Antennas use phase-changing material to alter shapes, frequencies
Two antenna prototypes have been developed using a special class of thin film material which allows them to alter their shape using temperature and radiate at varying frequencies within the GHz range. A single reconfigurable antenna could replace two or more traditional antennas, including those in cell phones, Wi-Fi, and numerous military devices.
The antennas developed at the South Dakota School of Mines & Technology, in collaboration with Michigan State University, were documented in the IEEE Antennas and Wireless Propagation Letters in February 2015. They are made by integrating vanadium dioxide thin films, a type of "phase-change" material, meaning it is an insulator at room temperature and becomes metal when heated above 68 C. The heating-cooling cycle is repeatable and the phase-change is reversible. Principal investigator Dimitris Anagnostou, Ph.D., of the South Dakota School of Mines & Technology, led the research with graduate student Tarron Teeslink, collaborating with Nelson Sepulveda, Ph.D., and student David Torres, from Michigan State University.
Anagnostou, associate professor in the Department of Electrical & Computer Engineering, has been working on reconfigurable and tunable antennas for the past 15 years. Common methods to date have resulted in non-linearities, high losses, expensive fabrication equipment and often complicated biasing mechanisms.
His exploration of vanadium dioxide has shown the material can be used in linear devices, has minimal losses and can be activated using a variety of heat transfer methods.
Linear devices for radio-frequency communications applications involve usually passive components such as antennas and (microwave) filters, as well as resistors, capacitors and inductors.
Often antennas are tuned or reconfigured using non-linear components such as diodes, but these distort the electrical signals, especially over a wide range of frequencies. Vanadium dioxide is a linear material, meaning it affects all radio frequencies by the same amount causing no distortion, and is therefore suitable for narrowband and wideband tuning.
Reconfigurable antennas is a concept that has been studied for years using different mechanisms. A simple example of an application where you use antenna reconfiguration is when you tune the modern radios in your car. For example, when you are listening to FM radio, your antenna is "aiming" forward to a transmission tower. When you switch to satellite radio, your antenna has to "aim" higher in the sky.
The current mechanisms used for achieving this rely on switching between separate antenna systems, and/or involve moving mechanical parts—a large number of switches with complicated wiring—and most of them are capable of reconfiguring a single parameter at a time. Furthermore, most of them allow discrete "on/off" reconfigurability.
The results show a way to achieve analog antenna reconfiguration of multiple antenna parameters in a simple and low-cost manner. Researchers are now able to tune (and program) the frequency, radiation pattern, and even the polarization of the antenna within a continuous range of values. This allows a single antenna to serve many purposes.
Vanadium dioxide was investigated as far back as 1959, when an article described the insulator-to-metal transition at approximately 68 C. Since then, it has been almost neglected, as the need for reconfigurable components, wireless communications and antennas was not as significant, Anagnostou said.
In the past decade vanadium dioxide has received widespread attention from researchers due to its properties for applications spanning from information storage to stronger artificial muscles and missile guidance.
This is the only known success achieving reconfigurability by altering the antenna’s geometry with the special class of material. Several other universities are currently working in the area, indicating the strong scientific interest in this area.
"The novelty lies in obtaining the know-how of the integration and application of the material in antennas in the GHz range. There are still many things to learn. These prototype antennas prove the material is capable for use and should be further investigated," Anagnostou said, adding the material can find application in general antenna and microwave component design but ultimately has the potential for many military uses. "Our ongoing experiments in using the material for cloaking and thermal camouflage are also very encouraging," he said.
The National Science Foundation funded the work over three years as a collaborative proposal between South Dakota Mines and Michigan State University. While Anagnostou and Teeslink did the design and characterization at SD Mines, Michigan State researchers fabricated the antennas at the clean room of the Air Force Office of Scientific Research.
South Dakota School of Mines & Technology