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An optical diffraction technique was developed for performing far field transformations of spherical near field data. The first goal of the development was to promote a better physical understanding of the phenomena of spherical near field transformation. Along the way, the limitations of this type of measurement became associated with real, optical physics, thus providing new considerations that might not be easily derived from the traditional, multi-pole expansion of spherical waves. In addition, new applications of spherical near field measurements are suggested by this approach. Specifically, the optical method allows for better understanding of the necessity and application of probe compensation. An optical transform eliminates the need for axially symmetric probes. Perhaps more importantly, this understanding leads to new considerations toward the applicability of single scan, spherical transforms which may lead to significant increases to the effective lengths of far field ranges. The purpose of this paper is to present a conceptual foundation to the spherical transformation of near field data by optical means and the immediate, associated benefits.
The equivalent source approach [ 1-4] has been presented recently as an advanced antenna diagnostics tool. The equivalent source approach is a true 3D approach as opposed to traditional methods based on plane wave expansion using hemispherical field information. This method is therefore highly suitable for diagnostics on low and medium directivity antenna and even allows the possibility to isolate, identify and filter unwanted effects close to the antenna from the measurements such as cable interactions. Spatial filtering is used in spherical near field measurements through the truncation of the mode spectrum. From knowledge of the minimum sphere enclosing the Antenna Under Test (AUT) the minimum number of spherical modes needed to represent the antenna can be determined and information resident in the higher order modes are eliminated as associated to sources outside the spatial domain of the source. Due to the nature of the spherical waves, spatial filtering truncation cannot be performed too close to the physical minimum sphere enclosing the antenna. Since the equivalent current approach is based on an accurate reconstruction of the physical currents on an object slightly larger than the physical antenna, a more acute spatial filtering can be performed. This paper discusses the advantages that can be obtained from the 3D spatial filtering on spherical near field antenna measurements.
The spherical near-field antenna measurement system StarLab has recently evolved to cover the entire frequency band from 800MHz to 18GHz [1, 2, 3]. The system is based on patented probe array technology and the Advanced Modulated Scattering Technique (A-MST) [4, 5]. The StarLab system combines the speed advantages of a probe array with mechanical rotation in elevation, thereby allowing for unlimited angular resolution over the full 3D sphere. By adding a linear scanner, the StarLab system can be converted to a compact cylindrical near-field measurement system (Starlab BTS), as shown in Figure 1. This system is particularly well-suited for measurements of base stations and other sectorial type array antennas. This paper describes the innovative design aspects of the StarLab portable antenna measurement system, with emphasis on the cylindrical near-field measurement capabilities. The system validation testing has been performed with comparative measurement campaigns, including both multiprobe and other traditional (single probe) measurement facilities at customer locations.
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.
Anechoic chambers utilized for far-field antenna measurements at VHF/UHF frequencies typically comprise rectangular and tapered designs. The primary purpose of conventional far-field chambers is to illuminate a test zone surrounding the Antenna Under Test (AUT) with an electric field that is as uniform as possible, while multiple reflections from the side wall absorber assemblies are kept to a minimum. The cross section dimensions of far field chambers at VHF/UHF frequencies can be electrically small, often as little as 3.. In this paper the side wall reflections at VHF/UHF bands are studied in more details for elongated rectangular and tapered chambers. In particular, the reflectivity is evaluated in rectangular chambers as a function of electrical dimensions of the chamber cross – section and of the ratio W (width of the chamber) or H (height of the chamber) to L (length – separation between antennas) for values ranging from 0.5 to 2. The methods of reflectivity improvement are presented and compared. In particular, the conventional chamber design is compared with a “Two Level GTD” approach [4,5,7] and the latter one shows significant reflectivity improvement in the test zone, even at longer source antenna AUT separations. The side wall reflections are examined in tapered chambers as well. The back wall reflection mechanism, which assumes multiple incident waves – direct from the source antenna and reflected from the side walls, floor and ceiling, is offered and confirmed by the simulation, which, in turn, yields an optimized back wall chamber design (see also [6]).
The in-flight pattern measurements of a sub-millimetre space telescope may be improved by de-termining the actual reflector anomalies and then in-clude the knowledge to these in the final pattern de-termination. The pattern measurements with a celes-tial object as source often have an insufficient signal-to-noise ratio for a pattern prediction outside the main lobe. Repeated measurements may improve this but even better is the possibility to extract data from different detectors operating at different frequencies. With the Planck Space Telescope as example, simulations of a displaced and distorted reflector have been carried out for noise contaminated amplitude measurements of Jupiter by 5, respectively 10, differ-ent detectors. First, the main beams of the antenna patterns are retrieved in a regular grid. Here the ac-curacy is limited by the noise level. Then, by a Physi-cal Optics optimization the actual distortions of the telescope's reflector are determined so that the calcu-lated radiation patterns of the antenna are correlated to the measured main beams. The patterns for the optimized and retrieved reflector geometry are shown to be precise at levels far below the noise floor in the direct measurements1. 1 The work presented in the paper has been carried out under ESTEC Contract No. 18395/NL/NB
This paper describes the principles of operation of high performance absorbing materials and the criterion for its performance optimization at UHF/VHF frequency bands. The optimization criterion is intended to determine the optimum carbon loading of the foam based absorber components, thus delivering optimal reflectivity of the full absorbing assembly (foam based absorber components on a metallic backing plate) at the lowest possible operating frequency. The optimization is based on equalization of reflections in the time-domain from the front face surface of the absorbing component and from the backing metallic plate. Validity is confirmed by measurements of the reflectivity of pyramidal absorbing components of varying heights, (3’, 5’, 6’ and 8’) in a 40’ long coaxial line terminated in a metallic back wall. In addition, it is shown that the “aging” process of the absorbing components can be characterized by the change of the effective reflectivity in the time-domain of the components as a function of aging time. It is possible to determine whether the absorber performance is stabilized and the “aging“ process is complete, and whether the loading of the absorber carbon mix is optimum, or is otherwise under-loaded or over-loaded. In particular, it is possible to determine prior to the time when the “aging” process is stabilized whether the loading is excessive.
In this paper, a symmetrically feeding structure for linear dual-polarized feeds is put forward. Due to its electrical balance and modes within the circular feeding waveguide are compressed. The polarization purity of feeds is then improved. Thus the level of cross-polarization can be very low, which is the key performance requirement for CATR (Compensated Compact Test Range). Besides, the return loss equation derived from equivalent microwave network for this symmetric structure is different from the ordinary single port feeding structure. In the low frequency range application, reflection at interface between coaxial and circular waveguide can reach a high level, a new transition probe is designed to depress it. Simulation results show that VSWR and cross-polarization performances of symmetric feeding feeds both are better than dual-polarized quadruple-ridged horn. 01TM21TE
From measurements of RCS of a target as a function of the frequencies and the bearings, it is possible to make RADAR imagery. A common way is to use a bi-dimensional Fast Fourier Transform (FFT2) while this algorithm being very fast. Yet this algorithm demands that the grid on which the RCS is known fulfils some particular conditions. Now such conditions are not respected by the grid of measurement. Consequently an interpolation of this grid is necessary in order to be able to apply the FFT2 algorithm. The choice of the method of interpolation will directly impact the quality of the calculated RADAR image. In this article we propose to study this impact while giving the analytical expression of the interpolation then while giving the analytical expression of the RADAR image calculated from the interpolated RCS and while specifying eventually the method interpolation which limits the degradation of quality of the calculated RADAR image.
This paper discusses the designing of circular waveguide antenna, mode & field propagation analysis with in circular waveguides, cutoff frequency analysis & radius along with calculations for millimeter region Electromagnetic waves ranging between 1GHz-40GHz.This analysis will be based on Finite Element Method using Ansoft HFSS, therefore Finite element Method has also been briefly discussed. Circular waveguide’s cutoff frequency & radius can be directly calculated by using SAND’s constant; a method generated through the optimization of approximated cutoff frequency equation refined by using FEM numerical technique. Graphical analysis for cutoff frequencies ranging between 1GHz-40GHz against waveguide radii has also been discussed. SAND’s constant variation for entire frequency range of 1GHz-40GHz have also been discussed.
A stand-alone commercial program, performing advanced electromagnetic processing of measured data, is being developed by TICRA. The program reads the measured field and computes the extreme near field or the currents on the antenna surface. From the inspection of the extreme near field or currents, the program will solve typical antenna diagnostics problems, such as identification of array element failure and antenna surface errors, but also allow artificial removal of undesired contributions, such as currents on cables and fixtures, thereby saving valuable time and resources in the antenna design and validation process. The program will be based on two field reconstruction techniques, the SWE-PWE presented at AMTA in 2007, and a new and more accurate inverse higher-order Method of Moments (INV-MoM). The paper will illustrate the theory behind the two techniques and present numerical cases with simulated data.
The calibration and measurement of bistatic reflectivity at short range (3.3 m) presents challenges that are significantly different from the usual test range measurements (typically monostatic at 100 to 150 m). In order to overcome this an object-free calibration procedure has been applied, eliminating crosstalk, reducing other interferences and removing errors associated with the RCS and alignment of calibration objects. It is based on calibrating the transmitter and receiver antennas as a pair by directing the antennas toward each other. The method thus requires that the antennas can be separated. Furthermore the signal level needs to be handled e.g. by the separation distance or attenuators. The bistatic reflectivity measurements were performed by placing a clutter sample on a turntable which is located at the centre of a bistatic arc. This configuration enables us to do ISAR images. Background contributions were discriminated using a combination of synthetic resolution and zero-doppler filtration. The sensitivity variation across the antenna footprint was handled by calculating an equivalent area from measured off-axis antenna sensitivities. Reflectivities have been measured for a metallic test surface and for grass. The metallic test surface had been manufactured to correspond to typical theoretical bistatic clutter models.
In the last decades radar imaging techniques have been widely studied. Electromagnetic imaging is a very promising technique for many practical application domains (medical, surveillance, localization …). As an example, many RCS imaging systems have been developed for compact range indoor RCS measurement layouts. In this paper, a preliminary comparison of near field RCS images from Multiple Input Multiple Output (MIMO) arrays and monostatic radar is presented. The main objective of this study is to make use of efficient radar imaging algorithms, which were originally conceived for SAR systems, with MIMO arrays (ex. back projection) in order to develop real-time imaging applications based on MIMO array systems. The study was conducted with a one-dimensional MIMO array composed of 14 transmitting and receiving antennas. The goal of the optimization is to obtain radar images as similar as possible to those from monostatic radar. This paper presents the experimental layout, the imaging algorithms and the experimental results. As a conclusion, the imaging capabilities of MIMO arrays are discussed.
Compact ranges are well suited to perform accurate indoor RCS measurements. These facilities are limited at the lower end of their bandwidth by the size of the parabolic reflector. Therefore, when RCS characterizations are required in the UHF band, RCS measurement facilities usually operate large horns or phased array antennas in a near field measurement layout. However, these calibrated near field measurements cannot directly be compared to the plane wave RCS characteristics of the target. One way to compare simulation and measurement results is to take the near field radiation pattern of the antenna into account. This paper first presents the design of a phased array antenna developed for indoor UHF RCS measurements. Then a model of this antenna is derived and a simulation of the experimental layout is performed. In parallel, near field RCS measurements of a canonical target were performed with this phased array antenna in an anechoic chamber. As a conclusion, a comparison between simulation and experimental results on this particular canonical target is discussed.
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%.
A method to reduce the noise power in far-field pattern without modifying the desired signal is proposed. Therefore, an important signal-to-noise ratio improvement may be achieved. The method is used when the antenna measurement is performed in planar near-field, where the recorded data are assumed to be corrupted with white Gaussian and space-stationary noise, because of the receiver additive noise. Back-propagating the measured field from the scan plane to the antenna under test (AUT) plane, the noise remains white Gaussian and space-stationary, whereas the desired field is theoretically concentrated in the aperture antenna. Thanks to this fact, a spatial filtering may be applied, cancelling the field which is located out of the AUT dimensions and which is only composed by noise. Next, a planar field to far-field transformation is carried out, achieving a great improvement compared to the pattern obtained directly from the measurement. To verify the effectiveness of the method, two examples will be presented using both simulated and measured near-field data.
Impedance and gain measurements for electrically small antennas represent a great challenge due to influences of the feeding cable. The leaking current along the cable and scattering effects are two main issues caused by the feed line. In this paper, a novel cable-free antenna impedance and gain measurement technique for electrically small antennas is proposed. The antenna properties are extracted by measuring the signal scattered by the antenna under test (AUT), when it is loaded with three known loads. The tech-nique is based on a rigorous electromagnetic model where the probe and AUT are represented in terms of spherical wave expansions (SWEs), and the propaga-tion is accounted for by a transmission formula. In this paper the measurement results by the proposed technique will be presented for several AUTs, includ-ing a standard gain horn antenna, a monopole an-tenna, and an electrically small loop antenna. A com-parison of measurement results by using the proposed method and by using other methods will be presented.
General Purpose Graphical Processor Units (GPGPUs) provide increased processing capability for applications with a high degree of data parallelism. In the past the few years, GPGPUs have become readily available in the commercial market, and off-the-shelf programming tools (e.g. CUDA from the NVIDIA Corporation and Jacket from Accelereyes, LLC) have made them more accessible to the technical community. SAR and ISAR imaging algorithms are inherently computationally intensive. In order to overcome performance limitations of CPUs and traditional DSPs, simplified, computationally-efficient algorithms are often used, but at the expense of the phase information available within the raw data. We have demonstrated that GPGPU acceleration of SAR/ISAR processing has greatly improved processing times of a less-efficient (but more flexible) algorithm, making its use more practical. We have shown that GPGPUs can provide performance improvement in excess of 30X for a backprojection-based SAR/ISAR imaging technique.
In this paper, application of a modified Wheeler cap method for the radiation efficiency measurement of balanced electrically small antennas is presented. It is shown that the limitations on the cavity dimension can be overcome and thus measurement in a large cavity is possible. The cavity loss is investigated, and a modi-fied radiation efficiency formula that includes the cavity loss is introduced. Moreover, a modification of the technique is proposed that involves the antenna working complex environment inside the Wheeler Cap and thus makes possible measurement of an antenna close to a hand or head phantom. The measurement procedures are described and the key features of the technique are discussed. The results of simulations and measurements by the proposed method are pre-sented and compared.
Operating microwave receivers with remote mixers in a system requires the LO power to be flat over broadband frequencies. In large systems, this is difficult to attain due to long RF cables. Most systems require significant engineering to ensure the LO power level to the mixer is adequate. To help understand the problem, commonly used techniques have been evaluated while recommending a particular approach. Operating over a small fundamental frequency range with harmonic mixing has the advantage of lower RF cable insertion loss but results in high mixer conversion loss. Using negative slope equalizers and amplifiers, RF cable slope and attenuation can be sufficiently combated. However, this requires extensive system engineering and customization to match cable losses, thereby making it expensive. The approach is also designed to only work with a certain set of RF cables. A more viable approach includes independently controlling the attenuators and amplifiers for the signal and reference channels which can be configured to provide optimal LO power to the respective mixer. A simple setup file configures components in each channel to adapt to any set of RF cables. Positive experimental results of implementing this technique in different configurations are presented.