Engineers have figured out how to make antennas for wireless communication 100 times smaller than their current size, an advance that could lead to tiny brain implants, micro–medical devices, or phones you can wear on your finger. The brain implants in particular are “like science fiction,” says study author Nian Sun, an electrical engineer and materials scientist at Northeastern University in Boston. But that hasn’t stopped him from trying to make them a reality.
The new mini-antennas play off the difference between electromagnetic (EM) waves, such as light and radio waves, and acoustic waves, such as sound and inaudible vibrations. EM waves are fluctuations in an electromagnetic field, and they travel at light speed—an astounding 300,000,000 meters per second. Acoustic waves are the jiggling of matter, and they travel at the much slower speed of sound—in a solid, typically a few thousand meters per second. So, at any given frequency, an EM wave has a much longer wavelength than an acoustic wave.
The antennas are expected to have sizes comparable to the acoustic wavelength, thus leading to orders of magnitude reduced antenna size compared to state-of-the-art compact antennas. These miniaturized ME antennas have drastically enhanced antenna gain at small size owing to the acoustically actuated ME effect based receiving/transmitting mechanisms at RF frequencies. We note that the demonstrated ME antennas are pure passive devices, no impedance matching circuit, or an external power source was used during the measurement. And its maximum achievable bandwidth is within Chu–Harrington limit.
The trick is 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.
“This work has brought the original concept one big step closer to reality,” says Y. Ethan Wang, an electrical engineer at the University of California, Los Angeles, who helped develop the idea, but did not work on the new study. Rudy Diaz, an electrical engineer at Arizona State University in Tempe, likes the concept and execution, but he suspects that in a consumer device or inside the body the antennas will give off too much heat because of their high energy density. Wang notes that the acoustic antennas are tricky to manufacture, and in many cases larger conventional antennas will do just fine.
Sun is pursuing practical applications. Tiny antennas could reduce the size of cellphones, shrink satellites, connect tiny objects to the so-called internet of things, or be swallowed or implanted for medical monitoring or personal identification. He’s shrinking kilohertz-frequency antennas—good for communicating through the ground or water—from cables thousands of meters long to palm-sized devices. Such antennas could link people on Earth’s surface to submarines or miners. With a neurosurgeon at Massachusetts General Hospital, he’s also creating brain implants for reading or controlling neural activity—helpful for diagnosing and treating people with epilepsy, or eventually for building those sci-fi brain-computer interfaces.
In conclusion, they have demonstrated ME antennas based on NPR and FBAR structures with an acoustically actuated receiving and transmitting mechanism, which are one to two orders of magnitude smaller than state-of-the-art compact antennas. These ME antennas are designed to have different modes of vibration for realizing both VHF (60 MHz) and UHF (2.525 GHz) operation frequencies. Moreover, both NPR and FBAR based antennas can be fabricated on the same Si wafer with the same microfabrication process, which allows for the integration of broadband ME antenna arrays from tens of MHz (NPR with large W) to tens of GHz (FBAR with thinner AlN thickness) on one chip by the geometric design of device resonant bodies (Supplementary Note 6). A bank of multi-frequency MEMS resonators can be connected to a CMOS oscillator circuit for the realization of reconfigurable antennas45. These ultra-compact ME antennas are expected to have great impacts on our future antennas and communication systems for internet of things, wearable antennas, bio-implantable and bio-injectable antennas, smart phones, wireless communication systems, etc.
State-of-the-art compact antennas rely on electromagnetic wave resonance, which leads to antenna sizes that are comparable to the electromagnetic wavelength. As a result, antennas typically have a size greater than one-tenth of the wavelength, and further miniaturization of antennas has been an open challenge for decades. Here we report on acoustically actuated nanomechanical magnetoelectric (ME) antennas with a suspended ferromagnetic/piezoelectric thin-film heterostructure. These ME antennas receive and transmit electromagnetic waves through the ME effect at their acoustic resonance frequencies. The bulk acoustic waves in ME antennas stimulate magnetization oscillations of the ferromagnetic thin film, which results in the radiation of electromagnetic waves. Vice versa, these antennas sense the magnetic fields of electromagnetic waves, giving a piezoelectric voltage output. The ME antennas (with sizes as small as one-thousandth of a wavelength) demonstrates 1–2 orders of magnitude miniaturization over state-of-the-art compact antennas without performance degradation. These ME antennas have potential implications for portable wireless communication systems.