Design Of The Array Feed For The Satellite Antenna Scanned Electronically Within Limits

To meets the demand for the developments of satellite communication, satellite-borne antennas are required not only to produce spot beams with high quality, but also to have the flexible ability of beam scanning in their service region. Related tightly to a Pre-research project entitled "Multi-Beam and Steerable Spot Beam Antennas for Satellite", this dissertation presents a systematic investigation into the analysis and design of the array feed for the satellite-borne antenna. The author's major contributions on the subject can be summarized as follows:1. First the elements of the array feed are investigated. According to the design requirements such as dual band, receiving and transmitting circular polarization as well as symmetrical pattern in azimuthal angles, the spiral antenna, Archimedes plane spiral antenna and conic horn antenna are studied in detail. Simulations of these antennas are carried out by the electromagnetic simulation software, and the desired electrical performance can be achieved. A low sidelobe single wrap helical antenna is also presented, and its simulated and measured results are provided. These results show that for interested band, the VSWR of the antenna is less than 1.15 and its sidelobe level is less than -25dB. A dual band conical horn antenna is designed, which operates in dual band with high gain and meets the demands of satellite communication.2. Synthesis methods for the feed array, including the nonlinear least square method and the particle swarm optimization method, are investigated. The computational processes of the Gauss-Newton method and Levenberg-Marquardt method are introduced. The search routine and the convergence characteristics of the nonlinear least square method are shown by solving a practical problem. Especially the particle swarm optimization (PSO) algorithm is investigated, and the standard PSO algorithm is modified in some aspects in order to overcome some disadvantages, such as low efficiency and premature convergence. As r result, a new PSO algorithm, named the crossed PSO (CPSO) algorithm, is proposed. Simulation results show that the crossed PSO is superior to other kinds of PSO algorithms and can be used to solve a wide range of problems.3. Feed network of the array antenna is discussed. Techniques of electrically realizing multi-beam, beam isolation, beam-scanning, receiving and transmitting duplex, dual-band dual-circular-polarization are introduced. Main components of the feed network, including dual-band dual-circular polarizer, the orthogonal mode transducer (OMT), and the narrow band filter are presented. The electro-magnetic radiation theory, the microwave network theory, the wave-guide mode theory, filters theory and optimization are used for the design of the feed network. Simulation results show that with this feed network, the dual-band dual-circular-polarization can be achieved. The isolation between these two bands is more than 70dB.4. Synthesis of the array feed is addressed. Several kinds of array structures, including hexagon array and circular array, are introduced, and the CPSO algorithm is used to design such kinds of antenna array. The parameters affecting the array patterns, such as the amplitude and phase of the elements, the spaces between the elements as well as the positions of the elements in the array, are optimized in order to make the array pattern approach the desired pattern on each section of all azimuthal angles. Meanwhile, the dual frequency hexagonal array antenna is optimized. The scanning characteristic of the array feed is also analyzed. The scanning patterns of the array in the required region and the excitation phase of each element are presented. A hexagon antenna array is designed and measured. Measurement results show that for the interested band, the array patterns approach the desired patterns very well. A side-fed offset Cassgrain antenna with the array feed is analyzed. Simulation results show that this antenna system meets the engineering requirements.5. The mutual coupling effect between the elements in the array is analyzed. Patterns of the elements in the array are different because of the mutual coupling effect, which are related to the element locations in the array. The S-parameter matrix and mutual impedance matrix are also calculated. The optimal amplitude and phase of elements are given, and then the incident voltages can be achieved by solving the inverse of the S-parameter matrix, thus the mutual coupling effect can be reduced.6. Tolerance analysis of the array feed is introduced. The components of the system are analyzed by using the perturbation method, and the effects of the equipment precision and the environmental factor on electrical characteristic are reduced as much as possible. The effects of the stochastic and quantification error on the antenna gain, the sidelobe level and the beam direction are then analyzed by the probability theory. The scanning patterns of the feed array under the different quantification error are analyzed, and effects of the digital phase shifter and the attenuator on the array pattern are also discussed.