ANALYSIS OF THE DIRECTIVITY CHARACTERISTICS OF PHASED ARRAY ANTENNAS WITH AXIAL SYMMETRY WITH HARMONIC AND NON-HARMONIC EXCITATION

The advantages of utilising antenna arrays with axial symmetry in radar systems are considered in this paper. It is noted that the employment of such antennas, in conjunction with modern technologies for space-time spectrum management, such as Multiple- Input-Multiple-Output and Frequency Diversity Array, has the potential to enhance the performance of spatially coherent radar systems, particularly in the context of fire control radars and short-range and very short-range air defence systems. The paper presents a mathematical model of antenna arrays with axial symmetry in the form of a circular array consisting of elementary, non-interacting, electrically small electric dipoles. This model enables the evaluation of the directivity characteristics for emitting (receiving) the harmonic and non-harmonic waves. The presented model enables consideration of polarization and diffraction phenomena in designing antennas for spatially coherent radars. The chosen form of representation of the basic expressions of the mathematical model facilitates the estimation of the directivity characteristics at both uniform and non-uniform distributions of antenna array elements in both far-field and Fresnel regions for different types of array design. The diffraction phenomena on the conducted surface of the antenna array are considered by the image theory. The numerical analysis of the dependence of the directivity characteristics of circular antenna arrays on different types of amplitude-phase distributions and the values of the design parameters of the antenna system under harmonic and non-harmonic excitations is presented. It is shown that when choosing the amplitude-phase distribution at the middle frequency of the spectrum for sharply directed radiation (reception) of non- monochromatic narrowband signals in a ring antenna array with large electrical dimensions and parameters typical of FDA and MIMO radars, changes in the directivity and the level of side lobes are insignificant. The compromise choice between placing the array elements in free space or above the conductive surface requires a detailed evaluation of the mutual coupling.