Synthesis of arbitary antenna arrays

Abstract

Design of antenna arrays for present?day requirements has to take into account both mechanical and electrical aspects. Mechanical aspects demand that the antennas have low profile, non?protruding structures, and structures compatible with aerodynamic requirements, and so on. Electrical aspects may introduce several constraints either due to technical reasons or due to realizability conditions in practice. Thus, arrays of modern requirements may not fall into the category of linear or planar arrays. Further, due to the nearby environment, the elements will generate complicated individual patterns. These issues necessitate the analysis and synthesis of antenna arrays that are arbitrary as far as the orientation, position, or the element pattern are concerned. Such arrays, which may be called arbitrary arrays, are investigated in this thesis. These investigations have been discussed in different aspects as indicated below:

Radiation Characteristics of Arbitrary Arrays Radiation fields of an arbitrarily oriented dipole are obtained. Such fields are plotted for typical cases. Further, methods for transforming electromagnetic fields are discussed. Having obtained the field due to an arbitrary element, the fields due to an arbitrary array are obtained. Factors controlling the radiation fields-such as curvature in the array and element pattern-are investigated. Radiation patterns of circular and cylindrical arrays are plotted.

Synthesis of a Side?Lobe Topography Requirements of a narrow beam pattern generated by an antenna array are identified. A problem of synthesizing such a pattern using an arbitrary array is formulated. The envelope of the side?lobe region, called the side?lobe topography (SLT), is included in the computation of the covariance matrix. This problem, which is formulated as minimising a quadratic function subject to a system of linear constraints, is solved by the method of Lagrangian multipliers. An iterative procedure is used to satisfy all the requirements of the pattern synthesis. The procedure has been validated by synthesising linear arrays and is then used to synthesise circular and parabolic arrays. Patterns with tapered SLT and Taylor?like SLT have been synthesised. Asymmetric patterns have also been synthesised. The role of SLT is brought out.

Shaped?Beam Synthesis Synthesis of shaped broad beams is discussed. Amplitude constraints are formulated. Phase distribution is linked with the phase centre. Quadratic problems thus formulated are solved using the Lagrangian method of undetermined multipliers. An iterative procedure is used to synthesise flat?topped beams as well as cosecant?squared patterns using both linear and circular arrays. Reasonable excitation dynamics have been obtained. Optimum phase centres, obtained by trial and error, are used.

Effects of Frequency and Excitation on the Synthesised Patterns In general, synthesised patterns can be sensitive to specific parameters such as excitation or frequency. Several methods can be used to examine these issues. In this thesis, these effects are studied. Using a specific array configuration to synthesise a given radiation pattern:

frequency is changed by 10% from the design frequency and the pattern is recomputed, excitation phase distribution is quantised to the nearest level available in an 8?bit digital phase shifter and the resulting pattern is computed, excitation dynamic is controlled by boosting amplitudes of array elements below the permitted maximum.

Effects of these variations are recorded. It appears that reasonable patterns can be obtained despite significant variations in these parameters in most cases.

Reconfigurable Arbitrary Arrays It is highly useful if a single array configuration can be used for different applications, either for different phases of the same mission or for entirely different missions. Attempts are made to synthesise a variety of patterns from a single array. Such arrays, called reconfigurable arrays, are of great practical value. Obviously, the excitations differ for different patterns. Both narrow beams and shaped broad beams, with different side?lobe topographies, have been synthesised using a single array.