A polarization- and power-agile interrogator antenna for UHF RFID systems

We present an active interrogator antenna for UHF RFID (radio-frequency identification) systems which is capable of transmitting electromagnetic radiation at any desired polarization and with adjustable transmit power. We use a dual-polarized patch antenna originally radiating linear polarization in +45° and -45° direction and feed each input with signals that can be adjusted in both phase and amplitude by two vector modulators. The vector modulators are controlled by a microprocessor that provides an RS-232 interface. The antenna also comprises power amplifiers to achieve a maximum transmit power of 2 W at any desired polarization mode. For testing we perform measurements that determine the radiation pattern and polarization mode of a commercially available transponder and find very good agreement with simulation results.