JPH0347639B

PURPOSE:To use a frequency channel effectively and to eliminate a failure at radio interference, by informing a vacant channel detected at a reception unit of a command receiver of a command transmitter to a remote station and transferring a channel at radio interference automatically. CONSTITUTION:For example, electromagnetic waves fg from the command transmitter are normal and electromagnetic waves fc from the command receiver are interferred, the transmitter changes over channels of the reception unit 10 and a vacant channel is detected. The detection of the vacant channel is done with a squelch output from the unit 10. The squelch is set on when the strength of a reception electric field is on or below a level (no electromagnetic wave interferring with the reception frequency exists). The channel selection is stopped when the squelch output is set on and the channel information of the fc and the switching command are irradiated on the waves fg. The receiver changes over the channel of the fc based on the command from the transmitter.