Recording/playback apparatus with electric wave transmission

A communication system, comprising:

• a first communication device;
• and
• a second communication device,
• wherein the first communication device comprises an antenna, setting means for setting a resonance frequency of the antenna, detection means for detecting a transmission/reception state of the first communication device, and changing means for changing the resonance frequency of the antenna based on the detection result of the detection means, and
• wherein the detection means of the first communication device transmits an instruction to the second communication device to send predetermined data back to the first communication device, and detects the transmission/reception state of the first communication device based on whether the predetermined data has been received from the second communication device, and
• wherein the resonance frequency of said antenna of the first communication device set by the setting means is previously provided, and wherein the changing means sequentially switches the resonance frequency of said antenna, thereby changing the resonance frequency of said antenna set by the setting means.