A mobile device includes an antenna structure, a signal source, a tunable circuit element, and a tuner. The antenna structure includes a radiation element. The tunable circuit element is coupled to the radiation element. The antenna structure and the tunable circuit element are disposed in a clearance region of the mobile device. The tuner has a variable impedance value, and is coupled between the tunable circuit element and the signal source. The tuner and the signal source are disposed in a circuit board region of the mobile device.