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An adaptive antenna impedance matching system for mobile communication terminals is proposed. The system adaptively controls two varactors of the pi-network matching circuit detecting the change in the refection coefficient between the antenna combined with the matching circuit and the RF front end of a transceiver. This system consists mainly of analog circuits and employs only a simple algorithm for convergence, therefore does not need any complicated mathematical formulation for modeling of the system itself as well as its nonlinear control elements. Also the proposed system utilizes only simple digital circuits for timing generation. The performance of the adaptive impedance matching system is investigated by simulation. The input impedance of the antenna is alternated assuming the change of the environment due to a human head or a hand, and etc. In addition, a better performance is obtained in terms of the input power to the antenna by holding the adaptive control when the reflection is smaller than a certain threshold value. As a result, it is found that the input power to the antenna for transmission can be increased by about 2dB per second compared with a fixed capacitance matching system.