Enhancement of ISM band RFID system performance with varactor-loaded compact RFID tag antenna

Abstract

A novel method is presented for electrically tuning the frequency of a compact RFID tag antenna. A tuning circuit, comprising an RF switch and discrete passive components, has been completely integrated into the antenna element, which is thus free of dc wires. The low-profile printed antennas are fabricated together with the layouts of its DC control circuits and other RF/base-band circuit footprints. A surface mounted (SMT) varactor is applied as a frequency-tuning element at proper places of the meandered slots in compact antenna. Prototype designs show the in-band frequency (return loss is <10 dB) is tunable from 2.4 GHz to 2.8 GHz when the bias voltage varies from 4V to 7 V. The impact of the tuning circuit on the antenna's return loss and radiation pattern have been experimentally studied through comparison with the performance of a reference antenna not incorporating the tuning circuit. The proposed frequency tuning concept can be extended to more complex compact structures as well as other types of antennas to give enhanced electrical performance.