AbstractsCommunication

This thesis presents the design of three novel small wideband microstrip patch antennas. Microstrip antennas have the attractive characteristics of low profile, light weight and simple structure. However, its bandwidth is limited to a few percent which is not enough for most of the wireless communication systems nowadays. Some bandwidth enhancement techniques have been proposed such as using thick foam substrate, cutting one or two U-slots on the patch, stacking an additional patch on top of the original one and using shorting pins. For some applications such as indoor wireless communications, conventional patch antennas may not be suitable because of its large size especially when operating at lower microwave frequency. Therefore, many methods have been proposed in the literature to tackle this problem. One of the most effective methods is to use a shorting wall. This method can reduce the length of a half wavelength patch by half to a quarter wavelength patch. Based on a combination of these techniques, three novel antenna designs are presented here. Firstly, a folded patch with a shorting wall is studied. This antenna consists of a folded trapezoidal patch with a shorting wall and a shorting pin. By offsetting the feeding probe from the midline of the patch, one more resonance is found. Combining this technique with a two-sectioned feeding probe and a 50% thicker profile, the impedance bandwidth is improved from 29% to 56% (SWR≤2). To understand the relationship between the position of the coaxial feed and the bandwidth, an analysis will be presented. For the two-sectioned feeding probe, a parametric study will be carried out telling how the thickened diameter of the probe helps improves the bandwidth. Secondly, a folded shorted patch with two L-slots cut on the main part is investigated. This design adds the idea of half double U-slots patch into the folded shorted patch and the two L-slots contributes to the two extra resonances found on the SWR curve. Moreover, the distance between the folded part and the main part has to be increased for optimum performance but the height of the main part is kept low. This approach can prevent from the high input inductance caused by the coaxial feed. As a result, a very wide bandwidth of 99% (SWR≤2) and average gain of 6.8dBi are achieved. Finally, a double folding structure is introduced on a folded shorted patch. An additional folded part is introduced at the inner part of the half U-slot and can further enhance the bandwidth to 5:1 or 133% (SWR≤2). At the same time, to assist the impedance matching, a wings-liked wall is used to connect the folded part and the main part. An average gain of 5.4dBi is obtained over the pass-band. To summarize, three small wideband folded patch antennas have been successfully designed. xii, 77 leaves : ill. 30 cm. CityU Call Number: TK7871.67.M5 W655 2008