Analysis and design of the twisted loop antenna topology for mobile communications
The handset product has been styled in successive years to reach more compact sizes and there has as a result been a reduction in volume available to house antennas; therefore size/performance trade-offs have had to become accommodated. Some of the issues antenna engineers are currently confronted with include; frequency shifting due to the antenna not being isolated from the handset, far field pattern deformation due to close proximity effects from the energy absorbing human tissues, distortion caused by noise from electronic components that share the handheld platform. What is required is antenna technology, which maintains a high enough performance despite the escalating restrictions imposed by the demands of the market. Research is performed on a twisted loop antenna topology that possesses an integral balun as part of its structure. Two rudimentary designs are utilised in the research, a simple bifilar structure that can be adapted for GSM, peN, Bluetooth and W-LAN applications, and a quadrifilar helix structure for use in GPS. Both structures are based on existing industrial dielectricloaded antenna structures but are modelled as novel air-loaded structures using a commercially available Method of Moments (MoM) electromagnetic simulator. In this fashion, the antennas could be generated quickly with low computational requirements. A parametric study is performed on the bifilar antenna structure to gain an enhanced understanding of the twisted loop topology. Once this understanding is achieved proposed modifications to the structure are implemented to improve the performance of the antenna. The main subject of improvement is the broadening of bandwidth as normally dielectric-loaded antennas have inherent narrow bandwidth. Any improvements made on the air-loaded structures could be tested on dielectric structures in future research. The most successful novel approach attempted to increase the bandwidth in the twisted loop structure was the insertion of parasitic helices to create a coupled multi-pole filter response. In conjunction with the work performed on the bifilar, an air-loaded GPS quadrifilar helix antenna was also modelled. A method for inducing circular polarisation is proposed and then by the insertion of parasitics into the quadrifilar helix design a novel dual-band dual-polarised antenna is presented. Finally measurements are made to demonstrate the advantageous properties the dielectric-loaded GPS antenna has over conventional GPS antennas.