Zettl explains that the “nanotube does not act as an antenna in the conventional sense.” That is, instead of picking up electromagnetic waves electrically, it picks them up mechanically. This happens because of the nanotube’s natural resonance frequency. As soon as it encounters radio waves that match the frequency, the nanotube starts vibrating in step with the waves, effectively tuning in only to that radio signal. The nanotube’s vibrations change the field emission current, and the mechanical vibrations are converted into an electrical signal. An external battery powers the field emission current and amplifies the radio signal. The field emission is naturally asymmetrical–it allows current to flow only in one direction, just like the diodes and rectifiers used in demodulators. So the nanotube also acts as a demodulator and detects the music encoded onto the carrier wave.
To tune to a different radio station, the researchers change the resonance frequency of the nanotube. They do this by changing the voltage applied across the electrodes. “It’s like tuning a guitar string,” Zettl says. “The electric field pulls on the nanotube.” With the same nanotube, the researchers can cover the entire FM radio band.
Cees Dekker, a nanotube researcher at the Delft University of Technology, in the Netherlands, calls the new radio “an appealing demonstration that very simple devices can be used for everyday [tools].” Whether or not the device is used for sensors remains to be seen, he says, but for now, the simple demonstration is a good start.