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In this paper, we proposed a variable-impedance microstrip circuit using varactor diode as a control device. The proposed variable-impedance transmission line with a simple planar antenna can change the resonant frequency and the input impedance of antenna to achieve the DAM (Direct Antenna Modulation). As an application of the variable-impedance microstrip circuit, we have implemented the ASK modulation of RF signal for the low-power consumption RFID applications. The experimental results show that a patch antenna with a resonant frequency of 2.45GHz and a 30MHz bandwidth can transmit a 1Mbps digital modulated signal with the ultra low-power consumption.