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.

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).