A radio signal transmitter for an audio source generator includes a reception antenna for receiving a plurality of broadcasting signals, a decision unit coupled to the reception antenna for determining intensities of the plurality of broadcasting signals received by the reception antenna, a selection unit coupled to the decision unit for determining an operating frequency according to a decision result of the decision unit, a modulation unit coupled to the selection unit and the audio source generator for transforming an audio signal provided by the audio source generator into a broadcasting signal according to the operating frequency determined by the selection unit, and a transmission antenna coupled to the modulation unit for emitting the broadcasting signal outputted from the modulation unit in the air.