| Summary: | This thesis investigates various methods to improve the performances of antennas. For different applications, the antennas are expected to have corresponding performances and characteristics. The efficiency is one of the most important parameters that should be considered, and the losses determine the efficiency directly. For different types of antennas operating in different frequency bands, there exist different factors which influence the performances. For example, in the HF band (3 MHz-30 MHz), the performances depend on the mismatch loss and the size of the antenna. In the microwave frequency band, besides the mismatch loss, the dielectric loss of the substrate material is also one of the most important factors. Whereas in the millimeter wave frequency band, the loss in the feeding network is the factor that limite antenna gain and efficiency when the sub-antennas are all matched well.
Considering various influence factors, different techniques and methods are investigated to improve the performances.
First, the compact and effective ferrites are selected to improve the performances of the HF antennas. Effective methods of using ferrites are proposed to improve the performances of the HF antenna. By analyzing the current in the ground plane of the antenna, the ferrites can be arranged effectively and efficiently at areas with high current density, so the effect of image current is reduced. Moreover, with the ferrite loadings, the bandwidth of antenna is increased significantly, and the size of antenna is decreased.
For the microstrip antennas which work in the microwave band, the widely used high-loss material is the primary factor to influence the efficiency of the antenna. It is a commercial method to apply planar periodic structures in the ground plane or on the patch. When the periodic structures are arranged in the ground plane, the substrate does not perform the properties of the base material. The effective permittivity and permeability of material can be obtained by simulating the unit cell. With the periodic structures in the ground plane, the consumed power in the antenna is reduced, and the bandwidth of the antenna is increased. When the planar structures are arranged periodically on the patch, the periodic structures can work the same as the patch. More energy can be radiated from the slots in the patch. By analyzing the dispersion relation, the resonant frequencies can be derived. With the periodic structures in the patch, the bandwidth of the antenna is increased greatly.
For the millimeter wave antenna, the loss in the feeding network leads to the small number of antenna elements in an array, and the gain of antenna array is limited. A low loss transmission method, substrate integrated waveguide, is proposed to construct the feed network instead of the traditional microstrip line. Moreover, the geometry of the antenna array is simplified. The losses within the antenna array are decreased further. As a result, the gain of the antenna array is increased.
In this thesis, the techniques to improve the antenna performances are proposed over three different frequency ranges. Due to the different frequency ranges, different techniques for antenna optimization are required. The high-performance antennas are simulated, and the simulation results are compared with the measurement results. The antennas demonstrate high performances with good efficiency and small size.
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