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Title: Design and measurement of metamaterials based antennas
Authors: Ullah, Sadiq
Keywords: Metamaterials
Leopardi algorithm
Equi-area method
Equi-angle method
Rotation invariant
Maximum directivity
Polarisation agile
Issue Date: 2010
Publisher: © Sadiq Ullah
Abstract: The research work presented in this thesis consists of two main disciplines of antenna engineering. These are antenna radiation pattern measurement and design of metamaterial based Electromagnetic Bandgap (EBG) surfaces. In the first part of the research, an equi-area based sphere partitioning technique (based on the Leopardi algorithm) is used to get the coordinates of the sampling points. The radiation intensity pattern of the antenna is sampled using the equi-area coordinates for various sets of the sampling points. A simple formula which avoids weighted multiplications is introduced to determine the total radiation power of the antenna under consideration and hence the maximum directivity. The novel equi-area method incorporating the Leopardi algorithm and the traditional equi-angle method are compared w. r. t the analytical method. It is found that the equi-area method is superior to equi-angle method in terms of accuracy and computational complexity for the same number of sampling points. Furthermore, for the equi-angle method, the values of directivity deviate notably from the analytical value in the case of a beam-scanning (rotating) antenna pattern. On the other hand, the novel equi-area method demonstrates a relatively rotation invariant behaviour for a large number of sampling points so that the maximum directivity remains relatively stable for a given antenna with a known radiation pattern for all rotation angles. The term Rotation invariant used in this thesis means that the difference between maximum directivity (calculated using the numerical method) and the reference value (i.e. maximum directivity calculated using analytical method) as a function of rotation angle are negligible for very large number of sampling points such as 5000. The two methods are also quantified in terms of number of data points, and the worst case deviations in maximum directivity are computed and compared for different scan angles. It is demonstrated that the equi-area method require less number of data points as compared to the equi-angle method to calculate the directivity, particularly when the antenna radiation pattern is beam-scanned. The main reason for the nearly rotation-invariant characteristic of the equi-area method is the uniformly populated distribution of the sampling points on the measurement sphere. The numerical patterns of three standard antennas, such as dipole, broad side array and patch are used in the analysis. The second part of the research is focussed on the design of a novel 3.25 GHz, Polarisation Dependent Electromagnetic Bandgap (PDEBG) surface. The novel surface incorporates an inclined sheet via and can be used as a ground plane for low-profile, polarisation-agile antenna applications. FR4 is used as a substrate. The detailed parametric analysis of the surface is carried out in Microstripes. The surface is also illuminated with both linear and circularly polarised plane waves and it is found that polarisation conversion can be obtained at certain frequencies. Furthermore the outgoing CP (Circularly Polarised) waves conserve their sense. In other words the novel surface gives circular polarisation reflection wih the same sense, i.e. RHCP-RHCP (Right Handed CP) or LHCP-LHCP (Left Handed CP) within a specific band (Bandgap). The PDEBG surface is compared in performance with the traditional Perfect Electric Conductor (PEC) and mushroom type EBG ground planes. The traditional ground planes reverse the sense of the reflected CP waves. A prototype of the novel PDEBG surface is manufactured. A dipole antenna at 3.25 GHz with a bazooka balun is used to realise a low-profile single port CP antenna backed by the PDEBG surface. To summarise, this research is focused on the design of a novel planar PDEBG surface and the novel equi-area radiation pattern sampling technique, which is applied to traditional planar antennas and can be used in metamaterials (EBG) based antennas.
Description: Closed access. If you are the author of this thesis and would like to make it available on open access please contact the library. A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
URI: https://dspace.lboro.ac.uk/2134/7123
Appears in Collections:Closed Access PhD Theses (Electronic, Electrical and Systems Engineering)

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