| ETC (Electronic Toll Collection) System is an intelligent electronictransportation system for automatic toll collection without parking based on moderncommunication technology, electronic technology, computers and networktechnology. This system realizes vehicle automatic identification and data exchangethrough dedicated short communication between roadside antennas andvehicle-mounted electronic tags.In recent years, the technique of electronically beam-scanned antennas has beenwidely developed, and it has a broad application prospect in radar systems andmobile communication. Applying electronically beam-scanned antennas to ETCSystems solves the problems of car-following disturbance and narrowcommunication region in current fixed-beam ETC Systems. Microstrip leaky-waveantennas, with the superior features of low profile, simple structure, easy formatching and easy to realize electronic beam-scanning, are widely studied and used.Two electronic beam-scanning circularly-polarized microstrip leaky-wave antennasbased on the theory of CRLH TL are proposed for ETC Systems in this paper.The first proposed antenna is a microwave leaky-wave antenna with patchesloaded on one side of the transmission line. The proposed antenna realizesright-handed circularly-polarized radiation through the corner-cut square patches.Series varactors between patch units and shunt varactors between units and theground are loaded to the antenna to realize CRLH structure, and when the bias DCvoltage is varied, the main beam of the antenna scans between0°and-21°at thefixed frequency of5.8GHz. This antenna, though only scans within half plane, canbe applied widely in practical, for its characteristics of high gain, high radiationefficiency, simple bias circuit and steady performance for circularly-polarizedradiation. To achieve beam-scanning in both directions, a feeding networkcomposed of single-pole double-throw switch is designed to connect tworight-handed circularly-polarized antennas, which respectively scans to twodirections. By controlling the on-off direction of the switch, beam-scanningdirection can be selected.The second one is a CRLH-TL microstrip leaky-wave antenna. Thetransmission line of conventional leaky-wave antenna is divided into19periodicsquare units, two corners of which are cut off to realize circularly-polarizedradiation, and varactors are loaded between units and between units and the groundto form CRLH structure. When the bias DC voltage of the varactors increases, themain beam of the antenna scans from-20°to+35°, totally55°, which covers both the two half planes. This antenna has the advantage of scanning to both directions,but it also has the disadvantages of low radiation efficiency and difficult forfabrication. |
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