Contributions au controle du faisceau d'antenne reflecteur en utilisant les surfaces selectives en frequences et les reseaux reflecteurs

The antenna systems in satellites are generally based on parabolic reflectors. To cover a single region of the Earth with two distant frequency bands, two different sized antennas are used. The beam control of the aperture antenna would let us use the two different frequencies with the same parabolic antenna system and maintain the same beamwidth for the two frequency bands. This beam control can be done using frequency selective surfaces (FSS) or reflectarrays (RA).;On the other hand, we also present the steps to design a dual-band RA to reflect the signal of a frequency band (at 10 GHz) to the specular direction and to reflect the signal of the other band frequencies (at 15 GHz) to an off-specular direction. The measurements show that the 10 GHz signal is reflected to the specular direction and the main beam at 15 GHz is reflected in the desired off-specular direction. However, at 15 GHz, part of the signal is still reflected in the specular direction. Thereby, we present a thorough study of the specular reflection by a RA designed to reflect to a non-specular direction. Based on the assumption that this phenomenon is given by a periodic error of the phases of the RA elements, we show that when we add a linear periodic error to the desired phases, we get a good agreement between our model and the HFSS simulation. Thus, this simple model allows us to estimate the directions of undesirable sidelobes. In addition we present a study of the magnitude of the specular reflection as a function of the coverage of the RA by metallic patches.;In order to be able to use frequency selective surfaces to cover the aperture of the parabolic antenna, we show the steps to study a dual-band and multi-layer frequency selective surface (at 20 GHz and 30 GHz). By decreasing the cell size by changing the geometry of the elements we have delayed in frequency the appearance of the grating lobes and we avoided distortions in the frequency band of 30 GHz. Thereafter, we chose a two-layer frequency selective surfaces to ensure stability of the bandwidth as function of angle of incidence. We noticed an abrupt variation of the transmission coefficients of the frequency selective surface at about 23 GHz. These uctuations are the effect of the surface waves excited by higher order modes. In this study, we assume that the frequency selective surface is periodic and has an infinite size. This kind of study allows us to take into account the excitation of these surface waves. Furthermore, the excitation of the array guided surface wave can occur also when we study a finite size FSS. The diffraction of these surface waves at the edges of the finite frequency selective surface may modify the radiation pattern of the antenna. Thus, we are presenting a numerical method to analyze large finite size frequency selective surfaces taking into account the array guided surface waves.