Sub-wavelength Microwave Helical Resonators

Abstract

The work presented in this thesis is primarily focused on the influence of sub-wavelength metallic helical resonators on the electromagnetic response of metamaterials and other microwave structures. For each of the particular areas studied in this work, a relevant analytical study is provided and simulation data is obtained by means of numerical modelling. Based on these, an analysis is conducted to optimise the structures in terms of required parameters. Finally, the samples are manufactured, experimental results are obtained and possible applications of the study to particular metamaterial based devises are discussed. First, a fresh approach to the design of metamaterials formed from closely spaced helices is developed, which is based on the retrieval of values of electric and magnetic coupling coefficients between adjacent elements from analysis of numerical and experimental data. The coupling coefficients between both right- and left-handed helices in side-by-side and axial arrangements are then evaluated. The dependence of coupling strength and sign on separation distance, relative axial rotation, helical pitch, and the geometrical arrangement of the helices has been quantified both numerically and experimentally and a geometry giving zero net-coupling between very close helices is illustrated. This method and understanding have been further used to design and optimise different metamaterial-based microwave structures. First, it has been used to demonstrate superdirective end-fire radiation in the low GHz frequency range using magnetically-coupled structures of sub-wavelength metallic helices. Novel numerical, experimental, and analytical results are presented for superdirective dimers and trimers that are significantly smaller compared to the current state-of-the-art, and they provide close to theoretical maximum values of directivity without using complex feeding networks. The size, directivity, efficiency, and operational bandwidth of such structures are optimized. The second type of microwave application of helices studied in this work is connected with Artificial Magnetic Conductors (AMCs). An original analytical approach for size and bandwidth optimisation of AMCs constructed of magnetically coupled elements is proposed. Based on this, a novel AMC geometry constructed of magnetically coupled helical elements that is smaller in radiation direction than those previously suggested has been designed and optimised. Several original geometries have been manufactured and studied including a design that can be fabricated using printed circuit board technology. These designs are demonstrated for use at low GHz frequencies over a bandwidth very close to the theoretical limit. The final topic covered in the thesis is that of broadband negative index surface modes. A novel metasurface comprised of capped helices arranged as an hexagonal array that supports a broadband near-isotropic negative-index surface-wave is designed, manufactured, and experimentally characterised. The surface-mode dispersion is studied both numerically and experimentally with an operational band more than double the bandwidth of structures previously reported in the literature. From this, it is shown how one may provide a structured surface that uniquely supports just a negative index surface mode over a wide operational band with no forward-wave being simultaneously excited.