| Passive UHF RFID technology has been deployed in various industries including supply chain management, cargo tracking, ID badges and health care. These commercially used RFID tags are usually made up of a dipole antenna with a modified T match. These tags are expected to be physically small, function efficiently over a wide bandwidth, and be inexpensive. However, large values of Q factor for physically small antennas yield a narrow bandwidth. Therefore, a different approach to maximize the bandwidth for physically small antennas is necessary and is attempted to be presented in this thesis.;A circuit based analysis and synthesis of a UHF RFID tag is already known, where a circuit equivalent of the RFID tag (which is made up of a dipole antenna, matching circuit, and a chip) is modeled as a two-stage bandpass filter. From basic filter theory, the ratio of source and load resistance that is required to acquire maximum bandwidth of operation is determined. Here, Q factors of the antenna and the load are assumed to be approximately equal. Next, the technique of designing the matching circuit to get the above determined ratio of resistances is discussed.;In this thesis, we extend the above model to a circuit with antenna Q (≈ 50) significantly greater than that of the load (Q ≈ 10). The mismatch between the Q of the antenna and that of the IC is then analyzed to get the maximum bandwidth condition. When the ratio of the Q factors of antenna and load is approximately 2.1, it is found that the maximum bandwidth is acquired . Since decreasing antenna Q is not possible, we have to increase the Q of the load to get maximum bandwidth. Hence, in addition to designing the matching circuit to get the proper ratio of resistance, a parallel plate capacitor is added in parallel to the IC to increase the Q of the load. To validate and quantify the tag with and without the external capacitor, the circuits are designed and modeled using a finite element method simulator. The designs are then fabricated and tested by a reader in a partial anechoic environment. One dB bandwidth of over 30 MHz (covering entire band between 902 - 930 MHz) is obtained using this method, whereas for 'naive' tags with antenna Q = 50, the bandwidth is less than 1 MHz. Analytical, simulated, and experimental results are then furnished and compared.;Thus, a design technique to obtain a wide bandwidth of operation for a physically small dipole antenna using an external capacitor is discussed and the design is tested for verification. |
