Development and Implementation of RFID Technology 14 2.1 Antennas in the RFID system According to the different functions in the RFID system, the RFID antennas can be divided into two classes: the tag antenna and the reader antenna. The tag antenna not only transmits the wave carrying the information stored in the tag, but also needs to catch the wave from the reader to supply energy for the tag operation. Since the tag should be attached to the identified object, the size of the tag must be

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... ll enough, and the antenna should be small in size. In most cases, the tag antenna should have omnidirectional radiation or hemispherical coverage. Generally the impedance of the tag chip is not 50 ohm, and the antenna should realize the conjugate match with the tag chip directly, in order to supply the maximum power to the tag chip. In common applications, the tag antenna should be lowcost and easy to fabricate for mass production. The reader antenna transmits the electromagnetic energy to activate or awaken the tag, realizes the data transfer and sends the instructions to the tag. Meanwhile, the reader antenna receives information from the tag. Generally the position or the orientation of the identified object is random, and the manner for attaching the tag to the identified object is unfixed. Thus the reader antenna should be a circularly polarized antenna, in order to avoid the polarization loss when the orientation of the identified object is changed. Meanwhile, the reader antenna should have low profile and realize miniaturization, some of which should operate at more than one band. In some special cases, multiple antenna technology or smart antenna arrays for beam scanning will be employed. In passive RFID system, the energy for maintaining the tag operation comes from the electromagnetic wave transmitted by the reader antenna. Here the passive system is mainly discussed to show the impact of the antenna parameters on the system performance (Keskilammi, Sydanheimo & Kivikoski, 2003). To double the reading range, the transmitted power, the antenna gain, or the sensitivity of the receiver should increase at least 12dB. First, the impact of the antenna gain on thte system performance is described. When the transmitted power is fixed, the maximum reading range of the RFID system is mainly limited by the antenna gain and the operating frequency. By the RF link analysis, the electromagnetic wave transmitted by the reader antenna radiates to the tag through the space loss, and then reversely propagates back to the reader, carrying the information stored in the tag. Suppose that the RF energy caught by the tag can be re-radiated into the space totally. Let the power transmitted by the reader be reader transmitted P , and the gain of the reader antenna be reader G . The power density at distance R where the tag is placed can be expressed as 1 2