Antenna array systems : electromagnetic and signal processing aspects

Licentiate thesis, 2005

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.