To make the electric characteristic of the antenna variable stable, so as to be tuned to its own radio channel and not tuned to other channels in order to prevent inter/modulation noises through the reception of a radio signal conducted in the vicinity.

A closing circuit 5, made of a copper wire or an iron wire including a nonlinear element 3 such as a diode, is placed in the vicinity of an exciting element 1 which is a major part of the antenna, a bias voltage is applied to the nonlinear element to change a DC resistance of the element and the electric characteristic of the antenna 1 is changed, corresponding to a change in the DC resistance of the nonlinear element 3 such as a diode. Thus, in a system having numerous radio channels, tuning is attained corresponding to respective radio channels in use, resulting in that each tuning is not susceptible to the effects of intermodulation from other radio channels making reception in the vicinity, and good communication becomes then executable.