Hybrid Antenna System And The Scattering Matrix-based Beamforming

In the last few decades, much effort has been given to the development of methods for scanning reflector antennas. But most of them are accomplished by physically moving a single feed element or subreflector. The horn antenna or open-ended waveguide are usually used as a single feed element. Physically moving the position of feed element can steer the main beam in the desired direction. The main beam of a parabolic reflector can be steered by translation of a single feed element; however, a coma aberration will appear when a larger scan angle is required.In this thesis, a hybrid antenna system which is composed by placing a transverse static feed array near the apex of a compound reflector has been discussed. The advantage of hybrid antenna system is combining the electric scanning ability of feed array antenna with the structural simplicity of reflector antenna. Electric scanning can be achieved by adjusting the amplitude and phase between feed array elements instead of physically moving the feed array. When the scan angle is small, the electric scanning can be realized by using only a few feed arrays in the hybrid antenna system. Therefore, the cost of electric scanning is greatly reduced. Based on the physical optics (PO), the free space dyadic Green's function is applied to the computation of transmitting and receiving field. Moreover, a scatter matrix which establishes the transformation between fields of the feed array and that of the aperture of hybrid antenna system has been discussed. A N-port network model is developed for the hybrid antenna system, some ports are located at feed array, others are related to the reflector aperture. Meanwhile, the electric or magnetic field at the feed array and aperture has been computed using the dyadic Green's function in order to get the scatter matrix. The transmitting and receiving beamforming is analyzed by using the scatter matrix. In this thesis, the optimized hybrid antenna system which is brought up by reference, i.e.: the hybrid antenna system, in which the diameter of aperture is 60λ, the diameter of feed array is 30λ. From the results, we can conclude that when the scanning angle is within the range of ±20°, the beam keeps the same and the system realizes wide-scan. Therefore, through proper design of the hybrid antenna system, the beamscanning can reach hundreds of beamwidth and the elements of feed array are much less than the regular feed array.