của sự tương tác giữa các thẻ trên hiệu suất tổng thể bởi vì tần số cộng hưởng tổng thể của hai thẻ liền kề trực tiếp với nhau là luôn luôn thấp hơn so với tần số cộng hưởng của một thẻ duy nhất [2]. Tần số cộng hưởng của mạch cộng hưởng song song có thể được tính bằng cách sử dụng phương trình của Thomson: f = 1 √ 2 L - C (3,2) | 74 RFID Tag Antennas Microchip a Figure a Typical inductive coupling tag and b its equivalent circuit. of the interaction between tags on the overall performance because the overall resonant frequency of the two tags directly adjacent to one another is always lower than the resonant frequency of a single tag 2 . The resonant frequency of the parallel resonant circuit can be calculated using the Thomson s equation f -----. IkJlc Inductance For tag antenna design with a prior selected microchip with an internal capacitance Cr the task is to configure a coil antenna to resonate at the operating frequency. If Cp is known compared to Cr Cp is normally very small in the HF band so C Cr the required inductance of the coil antenna is given by L f The coil antenna is usually structured on a substrate typically made of polyethylene terephthalate PET polyvinyl chloride PVC or polyamide and consists of wound wire or Design Considerations 75 etched copper aluminum strips. Conductive polymeric thick film pastes can also be used for the coil antenna by screen printing or dispensing for cost reduction. Etched or screen printed coil antennas are suitable for HF systems because low inductance is required. There are many types of inductors that can be used to realize the required inductance and the spiral inductor is widely used in HF RFID systems. A typical spiral inductor is illustrated in Figure . The conductive strip is wound either clockwise or counterclockwise. This configuration ensures that the current in adjacent tracks is in phase. The resulting mutual inductance yields a significant increase in the spiral inductor s self-inductance. Connecting the microchip to the open ends of the spiral antenna forms a tag. The microchip can be directly connected to the inner and outer ends of the spiral inductor. It is more convenient to use an underpass to connect the end of the outer turn to the centre for microchip assembly if more windings are required for
