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- Lanne, Maria, 1975
(författare)
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Antenna array systems : electromagnetic and signal processing aspects
- 2005
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A simple and commonly used model of an array antenna consists of identical sensors, which are equidistantly located along a line. But in reality the array antennas do not look this way. The elements are not perfectly positioned, and sometimes they are even placed on a curved surface (conformal arrays). The antenna elements are not identical, and there are active components within the array which change with time and temperature. Due to the mutual coupling, the elements will also interact. The excitation of one element will change the excitation of the neighboring elements, which means that the elements near the edges of the array will perform differently from the ones in the middle. The real world array antennas are non-planar, imperfect, finite and their performance is changed by the mutual coupling. The purpose of the present thesis is to study these aspects. The papers which this thesis is based on includes the following topics. Different shapes of conformal array antennas have been studied with an optimality criterion of maximizing the gain over the scan region, to the smallest possible cost. A calibration method for an active array antenna with replaceable transmit-receive modules has been studied. The calibration method together with the exchange of the transmit-receive modules (which are not identical) are shown to give rise to a substantial degradation of the radiation pattern. Methods to avoid these problems are suggested. The mutual coupling for an infinite array of strip dipoles has been calculated and compared to measurements on a finite array of dipoles. The performance of the elements close to the edges of an array of dipole elements has been estimated using measurements for the elements in the center of the array. The difference between the real and the ideal array is a question of electromagnetics, but in this thesis it is also studied from a signal processing perspective. Whether or not the difference is a problem is determined by if the signal processing still gives the desired result. To determine the differences and to compensate for the differences are also signal processing problems. Future work will include studies of the requirements of the mutual coupling modelling in different beam forming methods.
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- Lanne, Maria, 1975
(författare)
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Phased array calibration and beamforming using signal processing
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Array antennas have a wide range of applications which includesground and airborne radar, weather radar and anti-collision radar.In addition, equally important applications exist forcommunications, both on earth and to satellites in space, as wellas for medical applications such as the detection and treatment oftumors.The modern trend in array design is to use digital beamforming andto integrate the antennas with the rest of the system moreclosely. This adds new possibilities, but also contributes to makethe system expensive. To make full use of the new technology, thisthesis presents a direction dependent signal processing arraycalibration method. The new method, called local calibration, alsoallows for larger array imperfections and, possibly, a loweroverall array cost. In contrast to other non-parametriccalibration methods, the proposed method can handle positionerrors and shows good performance also for sparse calibrationgrids, which is demonstrated by high resolution direction ofarrival estimation. The local calibration has also been includedin a convex beampattern optimization, which makes it possible toachieve a uniform sidelobe level also with unknown sensor positionerrors in the array. The embedded element pattern is included inthe optimization by extracting the direction dependence of thereceived power from the calibration. Another application for thelocal calibration method is to reduce the number of necessarycalibration measurements by calculating pseudo calibration datafrom a sparse grid of measured calibration data.Array signal processing is often performed assuming Uniform LinearArrays (ULAs), although this model is far too simple to reveal thetrue behavior of array antennas. Throughout this thesis, morerealistic models including mutual coupling and array imperfectionsare used. Specifically, the impact of mutual coupling and positionerrors on the adaptive beamforming performance in linear andcircular arrays is studied. In the studied cases, the circulararray is found to be more sensitive to neglecting the mutualcoupling. Robust adaptive methods are commonly used to handlemodel uncertainties, such as unknown mutual coupling, but evenbetter performance can be achieved if these methods are combinedwith a calibration.
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