COLUMBUS, Ohio -- Researchers at Ohio State University have taken the first step toward developing adjustable antennas for satellite communications.
Such an antenna could change the shape of its reflector while in orbit to improve signal quality. It could also replace several traditional antennas by delivering a variety of signals for cellular phones, pagers, and global positioning systems.
Scientists had previously found that adjustable reflectors made of plastic were light enough for use in space, but they were too flexible to afford good shape control.
Gregory Washington, assistant professor of mechanical engineering, and Hwan-Sik Yoon, a graduate student, have proven through computer simulations that thin piezoceramic patches spaced around the back of a reflector will reinforce the plastic while controlling its shape. The research appeared in a recent issue of the journal Smart Materials & Structures.
A piezoceramic material is a ceramic that changes shape when a voltage is applied to it, or releases a voltage when its shape is changed manually. Researchers sometimes call these 'smart' materials. "When we attach this piezoceramic material to another surface and it expands, the surface bends. When it contracts, the surface bends the other way. With that movement, we're able to change the overall shape of a structure. In this case, the shape change alters the properties of the reflector or antenna itself," explained Washington.
Satellites normally must move the entire mechanism beneath an antenna to change its direction. "It's as if our eyes could only stare straight forward -- to see other directions we'd have to turn our whole head," said Washington. "But an adjustable antenna can make fine movements on its own, like we do when we look around with our eyes."
When satellites move to adjust the direction of standard antennas, they create inertial forces that throw off the orientation of the entire satellite. But an adjustable antenna with actuators made of plastic and piezoceramic material would be light enough to generate very little inertia.
"That's one of the significant benefits -- the satellite wouldn't have to constantly reorient itself," said Washington. "Plus, instead of having three or four antennas on a satellite, we could have one or two multifunctional antennas."
Washington and Yoon developed a series of lengthy and complex equations to model the movement of the piezoceramic actuators. Then they put those equations into a computer code called POMESH, short for Physical Optics Mesh.
Researchers at the Jet Propulsion Laboratory in Pasadena, CA, previously developed POMESH to model the radiation pattern of rigid antenna reflectors. Washington and Yoon modified the code to suit an antenna that changes shape.
Through computer simulations with POMESH, Washington and Yoon found that the plastic reflector materials must be molded to a particular shape and the actuators must be bonded to the structure at specific temperatures and pressures for the design to work. The researchers have now obtained a series of molds to begin making adjustable antennas. "Nobody has ever tried to build these antennas before, so we're doing everything from the ground up," said Washington.
He and Yoon have also written control algorithms by which these new antennas will be able to self-tune in response to commands from Earth. "Let's say someone wants to find a shape that gives maximum power to a signal in the presence of atmospheric disturbances," said Washington. "An adjustable antenna could configure itself to send out the maximum amount of information."
The ability for antennas to change shape solves certain problems. For instance, sunlight heats antenna materials and warps them; when that happens, adjustable antennas could simply self-correct.
Another common problem: Earth's atmosphere scatters satellite signals the same way water scatters a beam of light. For this reason, not all transmitted information reaches a target. Standard antennas can't correct for that, but an adjustable antenna could even navigate signals through turbulent atmospheric conditions like storms. It could deliver more information using the same amount of power.
The technology may also lead to advanced membrane optics for telescopes and microscopes -- that is, thin and flexible lenses and mirrors that change shape to change focus.
Currently Washington and Yoon are working with Willie Theunissen, an engineering graduate student at the University of Pretoria, South Africa, to continue to modify and develop physical and geometric optics code for advanced antennas. "We're generating new code because we want to be able to choose the right amount of actuators and find out just where to place them for the different shapes we want to obtain," said Washington.
This work was funded by a grant from the National Science Foundation and the U.S. Army Research Office. In the future, the researchers may work with NASA on smart, adjustable antennas for wireless communications networks. Such networks could transmit signals for telephones, televisions, and faxes without the need for cables and phone lines. Washington hopes they will be able to finish building an antenna by March 1999, when the space agency will run a series of field tests.
Journal
Smart Materials and Structures