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This is part three of a series of talks on the development of a new type of near field antenna range. The range is designed to measure the VHF and UHF antenna characteristics of a vehicle over a realistic ground. This means that the spherical symmetry has been lost and the classical spherical mode expansions are not appropriate. We have previously demonstrated a plane wave synthesis approach to the far field transformation, including the lossy dielectric half-space representing the ground. This third yearly presentation will discuss the compensation needed for realistic probe antenna, probe arm and turntable imperfections. Results for actual experimental measurement data with sample space and probe antenna time dispersion compensation will be shown. Comparisons with the theoretical far field computations and the spherical mode expansion results will be included.
This paper discusses the process of optimization of a spherical near-field range for measurement of large UHF antennas used in space applications. Results of a study undertaken to understand and optimize range performance in presence of multi-path errors and mutual coupling are presented. Data is presented showing variation in measured patterns of a generic UHF antenna as a function various parameters such as a) use of probes of different gains, b) separation distance between the probe and the antenna and c) absorber rearrangement. Use and effectiveness of software post processing approaches such as spherical mode filtering, time domain gating and use of proprietary algorithms (e.g. “MARS processing” developed by NSI Inc.) is illustrated. Practical implementation of these approaches and corresponding impact on data density, test duration and computational effort are also discussed.
This paper describes some improvements in the measurement process of the NIST 18 term error analysis that reduces the required measurement time and also improves the sensitivity of some of the tests to the individual sources of uncertainty. As a result, the measurement time is reduced by about 40 % and some of the estimated uncertainties are also improved without a reduction in the confidence levels. The reduction in measurements is accomplished by using one measurement for two or more error terms or using centerline rather than full 2D data scans for some of the terms.
In a FF antenna range the DUT mechanical Boresight can be aligned with the Range Boresight simply by using a Boresight scope to transfer the DUT mechanical Boresight to the Range coordinate system. This is not applicable to a PNF Range; hence, another transfer device and different transfer methods are required. This paper describes the development, testing and referencing of an existing PNF range to a reference optical cube that serves as the coordinate system transfer device. The optical measurement system employs an automated total station Theodolite system, incorporating true 3D positioning of the NF probe along defined axes of movement. The data collected is processed to best fit a straight line defining the vector representing the axis. The scanning PNF plane is defined with high accuracy, by a geometrical representation of two (or more) axes in that plane. Thus, the scan plane coordinate system was transferred by auto collimation methods to the reference optical cube. A second optical cube must be placed on the AUT to be used as a reference for its mechanical Boresight. When the AUT is set up for testing, the coordinate systems are transferred from each cube to the other by means of co-collimation using a temporarily positioned Theodolite combination.
Dual polarized probes for modern high precision near field measurement systems have stringent performance requirements in terms of pattern shape, on-axis and off-axis polarization purity, return loss and port-to-port isolation. A further requirement to the probe is that the useable bandwidth should exceed the antenna under test. As a consequence, the probe design is often a trade-off between performance requirements and the usable bandwidth of the probe. Current high performance designs are based on corrugated horns with balanced capacitive orthogonal excitation achieving close to 25% bandwidth [1]. This technology is well suited for near field probes in the L to Ka band range. Although attractive for compactness, simplicity and excellent performance, probes with external balanced feeding require high precision couplers and manual tuning that impact the overall complexity and manufacturing cost of the final probe. A reduction in cost and complexity can be achieved while maintaining the high performance standards. SATIMO has developed an innovative near field probe with self-balanced feeding maintaining high performance on a wide bandwidth. The overall simplicity makes the new technology very attractive for probe designs in the L to Ka band range.
Typically radome tests are performed on outdoor far field ranges or compact ranges. ORBIT/FR has designed, build and qualified a unique spherical near-field radome test facility for the nose-mounted radomes of commercial traffic aircraft for the so-called “after repair” tests according to the international standard RTCA/DO-213, as well as the aircraft manufacturers Component Maintenance Manuals. The facility is extremely compact (chamber size 5.7 m x 5.2 m x 3.2 + 0.7 m, L x W x H), can handle radomes as small as used on the Canadair and as large as used on the Airbus-380 and can be installed directly in the repair workshop for such radomes. The tests performed are transmission efficiency and side lobe level increase. The system is completely automated, so that a workshop technician can operate the facility. Utmost attention has been paid to operational aspects and both operator and equipment safety. After the measurements are done, a test report is fully automatically generated according to RTCA requirements and classifications. The facility is equipped to test all standard Airbus, Boeing, Canadair and Dash nose radomes.
A new algorithm for the amplitude-only characterization of Compact Antenna Test Ranges (CATRs) is presented. The algorithm applies a successful strategy to retrieve the missing phase of the field in the quiet zone. Particular care is devoted to facing the issue of the typically large electrical dimensions of CATRs and to obtaining the necessary accuracy by the use of an “efficient” representation of the radiated field. This is accomplished through a Jacobi-Bessel expansion of the aperture field which allows to keep low the overall number of unknowns and to improve the accuracy and the reliability of the algorithm. The presented numerical analysis, based on realistic CATR simulations by means of GRASP8-SE, shows the feasibility of the algorithm to estimate amplitude and phase of the quiet zone field within an acceptable accuracy.
A large rolled edge compact range system featuring a 12’H x 16’W quiet zone has been designed, fabricated, installed, and tested in a large aerospace test facility. During the program, a high precision alignment methodology was utilized in conjunction with electromagnetic prediction capability to verify both mechanical and electrical performance while still under trial assembly conditions at the factory. A coherent laser radar (CLR) was utilized to measure the reflector surface on a very fine grid, and the electromagnetic (EM) quiet zone performance was calculated from the raw CLR data using a Physical Optics (PO) model. Despite extremely high surface accuracy of the panels, this evaluation methodology highlighted systematic alignment errors in the reflector system, and guided the process of correcting these errors to achieve a final factory verification assembly for the entire 20’H x 24’W reflector system of better than 0.001” over the quiet zone section of the reflector, and 0.004” rms over the entire reflector. This procedure was also utilized for the on-site installation to achieve alignment of the reflector to an AUT positioning system using the CLR, as the positioning system and chamber were already existing and operational. Thus, it was required to align the reflector to the positioning system, and not the positioning system to the reflector as is usually the case. A unique vertical carousel feed system was also aligned using this procedure. Predicted EM results were again used to finalize alignment on site prior to quiet zone field probe evaluation. This paper summarizes the overall alignment and EM evaluation process, and presents results for the installed compact range reflector system.
Satellite TV reflectors for home use, provided to the public by service companies such as DIRECTV, have many features which must be adequately characterized prior to design release, including: • Multiple Beam Frequency Re-use • FCC Sidelobe Envelope Verification • Circular Polarization Isolation These features must be adequately tested at frequencies up to Ku band and beyond. The use of a far-field range is impractical, as some of the reflectors measure several feet in diameter, and thus requires a range length of several hundred feet at Ku band. Near-field testing requires a full scan to determine a single cut for evaluation of FCC compliant sidelobe performance. Thus, a compact range is a logical alternative for measurement of this class of antennas. The compact range can provide a quick assessment of multiple beam coverage performance and pass/fail analysis against FCC sidelobe curve specifications. In addition, the feeds for these antennas often use Low Noise Block (LNB) Downconverters that are built in as part of the feed assembly. Measuring the output of an LNB does not yield the phase information required to determine all polarization parameters. A spinning linear measurement with some unique processing was implemented on this range to determine the full polarization characterization, using some elementary assumptions about polarization sense. This paper describes the implementation of a compact range based measurement facility for satellite antenna testing, with emphasis on the circular polarization measurement of the LNB assembly, capability for comparison against FCC sidelobe levels, and measurement of offset beams featuring frequency re-use capability.
This paper describes a new approach to improving the low frequency reflectivity performance of geometric transition radar absorbent materials through the use of impedance loading in the form of one or more included FSS layers. The discussion includes theoretical predictions and measured data on modified commercially available RAM which confirm the validity of the concept.
A common apparatus for microwave characterization of intrinsic material properties is the coaxial airline. Often the largest source of measurement uncertainty in the coaxial airline is from air-gaps between the sample and fixture. Previous analyses of air-gaps in these fixtures have been restricted to analytical quasi-static approximations that assume very small air-gaps. In this work, finite difference time domain (FDTD) simulations were used to study the systematic error caused by air-gaps in coaxial airline measurements of dielectric permittivity. The fundamental mode in a coaxial airline is a circularly symmetric TEM wave. Thus body of revolution (BOR) symmetry was assumed, reducing the required computational effort. Comparison of two-dimensional BOR-FDTD to three-dimensional FDTD showed excellent agreement. BOR simulations were conducted for a variety of gap sizes and sample permittivities to catalogue systematic ‘measurement error’. The quasi-static gap models were evaluated with these simulations, showing that traditional corrections are effective only with small gaps and low to moderate permittivity. The conventional wisdom for non-magnetic samples is that dielectric inversion from the transmission coefficient is the most accurate. The transmission-only inversion was compared to the Nicolson-Ross-Weir algorithm, showing that the opposite may be true – that inversion of dielectric properties from both transmission and reflection can be more accurate when gaps are present.
A material measurement technique is developed to simultaneously characterize electric and magnetic properties for homogeneous lossy material in a parallel-plate region. The material is excited by a rectangular waveguide which interacts with the parallel-plate region through a slot. In order to extract the complex constitutive parameters from the material two independent measurements are required. If the material is attached to a PEC surface and is unable to be removed, the most obvious manner to characterize the material is a parallel-plate region. This paper demonstrates through the use of a magnetic field integral equation (MFIE) how a rectangular waveguide interacting through a slot with a parallel-plate region can be used to obtain two independent measurements, which are necessary for characterizing the homogeneous lossy material. Experimental results for various lossy materials are compared to stripline and waveguide measurements to verify the MFIE
Microwave microscopes that measure surface impedance or roughness have been demonstrated with fine spatial resolutions of less than a micron. These microwave probes are practical only for samples less than a few inches in size. However, composite materials in applications such as multi-layer radomes, embedded frequency selective surfaces, or integrated EMI shielding, have larger length-scale features embedded within a multilayer laminate. Diagnosing larger-scale, subsurface features such as joints/seams, periodic elements, imperfections, or damage is driving a need for methods to characterize embedded electromagnetic properties at mm or cm length-scales. In this research, finite difference time domain (FDTD) simulations and experimental measurements were used to investigate a probe technique for measuring sub-wavelength sized features embedded within a dielectric composite. For these applications, the probe interacted with the sample material via both evanescent and radiating fields. A dielectrically loaded, reduced size, X-band waveguide probe was designed in a resonant configuration for improved sensitivity. Experimental measurements demonstrated that the probe could characterize small gaps in ground planes embedded within a dielectric laminate. Simulations also demonstrated the possibility of detecting more subtle imperfections such as air voids.
Nondestructive evaluation (NDE) of a conductor-backed lossy material plays an important role in electromagnetic shielding applications. Current NDE techniques limit such evaluation to non-magnetic materials and/or use of approximate solutions. In this paper a technique employing a flanged opened-ended rectangular waveguide and a rigorous integral equation model is presented allowing simultaneous extraction of both permittivity and permeability from a lossy magnetic shielding material. The technique reduces sample preparation time leading to rapid measurement. Calculation of both material parameters is accomplished using reflection measurements from two different thicknesses of a given material sample. A theoretical solution to the reflection coefficients is developed using a magnetic field integral equation (MFIE) formulation. The theoretical solution, along with experimental results of the reflection measurements, allows extraction of the material parameters using a two-dimensional Newton root search. Results using single-mode calculations are presented and compared to those obtained using traditional waveguide material characterization techniques. Future work will also be discussed.
This paper describes the method and hardware implementation of a test bed that was designed and built to characterize the reflection characteristics of various types of reflector materials. The system described measures reflection amplitude and phase from flat test panels relative to a metal panel standard at normal incidence and for dual linear polarizations simultaneously. The measurement’s theoretical concept is based on a focused free space technique with time domain gating to remove the effect of multi-path coupling between the test panel and the feed assembly. The system as a whole demonstrates a novel method for measuring the reflection from reflector materials and characterizing their potential impact on polarization purity. The measurement system consists of: 1) A fixed reflector, 2) An alignment fixture accommodating feed assemblies, which include corrugated horns that operate over a 40% bandwidth that may be swapped out in order to cover a continuous frequency band from 18 to 75 GHz and Orthomode Transducers (OMT) in order to measure dual linear polarizations simultaneously, 3) An additional alignment fixture for mounting the flat panels under test, and 4) A Vector Network Analyzer (VNA) and computer for data collection and processing. The system is assembled on a bench top and aligned utilizing a Coordinate Measurement Machine (CMM). Sample results demonstrating the measurement of various types of reflector materials including composite reflector lay-ups with graphite face sheets and mesh samples for deployable reflectors are presented.
Some radar applications require a system to acquire range profile or S11 network analyzer data through a lossy dielectric layer to measure something behind that lossy dielectric layer. It is often difficult to specify the dynamic range requirements of such a system due to the “flash” of initial reflected transmitted energy from the lossy dielectric layer. It is also difficult to determine the most effective architecture for such a system, such as pulse IF, ultra-wideband impulse, FMCW, or another more exotic architecture. In this paper a theoretical model is developed of a lossy dielectric layer, a radar transmitter and receiver, and a standard radar target on the other side of the lossy dielectric layer. The theoretical results from this model provide insight into the dynamic range requirements for any radar system that must acquire range profile data or S11 network analyzer data through a lossy dielectric of any permeability, permittivity, and conductivity at any microwave or RF frequency range in order to measure something behind that lossy dielectric layer.
A computational technique based on near-field to far field transformation is presented. This can be more versatile and accurate than the conventional modal expansions. The established method for near-field to far-field transformation has been the modal expansion method. The primary drawback of the technique is that when a Fourier transform is used, the fields outside the measurement region area is assumed to be zero, particularly in the planar and cylindrical case. Consequently the far-fields are accurately determined only over a particular angular sector which is dependent on the measurement configuration. A simple and accurate integral equation solution which represents an alternate method for computing far-fields from measured near-fields is presented. The basic idea is to replace the radiating antenna by equivalent electric and/or magnetic currents which reside on a fictitious surface and encompasses the antenna. These equivalent currents are assumed to radiate identical fields as the original antenna in the region of interest. Using the surface equivalence principle different types of the E-field integral equation (EFIE) have been developed. The method of moments (MoM) has been utilized to transform the integral equation into a matrix one and the conjugate gradient (CG) procedure has been applied to solve it numerically. Hence, this procedure is not limited by the Nyquist sampling criteria nor by the presence of evanescent waves which may make source reconstruction using current procedures unstable. Accurate far-fields over large elevation and azimuthal ranges have been calculated from simple measurements based on planar and spherical scanning.
Planar power dividers with a good match over a wideband of frequencies are designed using Klopfenstein impedance taper. To validate the proposed design procedure a 2-way stripline and a 2-way microstrip power divider are designed based on simulation, fabrication, and measurement. The measured return loss reveals better than –24 dB (from 4.3 GHz to 19.5 GHz) for a stripline configuration and –27 dB (from 2.2 GHz to 12 GHz) for a microstrip line configuration. Guidelines for accurate simulation and experimental verification are also presented.
In this paper we present a direct optimization procedure which utilizes phase-less electric field data over arbitrary surfaces for the reconstruction of an equivalent magnetic current density that represents the radiating structure or an antenna under test. Once the equivalent magnetic current density is determined, the electric field at any point can be calculated. Numerical results using experimental data are presented to illustrate the applicability of this approach for non-planar near field to far field transformation as well as in antenna diagnostics.
Nowadays near-field measurement techniques are widely used for detecting the characteristics of the radiated pattern for a large variety of antennas. The core of any near-field measurement is the near-field to far-field transformation. Such transformations use different coordinate systems, like planar, cylindrical, or spherical, and may utilize special solutions. They are already well known for many years. The common feature of all mentioned near- to far-field transformations is the usage of regular measurement grids on planar, cylindrical, or, respectively, spherical surfaces. Future applications, like the Airborne Near-Field Test Facility (ANTF) are expected to lack this characteristic of regular measurement grids, since the flying or floating probe platform cannot be guided sufficiently accurate. This requires the utilization of advanced data processing methods for interpolating measured data on an arbitrary irregular grid to a nearby regular grid, or direct transformation to the far-field. It will be shown that this data processing can be performed by using the Stratton-Chu representation formula utilizing equivalent currents on a well-chosen artificial surface or the classical mode expansion method with additional pre-processing. This paper describes briefly the principles of the ANTF, discusses the application of the equivalent current method and compares it with the widely used mode expansion method. Measured and processed data examples will be presented.