Antennas receive information by resonating with EM waves, which they convert into electrical voltage. For such resonance to occur, a traditional antenna’s length must roughly match the wavelength of the EM wave it receives, meaning that the antenna must be relatively big. However, like a guitar string, an antenna can also resonate with acoustic waves. The new antennas take advantage of this fact. They will pick up EM waves of a given frequency if its size matches the wavelength of the much shorter acoustic waves of the same frequency. That means that that for any given signal frequency, the antennas can be much smaller.
The trick is, of course, to quickly turn the incoming EM waves into acoustic waves. To do that, the two-part antenna employs a thin sheet of a so-called piezomagnetic material, which expands and contracts when exposed to a magnetic field. If it’s the right size and shape, the sheet efficiently converts the incoming EM wave to acoustic vibrations. That piezomagnetic material is then attached to a piezoelectric material, which converts the vibrations to an oscillating electrical voltage. When the antenna sends out a signal, information travels in the reverse direction, from electrical voltage to vibrations to EM waves. The biggest challenge, Sun says, was finding the right piezomagnetic material—he settled on a combination of iron, gallium, and boron—and then producing it at high quality.
The team created two kinds of acoustic antennas. One has a circular membrane, which works for frequencies in the gigahertz range, including those for WiFi. The other has a rectangular membrane, suitable for megahertz frequencies used for TV and radio. Each is less than a millimeter across, and both can be manufactured together on a single chip. When researchers tested one of the antennas in a specially insulated room, they found that compared to a conventional ring antenna of the same size, it sent and received 2.5 gigahertz signals about 100,000 times more efficiently, they report today in Nature Communications.