Alex Zettl at the University of California at Berkeley has invented an interesting radio antenna made from a single conductive carbon nanotube (less than a micron long and ten nanometers wide) positioned between two conductive plates. He has used the antenna to receive songs transmitted by radio, and has posted the results for your listening pleasure. There is a gap between one plate and a free end of the nanotube, across which electrons tunnel. When a voltage is placed across the two plates, the nanotube's free end becomes electrically charged oppositely from the nearby plate, and the electrostatic attraction keeps the nanotube under mechanical tension.
The nanotube's electrically charged free end moves in response to an ambient radio frequency electric field. This changes the gap size, and therefore the measured tunneling current across the gap, just as with a scanning tunneling microscope. The resonant frequency of the antenna is simply the mechanical resonant frequency of the nanotube under tension. The tension can be changed by changing the voltage across the two conducting plates, and in this way the radio can be tuned. The bandwidth of the antenna is determined by the nanotube's stiffness, and (I think) would depend primarily on the length of the nanotube. The space between the two plates should be a vacuum so the nanotube can move freely, and so that Brownian motion does not detune the radio.
The value of a radio antenna this size is that one can communicate with and control nanorobots, for instance in the human body. One could use these nanorobots for diagnostics, reading out blood chemistry or information about various kinds of cell damage, and could send them instructions to intervene.
There are lots of interesting things happening in the area of nanofabrication, such as Andrew Turberfield's tetrahedra discussed in the previous posting. Presently such things are "controlled" by adding solutions of different DNA sequences to the liquid the structure is sitting in, and the new sequence interacts mechanically with the structure to alter it, by binding selectively with some part of the structure already in place. But each step takes tens of minutes as molecules diffuse through water and position themselves to bind correctly. A signal received by a radio antenna might make things happen much quicker.