| In modern society,the demand for wireless communications is increasing and as an important part of the communication systems,high-performance antennas are highly desirable.For applications such as satellite communications,radar,and deep space detection,the high-gain,wideband,and low-profile antennas have become a popular research topic and draw considerable attention in both academia and industries.The transmitarray antenna is a new type of antenna combining the advantages of a planar microstrip array antenna and a lens antenna.It has appealing features such as high-gain,simple feeding,low-mass,and mature fabrication process.However,its narrowband operation and relatively high-profile structure are still challenging issues to be tacked.In this thesis,various novel types of elements are proposed and studied for low-profile wideband transmitarray antennas.By optimizing the key geometric parameters of the elements,good performances in terms of transmission magnitude and phase are achieved.Based on the novel elements,the transmitarrays are designed and developed.The measurement results show that the proposed transmitarrays can realize a wideband operation with a low-profile structure.The main contents of this paper are summarized as follows:1.A low-profile transmitarray antenna employing square patches with cross dipole slots and vias is proposed at X-band.Different from traditional transmitarray antennas with multi-layer frequency-selective surface type elements that feature a thick anenna profile,in this work,a novel transmitarray antenna consisting of only a single-layer substrate with dual metallic layers connected by vias is presented.The thickness of the element is 3 mm,corresponding to only 0.1λ at the design frequency.Metallic layers contain square patches embedded with cross dipole slots.By varying the length of the square patch and adjusting the dimensions of cross dipole slots as well as the locations of the vias,a phase shift of 340o is obtained.A transmitarray adopting the novel elements is simulated,constructed,and measured.The experimental results show that the antenna can realize a 9% 1-d B gain bandwidth with a 49% aperture efficiency.More importantly,compared with the multi-layer frequency-selective surface-type transmitarray antennas whose thickness usually exceeds 0.5λ,the profile of the designed transmitarray is significantly reduced.2.A novel single-layer element is proposed at 20 GHz for low-profile wideband transmitarray antennas.The unit cell is formed by dual layers of cross rings and dipoles etched on the substrate and connected through metallic vias.The location and amount of vias as well as the dipole lengths are varied to realize a reasonable transmission phase swing and magnitude.Based on the novel unit cell,a transmitarray comprising 441 elements is designed and developed.The measured results show that the proposed trnasmitarray can achieve a reasonable behavior.3.Transmitarray antennas featuring only metallic structures are designed at 10 GHz.Firstly,metal-only quadruple-layer frequency-selective surface-type transmitarray antenna using double square ring slot elements is designed.Secondly,by employing only dual metallic layers with two connecting vias,a low-profile transmitarray design with wideband performance is proposed.Finally,the metal-only low-profile transmitarray unit cell is designed to feature dual-functionality with one polarization incidence for transmission and phase changing and the other orthogonal polarization incidence for reflection.Based on the ray tracing principle,a folded transmitarray antenna comprising the novel unit cells and polarization rotated reflection surface is presented.The proposed folded designcan reduce the antenna profile to one-third of the traditional design and still maintain reasonable radiation characteristics. |
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