A two-way data communications link for traffic applications is currently under development by the Communications Research Group, Newcastle Upon Tyne Polytechnic and the Department of Transport Operations Research Group at Newcastle University. An integral part of the system has been the development of a miniature passive microwave transponder for location within a moving vehicle. Data is transmitted to the transponder by means of polarisation shift keying using two orthogonal circular polarisations. To minimise space requirements a single antenna capable of producing two orthogonal circular polarisations was developed. A proposed model for this antenna together with preliminary theoretical and experimental results are outlined.
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