Tehnici de transformare a polarizării pentru antene directive / Polarization transformation techniques for directive antennas

In space communications, ensuring the highest possible signal-to-noise ratio is one of the most important aspects considering the long distances at which such radio links are made, but also the low powers of the transmitters located on the space vehicle. For this reason, antennas with high directivity are used in most applications, among which horn antennas are the most common. Another challenge in designing antennas for such applications is to ensure circular polarization, as the upper layers of the atmosphere cause the polarization of radio waves to rotate. The goal of the doctoral thesis is the development of methods that allow changing the polarization of some directional antennas of the pyramidal horn type, without interfering in the excitation system of the antenna. Compared to the solution proposed in most approaches in the specialized literature, which assume the introduction of a polarizer (for example a diaphragm provided with a slot) inside the waveguide that excites the antenna, in this doctoral thesis the possibility of obtaining the rotation polarization by adding multiple layers of frequency selective surfaces in front of the pyramidal horn aperture. The PhD thesis presents an alternative way for transforming antenna polarization, basically by adding multiple frequency selective surface structures in front of the pyramidal horn aperture, in order to rotate the direction of the electric field. The proposed horn antenna with multiple frequency selective surfaces can be used as a feed element for a parabolic reflector antenna for maritime satellite communication systems in the X-band or in the lower part of the Ku-band, or as part of a sensor for finding the direction of arrival of a wave, in order to orientate an antenna system. The concept was successfully validated through simulation and measurements.