In the last two decades, wireless communication systems have successfully penetrated the commercial market. This is confirmed by the increased number and usage of mobile phones, PDAs (Personal Digital Assistant) with internet access, wireless access laptops, hand-held GPS (Global Positioning System) devices and wireless LAN (Local Area Network). Along with present and new services they deliver, these types of communication systems are expected to grow even further. In anticipation of this growth, research is being conducted to increase their capacity. A smart antenna (an array antenna with a suitable signal processing algorithm) that uses a directive beam to mitigate interference and multi-path signal propagation is envisaged to achieve this task.
The work presented in this thesis involves the design of a smart antenna system for indoor wireless LAN applications. The array antenna, which is investigated here, is of switched beam type. It is formed by a circular array of N + 1 monopoles with one central monopole being active and N peripheral monopoles being passive that are supported by a circular ground plane. By varying the loads at the peripheral passive elements this array is able to steer its radiation beam in azimuth direction.
In order to design this antenna system, an electromagnetic simulation method called Radial Guide Field Matching Method (RGFMM) is developed. The formulation of this method is derived in the thesis project. The one advantage that RGFMM offers is speed of analysing the behaviour of a monopole array, which eases the design process of this antenna system. By using RGFMM, the monopoles parameters are optimised so that the central active monopole has an input impedance of 50ft The development of the RF switch circuits which enable the load varying of the peripheral monopoles completes the switched beam array antenna design. This switched-beam circular array antenna is manufactured and tested experimentally. The measured results show good performance in terms of return loss bandwidth and radiation patterns.
This thesis also looks at the effect of finite ground plane on the performance of this array of monopoles. It is known that when an antenna on finite ground plane starts radiating, the induced surface current and the diffraction from the ground plane edge interfere with the primary radiation pattern. Recently, an Electromagnetic Bandgap (EBG) structure has been introduced to reduce the undesirable effects of a surface current induced by primary radiator in a supporting ground plane. In this work, a structure called Ringed Ground Plane (RGP) is introduced to create a simple EBG. By utilising RGP, it is shown that less back radiation, lower levels of sidelobes and more forward gain can be obtained for an array of monopoles supported by a finite conducting ground.
The considered circular array of monopoles is only able to steer its beam in the azimuthal plane. However, its radiation pattern in the elevation plane is fixed, which may be restrictive in some applications. This shortfall can be overcome by housing this array antenna system in a radial guide with a suitably shaped guide-to-free-space transition. Using this structure, the impedance bandwidth of the active monopole can be widened and an extra degree of freedom of modifying the radiation pattern in the elevation plane can be achieved. In the work undertaken here two types of transitions, one of radial-horn type and the other one with a dielectric taper, are investigated using a theoretical and experimental approach.
A smart antenna system operation is controlled by a suitable algorithm. As this thesis employs a switched beam system, simple software is able to do the job. The required algorithm has to activate the beam in the direction which provides the strongest signal strength. However, when the base station tracking a mobile unit performs the switching action the receiver encounters a problem. At that time, the signal reception is interrupted. To combat this problem, channel prediction software is investigated and developed. The investigated channel prediction algorithm is capable of estimating the signal when deep fades are about to occur. During these periods, the antenna system can execute its switching operation to determine the pattern with the strongest signal strength.
The work completed as part of this thesis has resulted in several publications in internationally recognised journals and refereed conference proceedings. This considerably high acceptance rate supports the originality and significance of the accomplished research.