Researchers at the Air Force Research Laboratory, working with Northeastern University, recently developed an ultra-compact antenna that uses a new approach in transmitting and receiving signals. This breakthrough could be a big step toward miniaturizing many military and commercial communication systems.
Typically antennas rely on size to work effectively in the electromagnetic spectrum. If the antenna is not long enough to resonate at the proper frequency, the antenna will not be able to efficiently transmit or receive the desired electromagnetic waves. Although strides have been made over the years in antenna miniaturization—with cellphones being a prime example—the quality of antennas still degrade as they become smaller. That’s why cellular carriers must place large numbers of cellular antenna towers to ensure consumers have adequate phone reception.
“We identified ultra-compact antennas as the critical last step in true device miniaturization,” says Brandon Howe, AFRL materials scientist. “Researchers had successfully shrunk most electronic components, but the true miniaturization of antennas was still a missing piece.”
The size of an efficient miniature antenna is typically about 10% of the received wavelength, whereas the ultra-compact AFRL antennas are as small as fractions of a percent of the wavelength. As a result, microwave antennas that were previously approximately a half-inch can now be reduced to an object smaller than a flea (less than one millimeter). Although not an immediate replacement for small antennas, this miniaturization could be a critical step toward incorporating antennas into applications for which they were previously impractical.
These ultra-compact antennas represent a new approach. Instead of using an electrically-conductive material to sense the electric field of microwaves, these antennas use special insulating materials, called “multiferroic composites.” These materials are composed of magnetostrictive materials which convert magnetism to strain, and piezoelectric materials which convert strain to voltages. Using multiferroic composites allows ultra-compact antennas to function by sensing the magnetic field of microwaves.
“We miniaturized the antennas by borrowing a trick from acoustic filters in cellphones, which convert microwave voltages to strain waves,” says AFRL materials scientist Michael McConney. “Strain waves travel much slower than the speed of light, and this lets us shrink the wavelengths while keeping the frequency the same, thus allowing us to make the antennas much smaller.”
By coating conventional bulk acoustic wave filters with a magnetic material, these slower strain waves can be converted into radiation, which lets them break the inefficient scaling laws associated with shrinking typical antennas to very small sizes.
This new approach let AFRL and Northeastern University researchers reduce the size of an antenna by over 90%, dramatically changing their potential design constraints. This new design lets antennas retain much more of their functionality compared to traditional antennas scaled down to the same size. This development could result in smaller devices, including wearable antennas, bio-implantable and bio-injectable antennas, smartphones, and wireless communication systems.
“The miniaturization of military electronics is of significant benefit to the warfighter, not only in terms of device size, but in transportability, space requirements, weight, and many factors,” says Howe. “It will let us fit more into a given space, whether in a field pack or on an aerial platform. It gives us greater capability in a smaller space.”
The team plans to continue its research by working toward matching the ferromagnetic resonance to the acoustic (strain) resonance, as well as by using a new low-loss, highly-sensitive magnetic material the group has pioneered. By doing so, the researchers hope to further enhance antenna efficiencies.