1,721,329 research outputs found
Lossy dielectric resonators for microwave absorber and antenna applications
No full textMicrowave absorbers are used in radio systems to attenuate the electromagnetic wave energy to reduce unwanted reflection, transmission, scattering, and coupling. In microwave circuits, absorbers are needed to reduce the intra-system interference caused by undesired interactions between the digital circuits and the radio frequency analog receivers. Absorbers also can improve the isolation between closely-placed antennas and reduce their sidelobes. In radar application, the scattering from metallic objects due to the illumination from incoming waves can be suppressed by attaching them with absorbers. In antenna chambers, the absorbers play a vital role in providing the required quiet zone for the measurement. A variety of absorbers have been developed and employed in those diverse applications. These absorbers can be classified based on their specific characteristics. Depending on the relative distance between the microwave absorbers and unwanted sources, microwave absorbers can be divided into free space and near-field absorbers. Based on the matching and absorption mechanisms, microwave absorbers can also be categorized into non-resonant and resonant absorbers. This dissertation focuses on the development of a planar resonant absorber. One of the typical features of such absorbers is that they are thinner than non-resonant ones. The matching in the planar resonant absorber is achieved by designing its surface impedance to be equal to 377 Ohm by generating resonance(s) within the structure and taking care of coupling among the resonator structures. The resonance is controlled by selecting proper physical dimensions and electrical properties of the materials. The planar resonant absorber in this work is also intended to perform, not only for free space but also for near field applications, in relatively broad bandwidth, e.g., in the X-band (8 GHz - 12 GHz). Within this band, several radar applications are dedicated: marine radar, synthetic aperture radar, and weather radar. The author proposes the use of the dielectric resonance mechanism to develop novel resonant absorbers. Dielectric resonators have been used since a long time ago for microwave filters, oscillators, and antennas applications. They are made of low losses dielectric materials and available in generic 3D structures: cylinder, half-sphere, cubic, and ring. For broadband free-space microwave absorber applications, low losses materials are not desirable since they will limit the bandwidth of the absorber. So, lossy materials are the natural choice for microwave absorbers. However, problems related to the worse matching should be tackled. A systematic design method has been proposed. It started by determining the initial dimension of the resonator diameter. Then, modes mapping (modes in frequency versus the radius-to-height-ratio) is plotted to obtain the possible range of resonator height. To extend the bandwidth and simultaneously decrease the reflection coefficient, the dielectric resonator dimensions are determined in a way that multiple resonances can exist within the structure in the intended frequency range. The last is finding the proper spacing. A prototype intended to have a minimum reflection coefficient of less than -20 dB within the X-band (40% bandwidth) has been fabricated and measured. The dielectric resonators with a thickness of 5 mm are constructed by gluing five layers of thin cylinders. The materials are from commercial magnetic sheet absorber that has both electric and magnetic losses. This commercial sheet absorber is also measured to provide a comparison. The measurements were performed using the NRL arch setup. In the antenna application, the dielectric resonator absorber can be integrated with a bandpass frequency selective surface (FSS) to serve as a radome absorber. This integration enables the radome to absorb unwanted waves above the antenna operational frequencies so that, not only the monostatic but also the back-scattering in other directions are significantly reduced. The low insertion loss within the antenna band is achieved. This radome absorber can also be used as the second superstrate layer for a partially reflective structure (PRS) antenna. With this integration, the PRS antenna can benefit from a reduced scattering cross-section, especially at higher frequencies (Xband and above). By comparing the measured input reflection coefficient, realized gain, and the radiation pattern, in the case of with and without using the radome absorber, it can be concluded that the radome absorber does not degrade the PRS antenna performance. Another application of the dielectric resonator absorber is to improve the isolation between pair antennas. These antennas operate in the X-band. The absorber is placed between those horn apertures. Although the absorber is optimized as a free space absorber, its capability as a surface wave absorber is demonstrated. Other planar resonant absorbers (i.e., ultra-thin absorber, magnetic sheet absorber), which are previously optimized as free space absorbers, are also examined. With the presence of the absorbers, the antenna isolations are improved. The ultra-thin absorber improves the isolation only in part of the X-band, while the magnetic sheet and dielectric resonator absorbers improve for the whole X-band. The highest average of isolation improvement is achieved if the dielectric resonator absorber is inserted between those antennas
Measurement techniques for highly integrated mm-wave antennas
No full textIntegrated antennas are a crucial component in the development of high-performance, fully-integrated transceiver chips. However, due to the small size of integrated antennas and their complicated contacting fixtures, existing measurement setups cannot be used for their characterization. Therefore, new approaches must be investigated to obtain reliable and accurate measurement results and thus to make optimized on-chip antenna designs possible.
This work addresses the challenges of measuring integrated antennas at mm-wave frequencies. It introduces a robotic measurement setup to accurately determine the radiation properties of on-chip antennas and quantifies the achievable measurement accuracy through an in-depth uncertainty analysis. The study also analyzes post-processing techniques to mitigate severe distortions that occur in the measurement environment
Fast near-field antenna measurements by application of compressed sensing
No full textWith a higher demand of communication systems and the ever-growing presence of connected devices, the need for antenna measurements increases accordingly. A particularly convenient method to measure antennas is to do it in their near field, which allows us to measure them in smaller facilities. Although procedures for different measurement geometries exist, the method that acquires a full, three-dimensional representation of the antenna's radiation characteristics is defined for a spherical geometry. The procedure consists of the acquisition of enough information to be able to calculate the weighting coefficients of each of the radiated spherical waves (also called modes), which form a multipole expansion. Thereafter, it is possible to calculate the same superposition when the radius of the spherical waves tends to infinity. Under this condition, the spherical waves become plane waves, and the radiation characteristics of the antenna in its far field are thus retrieved. The name of this procedure is 'spherical near-field to far-field transformation'. The inconvenience of this type of measurements compared to a direct acquisition of the antenna's far field is that, to be able to calculate the spherical mode coefficients, a high number of measurement points, and thus a long measurement time, is required. This can be explained by assimilating the problem to the resolution of a system of linear equations: each of the spherical mode coefficients is a variable, while each of the acquired measurement points results in an equation. While the number of variables is limited due to the band-limitation of antennas, the number of equations, or of acquired points, can be much larger, since many of the samples may result in linearly dependent equations, thus not contributing any additional information. In the case of spherical near-field measurements, a whole sphere enclosing the antenna under test is scanned. Typically, the Nyquist-Shannon criterion is used to define scanning strategies on the sphere that, due to the nature of the problem, inevitably results in oversampling the sphere. The vector described by the coefficients of the spherical waves, however, proves to have, for most physical antennas in typical measurement situations, a large number of zero-terms, meaning the full information is contained in only a few significant terms. Vectors that comply with this condition are said to be sparse. A sparse vector implies that the number of significant variables of the associated system of linear equations is lower than that of the defined variables, which enables the reconstruction from a set of fewer measurement points, i.e., from a set of fewer equations, which can potentially accelerate measurements. The field of compressed sensing studies the acquisition and reconstruction of sparse signals from a smaller set of measurement points than the Nyquist-Shannon criterion suggests, thus speaking of undersampled or `sparse' measurements. In an effort to retrieve the sparsest solution for the system, the reconstruction is done by application of l1-norm minimization algorithms. The typically used sampling strategy, however, is the random choice of sample points. This is due to the mathematical properties of random sampling matrices, which provide reconstruction guarantees for sparse systems. However, random sampling schemes are not practical for acquisition with standard positioner systems, normally requiring a longer measurement time despite acquiring fewer points. In this work, the minimum-coherence of the sampling matrix is used as criterion for reconstructability. To enable a time-efficient scanning with standard positioner systems, a cosine-symmetric distribution in elevation is chosen. The lower coherence bound of sampling matrices of the spherical near-field problem's basis functions is derived for the chosen distribution in elevation. The corresponding pairs in azimuth that reach this bound are calculated with a suggested iterative algorithm. To increase the efficiency of the proposed compressed sampling, the acquisition of additional samples on the scanning path is suggested. At the cost of worsening the mutual coherence of the sampling matrix, more information is acquired at no time-cost, which has practical relevance: the proposed sampling guarantees that a minimum amount of information be there, while potentially adding free information. These concepts are extensively tested numerically and validated with real measurements, with a focus on a reduction of the measurement time with respect to classical approaches. Likewise, the strain put on the computational part increases, so that expensive measurement time is exchanged for cheaper computation time. In an effort to apply the studied techniques to other geometries and, in particular, to already existing mechanical systems for the cylindrical geometry, the sampling points are projected onto this surface using the technique known as pointwise probe correction. This allows the application of the mathematical framework for spherical near-field measurements to other geometries. Numerical experiments show the validity of these concepts under certain conditions and the effect of truncation of the sampling surfaces. The suggested method not only enables the use of compressed sensing for already-existing positioner systems, but also for arbitrary surfaces in modern systems, such as industrial robotic arms. Further, other practical considerations are briefly discussed and a technique to adaptively choose the number of measurement points is suggested. This technique is based on the choice of a high initial number of points and checking the variation of the reconstructed results during a measurement. This is done using a quick solver and truncating the reconstruction after only a few iterations, sufficient for an estimation of the convergence. The process is repeated during the measurement as further points are acquired until the stopping criteria are met. Finally, the application of compressed-sensing techniques to phaseless spherical near-field antenna measurements is introduced and briefly discussed, paving the way for further research in this direction. The introduced techniques for the application of compressed sensing to near-field antenna measurements of this dissertation, thus, aim to lighten the burden put on measurement facilities at the cost of increasing the required computational cost. Moreover, the suggested projection to arbitrary surfaces enables using the method, developed for spherical near-field measurements, with already-existing equipment for cylindrical measurements, as well as for future mechanical systems that might be optimized for different surfaces. Additionally, the suggested adaptive-sampling strategy allows for an efficient application of the proposed sampling scheme
Influences of external error sources on measurements of room acoustic parameters
No full textCorrect and precise measurements of room acoustic parameters are of fundamental importance for subjective room impression characterization and for the physical description of the sound field. This thesis investigates external influences on acoustic measurements and errors of the resulting room acoustic parameters. Theoretical models have been developed to predict these errors and indicate the tolerable limits of the described influences. To validate these models, specially designed room acoustic measurements that separate the individual influence factors have been conducted. Noise has been identified as one of the main influence factors and occurs during every measurement. In this thesis the performance of five commonly used stationary noise compensation methods are systematically analyzed depending on the peak signal-to-noise ratio. Impulsive noise that could also occur during the measurement is investigated separately, as the previously introduced compensation techniques are unsuited to handle the influence. The second part of this thesis analyzes the influence of time variances during measurements. Inter- and intra-measurement temperature changes, air movement, and human-sized scattering objects have been investigated
Real-time auralization of aircraft noise considering curved sound propagation
No full textAircraft are one of the major noise sources affecting residents close to airports. Classical metrics for noise assessment are usually derived from energy-based quantities, such as known from noise maps. Thus, they only consider human perception in a very basic manner. To investigate the impact of aircraft noise in a more accurate way, it is desired to conduct experiments focusing on human perception by presenting audible signals. This can be achieved in virtual acoustic environments. In this context, auralization is an approach to synthesize these audio signals based on virtual scenarios. Compared to recordings of a specific scene, this has the benefit of giving significant control over the parameters affecting sound as perceived by a listener on the ground. In this way, it is possible to compare aircraft flyovers that only vary in a single parameter, such as the aircraft design or the weather conditions. Real-time capable auralization extends the diversity of experiments that can be carried out by allowing users to freely move through the virtual scene and interact with the aforementioned parameters. One important requirement for auralization is a model for sound propagation from the aircraft to the listener. Typical aircraft auralization approaches use the simplification of a homogeneous medium and a flat ground. However, the atmosphere is generally inhomogeneous and moving leading to sound propagating along curved paths. Furthermore, sound generally interacts with urban structures within residential areas, e.g. leading to reflections and diffraction or occlusion of the direct sound. This work aims to overcome these simplifications in order to allow the consideration of aircraft noise in more realistic acoustic virtual environments. For this purpose, an aircraft auralization approach considering curved sound propagation in the atmosphere is proposed. This includes a computationally efficient method to simulate the curved sound paths connecting a source and a receiver, while still assuming a flat ground. Similarly to an approach by Arntzen et al., this method is based on a ray tracing algorithm for inhomogeneous, moving media and uses a divide-and-conquer routine to significantly reduce the number of calculated rays. Compared to the aforementioned state-of-the-art approach, the run-time is significantly lower while considering an additional propagation effect, namely advection. Furthermore, a procedure for scheduling these simulations and interpolating respective results is presented, which enables to perform such auralizations in real time. Finally, an interface method is introduced allowing to combine the presented curved propagation approach with urban auralization methods that consider the interaction with urban structures, while typically assuming straight-path propagation. For this purpose, a virtual source position is defined based on a curved free-field path. Using this position for the urban simulation allows to maintain the atmospheric propagation effects which affect parameters, such as the incidence direction at the receiver. The method is designed for scenarios where the source is relatively far away from the urban structures, which is typically the case for aircraft flyovers. The method was tested in a scenario with an aircraft flying over a street canyon. It was shown that, while the simplification of using a single curved sound path might introduce audible changes in the auralized signals, the plausibility should not be reduced. The auralization result also highlighted the relevance of considering the additional sound paths resulting from interaction with urban structures. This is especially prominent when the direct sound is occluded and therefore, reflected and diffracted sound paths have a significant effect on the localization of the aircraft
Immissionsmonitoring elektromagnetischer Felder des Mobilfunks mittels crowdsourcingbasierter Smartphone-Anwendung
No full textMobile communications have become indispensable in today's society. The signals to be exchanged for communication between terminal and base station are transmitted using electromagnetic waves. For protection against adverse health effects, the electromagnetic field strengths are limited by legal limits. Existing methods for monitoring exposure levels are mainly based on short-term measurements taken by trained measurement personnel with professional equipment at a small number of locations, or on measurement systems permanently installed at one position for a longer period of time. Due to the effort involved, comprehensive and continuous exposure monitoring cannot be realized in this way. However, this would be desirable as a basis for epidemiological studies as well as for informing the population as part of risk communication.A crowdsourcing-based approach that makes use of signal strength measurements from ordinary smartphones could provide an innovative solution for this. It relies on the fact that the signal strengths detected by smartphones to connect to the mobile network are related to the underlying electromagnetic field strength. By combining the measurements of many smartphones in the same environment, the impact on the exposure estimation result can be reduced.The present work aims to address the fundamental questions regarding the feasibility and usability of the results of such a monitoring system. This includes on the one hand the theoretical examination of technical aspects, on the other hand extensive metrological investigations of the measurement properties of smartphones and finally the practical testing of the evaluation of smartphone crowdsourcing data. The following scientific contributions to the underlying research question have been achieved by this work: 1) The relationship between the signal strengths measured by smartphones and the underlying electromagnetic field strengths was elaborated and described using the definitions in the mobile radio standards. 2) It was shown that the necessary information for exposure estimation can be collected within the Android operating system on ordinary smartphones. 3) A method for the correct interpretation of the signal strength indicator RSSI within the radio service LTE to determine the load-dependent exposure was developed and demonstrated. 4) The measurement characteristics of smartphones relevant for deriving the exposure were investigated and evaluated experimentally in detail. 5) For the first time, conversion factors from signal to field strength were determined for different smartphones by metrological investigations. 6) For the first time, estimates of exposure levels were obtained from smartphone measurements collected by crowdsourcing and presented in the form of exposure maps.The results achieved have shown that the fundamental technical feasibility for smartphone-based monitoring of exposure by mobile radio is given, since the essential required parameters are collected by smartphones and can be retrieved within the Android operating system by an application. In addition, the measurement properties of smartphones provide a sufficiently stable correlation between signal and field strength. The fact that the required technical infrastructure can also be implemented with the existing possibilities and reasonable effort was demonstrated by including data from an existing crowdsourcing network and setting up an own test network.However, the practical evaluation of smartphone crowdsourcing data revealed that a fully comprehensive exposure estimation of all mobile radio services and frequencies cannot be achieved, as measurement data are obtained predominantly for the service currently most commonly used by smartphones, and not to the same extent for all frequency bands. Thus, results that agree well with field strength measurements could only be obtained for LTE
Signal recovery on the sphere from compressive and phaseless measurements
No full textIn this thesis, we investigate the possibility of reducing the number of measurements and using only their magnitudes to reconstruct spherical signals and their three-dimensional rotations. These problems appear, for instance, in spherical near-field (SNF) antenna measurements, where the number of samples to acquire a signal implies a long measurement time and occasionally unreliable phase information. We tailor the compressed sensing (CS) approach to reconstruct spherical signals from fewer measurements. Instead of considering fully random sensing matrices like in the typical CS, we restrict the randomness to follow certain structures derived from the properties of spherical harmonics and Wigner D-functions. In this setting, we provide a bound on the number of measurements required to allow stable and robust signal recovery as well as numerical evaluations to verify these results. Although we limit the randomness, applying random sampling on the sphere is still cumbersome since collecting samples requires the use of mechanical devices that move smoothly over the spherical surface, such as robots. Hence, a deterministic sampling pattern to construct sensing matrices is still desired in many applications and the notion of coherence is used to measure the sensing matrices’ quality. First, we show a class of commonly used deterministic sampling patterns on the sphere that produces the worst coherence sensing matrices and, thereby, is unsuitable for CS. Subsequently, we propose a sampling strategy on the sphere to construct low coherence deterministic sensing matrices from spherical harmonics and Wigner D-functions, and derive a coherence bound for both cases. Apart from dealing with compressive measurements, we also study signal recovery on the sphere from phase less measurements and identify some potential ambiguities under this setting. We analyse ambiguities which arise from the properties of complex and real spherical harmonics, as well as the ambiguity caused by the implementation of inappropriate sampling patterns. As a further contribution, numerical evaluations are conducted to compare several reconstruction algorithms as well as the effects of different sampling patterns on the sphere. Finally, these results are implemented using SNF data, where we numerically show that our proposed sampling pattern requires significantly less number of measurements to provide high-quality reconstructions of far-field patterns when compared to classical approaches
Quiet Zone Performance Qualification using Spherical Near-Field Scanning
No full textThe evaluation of the performance of an antenna measurement chamber becomes an ever more important part of both the validation and maintenance therein. Key antenna performance parameters such as gain, directivity, radiation pattern and polarisation depend highly on the planarity of the field distribution of the quiet zone. In order to meet the demands of current and future antenna measurements, modern measurement ranges have to meet strict requirements on the field distribution inside the quiet zone. In order to verify that the measurement range meets these strict requirements the measurement setup and algorithms used for the evaluation of the quiet zone performance need to be improved accordingly.Quiet zone spherical near-field scanning provides a solid basis for the performance evaluation of a far-field antenna measurement chamber. The reason is that spherical harmonic decomposition provides a complete description of the electromagnetic field distribution inside the quiet zone. This description provides all the information needed for the analysis of the field distribution inside the quiet zone and provides meaningful insight in the expected performance of the range during antenna measurements. For the spherical harmonic decomposition only the measurement of both tangential electric field components on the surface of a sphere surrounding the quiet zone is needed. Therefore, only two full sphere measurements are needed for each range setup. Then on the calculated Spherical Mode Coefficients, the weights of the spherical harmonic waves, special algorithms such as the calculation of the electric field on arbitrary locations inside the quiet zone, or angle of arrival estimation can be easily applied without needing further measurements.The measurement setup itself consists of a sturdy beam and probe antenna. The sturdy beam is mounted on top of a roll-over-azimuth positioning system which commonly is already available in an antenna measurement chamber. At the end of the sturdy beam the probe antenna is placed such that the tangential electric fields on a sphere surrounding the quiet zone can be measured. The resulting measurement data is decomposed into spherical harmonic waves using established algorithms from spherical near-field antenna measurements. The SMCs provide all the necessary information for a full reconstruction of the electromagnetic field distribution inside the quiet zone.Using the SMCs the electromagnetic field can be calculated on arbitrary coordinates inside the quiet zone. With a newly developed adaptation of the transmission formula a virtual receiving antenna can be placed on these positions. The resulting output of the calculation is the same as the output a physically placed antenna at the same location in the quiet zone would measure. As such the adapted transmission formula can be used to calculate similar data a field probing setup would measure without the need for the additional positioning equipment.A second and important part of the evaluation and maintenance of an antenna measurement chamber is the detection of unwanted sources of reflection. These sources of reflection cause extraneous signals impinging on the Antenna Under Test (AUT) causing an error in the measured radiation pattern. Although the SMCs provide a complete and highly accurate description of the electromagnetic field distribution inside the quiet zone, signals impinging into the quiet zone are superposed in the SMCs and cannot easily be separated. Due to the finite number of SMCs that can practically be considered, a back-projection into the far-field results in a highly convolved angular map in which small signals can not be discriminated due to the presence of the large main beam illumination. By combining the SMCs and the CLEAN algorithm, the superposed signals in the SMCs can be separated and as a result a high resolution angular map can be derived. Using one version of the presented algorithm it is possible to discriminate signals from opposing direction, even if one of both has a power level that is several magnitudes higher than the other.As a last step, a new performance indicator is derived from the SMCs directly, the quality factor. The quality factor is an important indicator of the planarity of the field distribution of the quiet zone. Since quiet zone spherical near-field scanning is able to determine the field distribution in the quiet zone independently of the used measurement setup, the quality factor can be used for measurement range inter-comparison and provides an upper bound for the error of antenna measurements inside the antenna measurement chamber.Based on the developed measurement setup and algorithms it can be concluded that quiet zone spherical near-field scanning presents a next step in evaluating and validating the performance of a far-field antenna measurement range. The developed set of tools provide enough information for the range maintenance to track the range performance during its lifetime using only a simple measurement setup
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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