In this thesis, we had studied several kinds of the ground planes beside a single arm spiral antenna that is a broadband antenna to remove the back lobes and obtain unidirectional antennas with a good radiation patterns. Simple, light weight and broadband antennas are required for some applications. First type of the ground planes is the conventional perfect electric ground plane beside a spiral antenna that cannot maintain the broadband antenna.Considering a cone as a ground plane, it has a more impedance bandwidth than Perfect Electric Conductor (PEC), but it cannot achieve low profile antenna and cannot suppress the surface wave.Second type is Artificial Magnetic Conductors (AMC) as a ground plane for a spiral antenna that can maintain the impedance broadband for a low profile antenna and has a band gap to suppress the surface wave, and so,its radiation patterns are good and smooth.There are several structures of AMC. One of them is a mushroom structure. A mushroom structure ground plane is one of the applications of metamaterials that exhibit specific electromagnetic properties. It can achieve a low profile antenna with a wide impedance bandwidth while suppressing the troublesome surface waves that can’t be achieved by a conventional perfect electric conductor (PEC). For a mushroom structure,each element consists of a conducting patch above a conventional ground with a via connecting them. This structure has been successfully used, but these numerous vias make the fabrication difficult. Therefore, we investigate alternative structures that are easier to fabricate. Specifically,we studied partially planar structures as ground planes for broadband antennas. The proposed alternative EBG ground planes maintain the III bandwidth enhancement and low profile advantages of the conventionalstructure, with a reduced number of vias for the partially planar structure.Three cases for the positions of vias had been studied. First case is where vias are near from the edge of an antenna. Second case is where vias are near from the center of an antenna. Last case is where vias are near from the edge and the center of an antenna.Other AMC types include square patch frequency selective surface(FSS) and square loop FSS structures. These types had been also studied to show the characteristics of mushroom structures. The square patch ground plane without vias can maintain bandwidth, but it cannot suppress surface waves like a mushroom structure. Last type of AMC is the square loop structure that has the least sensitivity to the incident angle, it has a good impedance and return loss bandwidth, but it cannot suppress surface waves. So, its radiation patterns aren’t smooth.The proposed antenna configurations are investigated using Ansoft Designer and Ansoft high frequency structure simulator (HFSS) that are simulation programs. |
