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Recently ORBIT/FR Inc. has introduced a far – field antenna measurement anechoic chamber design method called “ Two Level GTD “ , which combines shaped chamber walls with a specific absorber layout intended to achieve a better level of reflectivity in the test zone [1-3]. The sidewalls may have the shape of an “inverted open book “, while the back wall may be a pyramidal shape with a small subtended angle at the base. A wedge type foam absorber with a variable orientation of the wedge tips can treat the sidewalls in a “fishbone” layout, while the back wall may be treated by using conventional foam based pyramids. The’ fishbone’ like layout is intended to adverting the reflected waves by the sidewalls out of the test zone, while the back wall pyramid layout is applied to utilize both: the optimum pyramid reflectivity at almost normal incidence; the back wall shape diverting the reflected incident plane or quasi - plane wave out of the test zone. It well known that GO and GTD principles are widely applicable to electrically large structures, delivering a high quality simulation accuracy and good correlation with measurement results. Therefore, the application of the “Two –Level GTD “ is expected to deliver well predicted improvement in the reflectivity of anechoic chambers operating at relatively high frequencies , where the chamber sidewall characteristic dimensions may reach 30. where . is the wavelength at lowest operating frequency. The key question to be answered is - Can the method be successfully applied to cases where the chamber sidewall characteristic dimensions are only – 2-3.? This represents a typical situation in anechoic chambers designed for operation at VHF/UHF frequency bands. In order to answer the question, a full wave 3D simulation has been performed on two anechoic chambers having similar dimensions: 20’ x 20’ x 33’ (L). The two cases are a conventional anechoic chamber and a shaped wall chamber designed based on the “Two – Level GTD” principle. The simulation results were compared, and the “Two - Level GTD” has shown superior performance. Based on these encouraging results, the anechoic chamber was constructed and measurements were performed in the tests zone at a number of frequencies down to 100 MHz. The chamber construction, simulation and measurement results are discussed in the paper below.
A novel probe design for measuring complex permittivity of soils in-situ from 10 to 1000 MHz without taking soil samples is presented. The dielectric constant and conductivity of soil is derived from step-frequency reflection taken inside a small freshly bored hole. As a result, permittivity at various depths with in-situ moisture content and soil texture can be obtained in the fields. A novel calibration method was developed to account for the frequency- and material-dependent geometrical factor which causes bias errors in conventional calibration methods. Experimental measurement results and simulation results are used to prove the efficiency and accuracy of this method.
Collecting accurate gain measurements on antennas is one of the primary tasks for our community. One of the primary concerns in making gain measurements is choosing one of the well known gain measurement techniques to make the measurement. Each gain measurement technique has an inherent accuracy limit based on the measurements made, the measurement environment and the equipment required. The frequency band of interest may also have an impact on the gain measurement scheme employed. In addition, each technique can affect the throughput of the range in question. Balancing the cost of obtaining the gain versus the required accuracy of the gain measurement is a difficult task. This paper will discuss the basic accuracy limitations for several of the standard gain measurement techniques and will catalog the accuracy limitations of the various gain measurement techniques versus the cost associated with obtaining that quality of measurement.
Highly accurate spherical near-field measurement systems require precise alignment of the probe antenna to the measurement surface. MI Technologies has designed and constructed a new spherical near field arch positioner with a 1.5 meter radius to support measurements requiring accurate knowledge of the probe phase center to within .0064 cm throughout its range of travel. To achieve this level of accuracy, several key design elements were considered. First, a highly robust mechanical design was considered and implemented. Second, a tracking laser interferometer system was included in the system for characterization of residual errors in the position of the probe. Third, a position control system was implemented that would automatically correct for the residual errors. The scanner includes a two position automated probe changer for automated measurements of multi-band antennas and a high accuracy azimuth axis. The azimuth axis includes an algorithm for correcting residual, repeatable positioning errors. This paper defines the spherical near-field system and relation of each axis to the global coordinate system, discusses their associated error sources and the effect on global positioning and presents achieved highly accurate results.
A free space transmission line measurement method for dielectric constant and loss tangent determination in low-loss dielectric materials has been analyzed and implemented. This method utilizes dielectric materials with thicknesses greater than half the wavelength in the material to obtain greater sensitivity for determining intrinsic dielectric properties. An analysis of the process sensitivities and experimental measurements has been utilized to estimate the accuracy and lower limits of the dielectric property extractions from the reflection loss magnitude.
RF guided missile developers require flight simulation of their target engagements to develop their RF seeker. This usually involves the seeker mounted on a Flight Motion Simulator (FMS) as well as an RF target simulator that simulates the signature and motion of the target. Missile defense developers, whose job it is to defend against guided missiles, require a similar test environment adding the ability to insert decoy RF targets that can spoof the seeker. Both seeker development and counter-measure development can benefit from an RF test facility that can provide RF targets and decoys controlled by a real-time simulation. This paper addresses an RF Target and Decoy Simulator developed by MI Technologies that provides this test capability. The direction of the target and decoy emitters is independently controlled such that the centerlines of their radiated main antenna lobes are always directed at the RF seeker. Each emitter can be independently and simultaneously commanded along a spherical surface. High rates of acceleration and velocity are achieved all the way out to the ends of the test area to simulate the high line of sight rate that occurs at missile closure. The simulator is capable of safely stopping a decoy racing to the ends of the target area with minimal over-travel. Collision avoidance provisions prevent target and decoy from damaging each other during the simulation. The paper presents a description of the simulator, pertinent tradeoffs considered in the design and accuracy data of the simulator’s performance.
Calibration of probes for planer near field range measurements is generally required to obtain accurate cross-polarization (xpol) data; however, probe calibration is costly and time consuming. Using analytical models in place of calibration is generally much more cost effective, but may result in larger measurement errors. In a previous paper [1], we showed that simple models of copol probe patterns with zero xpol can give accurate measured results, provided that the probe xpol is much better, generally 10-15 dB better, than the Antenna Under Test (AUT). The next question is “Can a lower performing (and cheaper) probe be used if both the copol and xpol probe patterns are modeled?” In this paper, we compute AUT xpol measurement errors that result from probe xpol errors, and we compare far field AUT patterns processed using probe models with patterns processed with calibrated probe files.
A spherical near-field measurement range at Nearfield Systems Inc. has recently been used to measure gain, pattern and polarization of a multi-element helix array operating in the UHF band. Verification of gain performance over the operating band was of primary importance and so major efforts were made to obtain the best possible gain results and to quantify the gain uncertainty through a complete error analysis. A single element helix gain standard was first calibrated and the estimated uncertainty in this calibration was 0.35 dB. A double ridged horn was to be used as the probe for the spherical near-field measurements and so the patterns of the horn at all test frequencies were measured on the spherical range using identical horns as the AUT and the probe. From these measurements, probe pattern files were generated that could be used to perform the probe correction in the measurements of the helix gain standard and the multi-element array. The helix gain standard was then installed in a new spherical near-field range at NSI with the double ridged horn as the probe. The range used a specially designed phi-over theta rotator that could support and rotate the array and maintain the required position accuracy. The chamber was lined with 36 inch absorber. Spherical measurements were then performed and the data processed to provide the far-field peak amplitudes at each frequency that were necessary for gain measurements. The far-field peak values are equivalent to the far electric field for the gain standard and are compared to the same parameter for the multi-element array to produce the final gain results. The helix array was then installed in the spherical range and a series of measurements were performed to produce the far-field gain, pattern and polarization results and also to provide the data for the complete 18 term uncertainty analysis. The uncertainty in the gain measurements was 0.45 dB and the axial ratio uncertainty was 0.11 dB.
This paper presents a hybrid analysis algorithm, which is used at Radiation Group (UPM) to carry out the design of a conformal serrated-edge reflector for the mm-Wave compact range UPM facility. Main features of this algorithm involve its capability of handling conformal serrated rim parabolic reflectors, accuracy and computational efficiency.
This communication addresses the design and implementation of a low-perturbation and high dynamic range near-field (NF) imager with increased measurement speed. The imager is equipped with an array of optically modulated scatterer (OMS) probes, each incorporating a commercial-off-the-shelf photodiode chip and a minimum scattering antenna, i.e. short dipole. In the OMS probes, transmission of modulating optical signals is performed using an optical fiber coupled to the photodiode, which is invisible to microwave signals. The imager measurement speed is also improved as the OMS array eliminates the delays associated with probes translations, in addition to fast switching of modulating light between the probes. Fast switching is accomplished by an array of fiber-pigtailed laser diodes. Improved dynamic range and linearity in the NF imager are achieved by adding a carrier canceller within the imager receiver front-end, eliminating the carrier signal and leaving the sidebands intact. This canceller also improves the isolation between input/output ports of the imager providing a potential for higher signal amplification. The performance assessment of the NF imager, including its linearity and result accuracy is made by comparisons with a known field distribution.
In this paper, a Neural Network based methodology is presented to measure the complex permittivity of materials using monopole probes. A multilayered Arti.cial Neural Network, using the Levenberg Marquardt back propagation algorithm is used to back solve the complex permittivity of the medium. The proposed network can be trained using an analytical model, numerical model, or measurement data spread over the complete range of parameters of interest. The input training data for the non linear inverse problem of reconstructing the complex permittivity comprises the complex re.ection coef.cient of the monopole probe. For the results presented in this paper, the network is trained using the analytical model for impedances of monopole antennas in a half space by Gooch et al. [1]. In addition to computational ef.ciency, the proposed approach gives 99% accurate results in the frequency range of 2.55 GHz, with the values of permittivity varying across a range of 3-10 for the real part, and 0 -0.5 for the imaginary part. The accuracy and the effective range of real and imaginary components of the complex permittivity that can be reconstructed using this approach, depends upon the accuracy and robustness of the model / system used to generate the training data. The analytical model used in this paper has a limited range for the values of loss tangent that it can model accurately. However, the performance of the back solving algorithm remains independent from any speci.c model, and the scheme can be successfully applied using any reliable analytical or numerical model, or re.ection coef.cient training data generated through a series of measurements. The methodology is likely to be employed for experimental measurements of complex permittivity of dissipative media.
Obtaining a quantitative accuracy qualification is one of the primary concerns for any measurement technique [1, 2]. This is especially true for the case of near-field antenna measurements as these techniques consist of a significant degree of mathematical analysis. When undertaking this sort of examination, room scattering is typically found to be one of the most significant contributors to the overall error budget [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in quantifying and subsequently suppressing range multi-path effects in first spherical [3, 4] and then, cylindrical near-field antenna measurement systems [5, 6]. This paper details a recent advance that, for the first time, enables the MARS technique to be successfully deployed to correct data taken using planar near-field antenna measurement systems. This paper provides an overview of the measurement and novel data transformation and post-processing chain. Preliminary results of computational electromagnetic simulation and actual range measurements are presented and discussed that illustrate the success of the technique.
Recently, the smaller of the ESTEC CATR’s has been moved to a new location in the ESTEC Test Centre. In the frame of the relocation, the original reflectors of the range were positioned and aligned in a brand new anechoic chamber. The commissioning phase of the new range included a quiet zone field probing in order to verify the range performance in the new situation and to identify direction of arrival of major reflections. During this exercise, it was realized that the criteria for Quiet Zone dimensions are rather arbitrary. The paper addresses a new figure of merit for range comparison in terms of accuracy. Peak to peak values and RMS have been recorded depending on the size of a hypothetic AUT. This analysis resulted in accuracy nomograms that allow ESA staff to easily assess measurement accuracy depending on antenna size and operational frequency. Similar nomograms elaborated for different CATR’s could allow unbiased inter-range comparison. Moreover, a GRASP model of the facility has been developed based on the metrology measurement of the reflectors surfaces, relative position of range feed and AUT positioner.
Standard Gain Horns (SGH) are utilized frequently either as measurements antenna or as reference antenna in antenna gain measurements by comparison or substitution method [1]. They also find use as source antennas in anechoic test chambers and for many other purposes such as fixed site antennas. The most widespread SGH geometry has a rectangular cross-section and is pyramidal with optimized geometry to achieve maximum gain [2, 3, 4]. When used as a precision gain reference in antenna measurements the SGH is often calibrated by a reference facility or another third party. When external or internal calibration means are not available the SGH peak gain is often determined directly from the reference tables of the NRL report [2]. The quality of the original work is such that even today the associated uncertainty on these peak gain values are generally accepted to be within +/-0.3dB [1]. In this paper the accuracy of the NRL gain tables are investigated by comparison with a full wave numerical method based on FDTD [7] and measurements in different antenna test ranges. Performance variation of the SATIMO Standard Gain Horns due to the manufacturing and measurement accuracy has been also investigated with conducted and radiated experiments.
This paper shall discuss a method for measuring the current distribution – in both magnitude and phase - along the length of a floating antenna operating on the surface of the ocean. The method makes use of a novel toroidal current sensing device and balun arrangement, with a vector network analyzer serving as the measurement instrument. The current data obtained using this method can then be used to compute the far-field pattern of the antenna, both at the horizon and overhead, in a manner similar to near-field scanning of aperture antennas. This new method has significant advantages over the conventional far-field method of measurement in terms of accuracy, time, and cost, and can also be used to determine the realized gain of the antenna. Measured and theoretical data shall be presented on example antennas to illustrate the process of measuring the current distribution as well as computation of the far-field pattern.
RF measurement of the dielectric properties of very thin films (less than 1/100 wavelength thick) presents a challenge using traditional techniques. Many techniques, such as conventional transmission line-type measurements, are not sensitive enough to measure a single thin sheet of material. Moreover, in the case of waveguide, the method of mechanically fastening the material in place properly is challenging. In this paper, we explore several different strategies for measuring thin films and compare the merits of each. In particular, coaxial line measurements with stacked layers, waveguide measurements, and cavity measurements are discussed. The methods will be compared in terms of their accuracy and sensitivity. Measurements are carried out using the various methods on several low-loss thin-film materials. The measurements are then compared and validated using known reference materials.
A widely used time-gating technique can be effectively implemented in near-field (NF) antenna measurements to significantly improve the measurement accuracy. In particular, it can be implemented to reduce or remove the effects of the following measurement errors [1]: -multiple environmental reflections and leakage in outdoor or indoor NF ranges -edge diffraction effects on measurement accuracy of low gain antennas on a ground plane [3] In addition, reflectivity in the range can be precisely localized, separated and quantified by using the time – gating procedure with only one addition (a subtraction operation) added to the standard near-field to far-field (NF – FF) transformation algorithms. In this paper a step by step procedure is described which includes acquisition of near-field data, transformation of the raw near-field data from the frequency to the time domain, definition of the correct time gate, transformation of the gated time domain data back to the frequency domain, and the transformation of the time gated near-field data to the far-field. The time gated results, as already shown in [2], provides for more accurate far-field patterns. In this paper it is shown how the 3D reflectivity/multiple reflections in the measurement chamber or outdoor range can be determined by subtracting the time gated results from the un-gated data. This technique is illustrated through use of several measurement examples. It is demonstrated that the time gated method has a clear physical explanation, and, in contrast with other techniques [4,5] is less consuming (does not require mechanical AUT precise offset installation, additional measurement and processing time) and allows for a better localization and quantization of the sources of unwanted radiation. Therefore, this technique is a straightforward one and is much easier to implement. The main disadvantage cited by critics regarding use of the time gating technique is the narrow frequency bandwidth used in many NF measurements. However, it is shown, and illustrated by the examples, that the technique can be effectively implemented in NF systems with a standard probe bandwidth of 1.5:1 and an AUT having a bandwidth as low as 5% to 10%.
The cloud profiling radar (CPR) for the Earth, clouds, aerosols and radiation explorer (EarthCARE) mission has been jointly developed by JAXA and NICT in Japan. The development of CPR has required several technical challenges from the aspects of hardware designing, manufacturing and testing, because very large antenna reflector of 2.5m diameter with high surface accuracy, high pointing accuracy and high thermal stability had been required to realize the first space-borne W-band Doppler radar. In order to verify the RF design, we have just begun to perform antenna pattern measurement by using a CPR Engineering Model (EM). For this RF testing, we introduced a Near-Field Measurement (NFM) system with necessary capabilities for high accuracy measurement. This paper will present the summary of preliminary test results of the CPR EM antenna and the other technical efforts being taken for the antenna pattern measurement.
There are only a handful of commercially available antenna calibration laboratories in the US that are accredited to ISO-17025. Satimo has been operating an accredited laboratory in Atlanta since 2005 and an accurate and documented evaluation of measurement uncertainty has been a key element of the accreditation process. In order to develop the budgets, the parameters affecting the accuracy of the antenna measurement must be well understood. There are several references [2-9] that outline the method for preparing a measurement uncertainty budget, but few encompass the unique attributes associated with antenna measurements. Many of the papers published on the subject of the measurement uncertainty for antenna measurements address the characterization of a specific error term associated with an uncertainty budget, but few describe all of the terms contributing to the error budget nor how to practically determine their values. The intent of this paper is to outline and briefly describe the derivation of the uncertainty terms that contribute to the overall error budget for antenna measurements. Topics that will be discussed include: the uncertainty types, how to obtain or derive the error term for each uncertainty type, the distributions associated with each uncertainty type, the determination of the confidence level and coverage factor, how to combine the error terms as the references listed above are not in agreement on the method for combining the terms.
Hardware gating has been widely used to eliminate stray signals in the test range for single antenna. While testing the antenna on satellite, several issues should be considered to obtain accurate result. The difference come from several new conditions such as complicated electro-magnetic circumstance, desired stray signals from the satellite and varying of time delay due to antenna rolling. The width of hardware gating pulses and time delay of these two pulses are carefully set to ensure the measurement accuracy. Several methods are presented in this paper. These methods have been used in several test of antenna with satellite, which prove to be very efficient.