Metamaterials were made famous a few years back by opening
the door to invisibility
cloaks. Now scientists from the National Institute of Standards and
Technology , the University of Arizona (Tucson) and Boeing Research &
Technology (Seattle) have repurposed metamaterials to create a
"Z-antenna" design that is 50 times smaller than today's antennas—enabling
a cell phone antenna to be shrunk small enough to fit on a finger ring. The
researchers hope their novel antenna designs will enable pint-sized emergency
communications devices, ultra-small sensors and portable ground-penetrating
radar devices that can find underground tunnels, caverns and similar
geophysical features.
Z-antennas
designed by the National Institute of Science and Technology (NIST) are 50 times
smaller than today's antennas thanks to metamaterials.
Metamaterials use composites of conductors and insulators
(called dielectric elements) arranged in free-air structures that react to electromagnetic
signals in the opposite way to normal materials. For instance, to craft an
invisibility cloak for microwave frequencies, split-ring resonators are
periodically arranged in free space in a manner that bends microwaves in the
opposite direction from normal, essentially creating a material with a negative
index-of-refraction.
Metamaterials were invented by John Pendry, a physicist at Imperial
College in London,
who predicted composite negative index of refraction materials, which were
subsequently fabricated at a cell phone's microwave frequencies by Duke
University scientists David Schurig
and David Smith. The electrical and magnetic properties of the inhomogeneous
composites produced by Schurig and Smith demonstrated that a metamaterial's
negative index of refraction could redirect microwaves to flow around an object
undistorted, essentially cloaking an area from view by neither reflecting nor
casting a shadow.
Now NIST, Boeing and the University
of Arizona have shown that
transmission antennas can similarly harness metamaterials to enable their
antenna designs to radiate as if they were as much as 50 times larger—theoretically
capable of shrinking a full-size 150-mm cell phone antenna down to just 3 mm.
In tests at 300MHz, which would ordinarily require a meter-size antenna, the
researchers were able to demonstrate almost 95 percent of full-size performance
from a Z-shaped antenna just 30 mm square.
"The problem with antennas that are very small compared
to their wavelength is that most of the signal just gets reflected back to the
source," says NIST engineer Christopher Holloway. "The metamaterial
makes the antenna behave as if it were much larger than it really is because
the antenna structure stores energy and re-radiates it."
Conventional antenna designs add matching networks of
resistors, capacitors and inductors to boost efficiency, but using
metamaterials instead enables smaller, more frequency-agile tuning networks
than are possible with conventional designs.
Next, the researchers plan to field-test real antennas using
their metamaterial-powered designs, with funding from the Defense Advanced
Research Projects Agency (DARPA).
