Increased bandwidth microstrip antennas for road tolling applications

Evans, Huw (2004) Increased bandwidth microstrip antennas for road tolling applications. Doctoral thesis, University of Northumbria at Newcastle.

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Abstract

This thesis provides the results of research that has been carried out in order to develop the microstrip patch antennas required by a high speed digital communication
link for road traffic applications. The operating frequency is 5.8GHz.
Although a single compact broad band radiating element is developed for use within the vehicle, the main focus of the research is increasing the frequency bandwidth of the antenna arrays used within the road side unit of such a system. This is achieved by investigation into radiating element design, the orientation of the radiating elements within the array and the arrays feed networks geometry. A single layer circularly polarised dual feed microstrip patch antenna is optimised for use within the array. Using a genetic algorithm to aid design of the dual feed network, the input impedance at the dual feed point is increased beyond what can normally be achieved when a tradition design process is adopted. This results in a more compact structure that allows thinner tracks to be used in the arrays feed network.
These radiating elements are sequentially rotated to form the array. The traditional corporate feed network is replaced by a series feed network and the effect this has upon the performance of a two, three and four element array is investigated. The four element array is suited to a vehicle access control application; due to the complexity of the feed network an adaptation of simulated annealing is required to both generate and optimise the series feed line sections of this array. Not only does the new series feeding method result in doubling the VSWR 2:1 bandwidth of the four element array to over 10%, it also produces a more compact structure. Both mathematical modelling and experimental measurement are used to confirm the performance of these new arrays.
The four element array is then sequentially rotated to form a larger sixteen element array, that adopts the same series feeding method, suitable for motorway tolling applications. This new array demonstrates a VSWR 2:1 bandwidth of 14.7% and a 3dB axial ratio bandwidth of 12.4%, an improvement factor of two over the same array using a traditional corporate feed. The side lobes of this larger array are reduced by modifying the power distribution within the series feed network.
A single compact circularly polarised wide band microstrip patch antenna for use within the in-vehicle equipment is developed using an adaptation of the cavity model. The radiating patch is excited by a single feed line via a cross slot aperture. Air is used for the antenna substrate, increasing frequency bandwidth while simplifying the fabrication process hence reducing the manufacturing costs of the in-vehicle equipment. The inclusion of the wide band radiating element within the series fed four element array is also investigated. This new structure results in a VSWR 2:1 bandwidth of at least 22.8% and 3 dB axial ratio bandwidth of 17.5%, a significant improvement over traditional designs.
As a result of this research papers have been published in the lEE Electronic Letters (2000), the lEE Electronic Letters (2003) and Microwave and Optical Technology Letters (2003). Papers have been accepted and presentations have been made at the Ansoft microwave workshop in London (2001) and Los Angeles (2001). This work has also contributed towards publications in the lEE Electronic Letters (2000) and in the lEE Transactions on Antenna and Propagation (2001).
The antenna arrays developed during the course of this research have been used within a commercially viable traffic management system and installed at a large shopping centre. It is pleasing to note that the fruits of this research have been used in a 'real life' situation.

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