses the field expanded using an array of equivalent dipole sources especially useful in arbitrary geometries. Both approaches represent inverse methods and are ill-posed and require regularization to stabilize the reconstructions. We hope that these methods will provide high resolution new diagnostic tools for antenna analysis, as well as diagnostics for applications in EMC and EMI among others. Currently we are seeking partnership with other laboratories and universities to direct this technology towards problems that could benefit from its unique diagnostic capabilities. Work supported by the Office of Naval Research.
An error simulator based on virtual cylindrical near-field acquisitions has been implemented in order to evaluate how mechanical or electrical inaccuracies may affect the antenna parameters. In outdoor ranges, where the uncertainty could be rather important due to the weather conditions, an uncertainty analysis a priori based on simulations is an effective way to characterize measurement accuracy. The tool implemented includes the modelling of the Antenna Under Test (AUT) and the probe and the cylindrical near-to-far-field transformation. Thus, by comparing the results achieved considering an infinite far-field and the ones obtained while adding mechanical and electrical errors, the deviations produced can be estimated. As a result, through virtual simulations, it is possible to determine if the measurement accuracy requirements can be satisfied or not and the effect of the errors on the measurement outcomes can be checked. Several types of results were evaluated for different antenna sizes, which allowed determining the effect of the errors and uncertainties in the measurement for the antennas under study.
Electromagnetic Anechoic Chamber has recently been built by Alenia Aeronautica at Caselle South Plant: The Anechoic Chamber is a full anechoic chamber, and it has been designed to carry out electromagnetic vulnerability tests mainly on fighter and unmanned aircraft. In addition measurement can be carried out on many different vehicles that can be brought into the chamber through the main access door. A system to extract exhaust gas was installed in order to carry out tests on a wide variety of vehicles. The Anechoic Chamber has been designed to carry out both HIRF/EMC test and High Sensitivity RF measurement: in particular HIRF/EMC tests in the frequency range 30MHz ÷ 18GHz with the capability of radiating a very high intensity electromagnetic field and High Sensitivity RF measurement, including antenna pattern measurements on antennas installed on aircraft in the frequency range 500MHz ÷ 18GHz. During the design phase a 1/12th scale model of the chamber had been fabricated to assess the desired electromagnetic performance. In this phase of design the model was tested at the scale frequencies for Filed Uniformity, Site Attenuation and Free Space VSWR results. This study was published at the AMTA 2004 meeting. In addition to the physical model, during the construction phase, various computer simulations were performed to further define the detailed internal absorber layout and to define test acceptance methods for procedures not covered by the standards. The computer model analysis was conducted to identify areas of scattering that could be treated with higher performance absorbers to improve the chambers quiet zone performance. The identified “Fresnel Zones." have been treated with high performance absorbers optimized to provide improved performance at microwave frequencies. The absorber optimization was reported at the AMTA 2006 meeting. This optimization has allowed validation of the chamber according to the requirements of CIRSP 16-1-4 2007-02 in the range of frequency 30 MHz - 18GHz. The size (shield to shield) of chamber is 30m wide, 30m long and 20m high, and the 18m wide by 8.5m high main door allows the SUT access. The shielded structure is a welded structure of 3mm-thick steel panels which guarantees shielding effectiveness of more than 100 dB in the frequency range 100 kHz to 20GHz. The chamber includes a 10 meter diameter turntable to rotate a 30 ton SUT with an angular accuracy of ± 0.02° and a pathway to allow SUT access. Both the pathway and the turntable are permanently covered by ferrite tiles. A hoist system permits lifting of the SUT (max 25 tons) up to 10 meters from the turntable centre enabling EMC testing on aircraft with the landing gear retracted.
Advantages of Far-Field (FF) anechoic chambers utilized for antenna measurements, as compared to conventional outdoor ranges, such as security, interference-free radiation, and immunity to weather conditions allowing broadband antenna measurements on a 24/7 basis, are well known. The dimensions of an anechoic chamber are primarily determined by the lowest operating frequency and are, therefore, significantly increased if operation is required down to VHF and UHF frequency bands. As a result, the advantages of indoor chambers are often disputed when considering low frequency applications. The main counter-argument is the real estate required for chamber construction. In addition, such chambers require the use of high performance absorbing materials, and consequently, chamber certification is always a challenging task. Therefore, rigorous and accurate 3D EM analysis of the chamber is an important procedure to increase confidence, reduce the risk associated with achieving the required test zone performance, and to make the design more efficient. Thus, an accurate simulation of the chamber is even more important these days due to a dramatically growing number of antenna manufacturers supplying products at VHF and UHF bands. Such analysis is a standard procedure at ORBIT/FR, and is described below for the example of a chamber with dimensions of 6m (W) x 6m (H) x 10m (L), operating down to 150 MHz.
NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been used to improve performance in anechoic chambers and has been demonstrated over a wide range of frequencies on numerous antenna types. MARS is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical or far-field range or Compact Antenna Test Range (CATR). It has also been applied to extend the useful frequency range of microwave absorber to both lower and higher frequencies than its normal operating band. This paper will demonstrate the use of the MARS capability in evaluating the performance of anechoic chambers used for spherical near-field measurements, as well as in improving chamber performance.
This paper presented a theoretical and numerical investigation of the performance comparison of Compact Antenna Test Range (CATR) equipped with Hybrid and Super Hybrid modulated serrations. The performance of the quiet zone will be degraded for a traditional CATR without both an edge treatment of the reflector antenna and a high quality anechoic chamber. Usually, the ripples in both the phase and magnitude of the field intensity inside the quiet zone are caused by stray signals, which come from edge diffraction. In order to solve the edge diffraction problems, edge treatments such as a different shaped serrated edge are often applied along the edge of the reflector antenna. The quiet zone field analysis of such reflectors has traditionally taken the form of ray tracing using numerical integration of the reflector surface currents called physical optics (PO). PO technique is used to obtain Fresnel Zone field of a plane square aperture embedded with a hybrid and super hybrid serrated CATRs. It is observed that super hybrid serrated CATR gives lesser ripples and enhanced quiet zone width than hybrid serrated CATR.
Traditional passive antenna measurements result in well-known quantities like Directivity, Efficiency, and Gain. However, when testing over-the-air (OTA) performance of active devices, there are additional effects that cannot be lumped together as part of the antenna performance. Terms like gain and efficiency are defined based on transmit or receive signal levels at the antenna port relative to the radiation pattern of the device. Thus, OTA performance is often assumed to be equivalent to the conducted performance of the device combined with the passive radiation pattern. However, when that antenna port is attached to an active radio in a typical wireless device, interactions between the circuitry and the antenna can produce results that do not match that predicted by the conducted performance and the passive radiation pattern. The difference between the predicted and actual performance of a device can be quantified in terms of "interaction factors", which represent the often non-linear behavior of the active circuitry when operating in an OTA environment. These factors include such effects as variation in amplifier gain due to heating caused by antenna mismatch, and receiver desensitization due to platform noise that couples through the antenna of the device. This paper will discuss the concept of interaction factors and define a number of sub-components of these factors that may be useful in predicting the level of some interaction factors.
This paper describes the Rectangular Coaxial 40’ long measurement system recently designed and installed at AEMI with the primary purpose of measuring the reflectivity of its high performance VHF/UHF absorbing materials in the frequency range 30 – 510 MHz. The basic principles of the system are described in detail in [1] and are based on S11 – measurements of absorbing material reflectivity by a Vector Network Analyzer (VNA). In order to improve the system productivity and measurement accuracy it was enhanced by the time-gating software option – the standard option of ORBIT/FR Spectrum 959 automated measurement software package [2].The measurement system performance was thoroughly evaluated and validated by a number of tests performed in the “empty” coaxial line, and in the line loaded by absorbing materials. The list of RF uncertainties – various measurement error sources - was generated, the main measurement error contributors were identified, the corresponding errors – estimated and the overall RSS measurement errors were calculated for the absorber reflectivity varying in the range of -30dB to – 40dB.
This paper describes the details of a specialized data acquisition system developed at the David Florida Laboratory. The system acquires, monitors, records and performs post measurement analysis of passive intermodulation (PIM) and multipaction events observed during RF testing. This characterization of components and systems carrying radio frequency signals is an important element of space qualification of satellites and other space faring systems. A National Instruments PXI chassis equipped with a PXI-4462 acquisition card and a LabView based software application was implemented to digitize the resulting data. A second application provided by InfoBright permits the compact storage of hours of measured data in its entirety (multiple channels each sampled at over 200,000 samples per second) using a specialized real time data compression scheme. The application also permits quick retrieval of relevant data segments using SQL query processing. Performance of this solution is presented along with its effectiveness in detecting details of PIM and multipaction events.
The 2004 AMTA paper entitled “The “Cam” RCS Dual-Cal Standard” introduced the theoretical concept of the “cam,” a new calibration standard geometry for use in a static RCS measurement system that could simultaneously offer multiple “exact” RCS values based on simple azimuth rotation of the object. Since that publication, we have constructed a “cam” to further explore its utility. The device was fabricated to strict tolerances and its as-built physical geometry meticulously measured. Utilizing these characteristics and moment-method analysis, a high-accuracy computational electromagnetic (CEM) “exact” file required for calibration was produced. Finally, the “cam” was evaluated for its efficacy as a single device that could be utilized as a dual-cal standard. This development was conducted with a particular focus on the hypothesized improvements offered by the new standard, such as the elimination of frequency nulls exhibited by other resonant-sized calibration devices, and improved operational efficiency. In this follow-on paper, we present the advantages to and challenges involved in making the “cam” a viable RCS dual-cal standard by describing the fabrication, modeling and performance characterization.
Current means to improve position accuracy in antenna ranges are often expensive, consume important CPU time, and/or limit data acquisition speed. By taking advantage of axes with good repeatability, higher multi-dimensional positioning accuracy can be achieved directly by a controller to ease complexity and achieve real-time position correction. A product family of controllers brings this capability to fruition. Comparison analysis of field data demonstrates improved accuracy with no measurement speed degradation. Results indicated a considerable accuracy improvement limited by axis repeatability. Existing and new antenna ranges can benefit from this simple cost-effective approach to improved position accuracy.