Abstract
This thesis presents and develops the intelligent quadrifilar helix antenna (I-QHA) as one possible solution to the problem of antennas for hand-held terminals in third generation communication systems. The research has examined many different aspects of the I-QHA along with its implementation in order to highlight some of the key benefits of optimum control. The development of third generation mobile communication systems leads to a more integrated approach to mobile communications. A user expects to have a single terminal that will operate in every conceivable environment, encompassing in particular, both terrestrial and satellite based communication systems. The antenna is a major part of the challenge for an integrated terminal for all systems. It must operate in different frequency bands with very different radiation characteristics in each band. The increase in data rate required in third generation systems requires significantly larger signal-to-noise-plus-interference ratios to maintain suitable bit error rates. The main theme running throughout this thesis is the use of variable weight components applied to the four ports of the I-QHA in order to adjust the antenna characteristics. In addition, methods of changing the frequency of operation are also examined. The effects of the variable weight components have been examined in the following areas: Investigations, via simulation of increasing the up-link gain in a given direction by steering the main lobe of the radiated power to that direction. Significant gain enhancement towards the horizon and in the back-fire direction is seen, leading to a mean gain improvement in the ICO system of almost 3dB. Investigations of the diversity gain available, utilising both simulated and measured signals in areas of diffraction and reflection from buildings or trees. Diversity gains of up to 14dB were seen in areas of high shadowing. Theoretical analysis linking the values of the complex weights to the input impedance of the I-QHA element and potential methods for overcoming impedance matches.