| With the rapid development of radio technology,the electromagnetic environment of communication,radar,navigation and other electronic information equipment is increasingly complicated.Polarization,as an important characteristic of electromagnetic wave,is essential to its regulation.Polarization converter is a device which can rotate the azimuth of linearly polarized electromagnetic wave by a certain angle after transmission or reflection.The most commonly used polarization converter is the cross-polarization converter,which can convert the electromagnetic wave of a certain polarization into its orthogonal polarization mode.However,traditional polarization converters pay little attention to the filtering characteristics and the arbitrary polarization azimuth of the incident wave.Traditional polarization converters can only achieve cross-polarization conversion,but how to improve the selectivity of passband and achieve cross-polarization conversion of arbitrary polarization azimuth incidence remains to be studied,which plays a crucial role in the use of antennas and other equipment,and is also a difficulty in the current research in this field.In order to improve the frequency spectrum utilization,avoid the increase of device complexity and reduce the large amount of signal loss between transmitting and receiving antennas,in this thesis,aiming at the above two problems of existing polarization converters,two polarization converters with different characteristics are designed and implemented respectively.The main research contents of this thesis are as follows:1.A frequency selective polarization conversion metasurface(FSPCM)is designed.FSPCM is a new concept device which combines planar antenna and planar spatial filter.It is composed of a receiving antenna,a non-radiative filter coupler and a radiating antenna.The filter function is further realized on the basis of the cross-polarization converter,and the thickness of the device is greatly reduced by the planar circuit structure,and the resonance order can be controlled by simply adjusting the element.Under the excitation of x-and y-polarized waves,the structure can achieve two resonant frequency peaks in4.89-5.12 GHz passband,and the polarization conversion rate in the whole passband is greater than 99.5%.The selectivity at the lower and upper band-edge is 52 d B/GHz and 31d B/GHz respectively,and it has 30°angular stability.The overall thickness of the cell is about 0.028λ_c.Subsequently,the third-order resonant FSPCM was further adjusted.Due to the increase of coupling between the resonators,a transmission zero was introduced on the right side of the passband,so it has a sharper band-edge selectivity of 94 d B/GHz and 192d B/GHz,respectively.And the polarization conversion rate of the structure is greater than97%in the 4.85-5.12 GHz passband range.2.A transmission dual-frequency polarization-insensitive cross-polarization converter is designed,which also consists of a receiving antenna,a non-radiation filter coupler and a radiation antenna.The dual-frequency characteristics are achieved by grooving the single resonant patches and adding branches,while the polarization insensitivity is achieved by the patches in both x-and y-directions.It can realize cross-polarization conversion in4.58-4.69 GHz and 5.33-5.41 GHz dual-band,and is insensitive to the azimuth of polarization of incident electromagnetic wave.In other words,the polarization converter can rotate the polarization azimuth 90°for the incoming wave of any polarization azimuth angle after transmission,and the polarization conversion rate is greater than 90%in the two passbands,and up to 95%in the resonant peaks. |
