Antennas often need to trace circles in the sky. But spinning large objects nonstop takes a lot of time and mechanical energy.

One alternative to mechanical motion is using flat planes made up of miniature transmitters that each emit fractions of an overall signal—every fraction varied so that it all adds up to a single linear beam. These antennas, called phase-varied arrays, can also modulate the direction of that overall beam by altering the electronic properties of each individual signal source. However, packing multiple small-scale antennas into one surface adds up to costly and colossal devices, limiting their usefulness.

University of Wisconsin–Madison electrical engineers are working out a new strategy to create antennas that spin their beams in circles while the devices stand still.

“Our approach doesn’t depend on exotic materials that bend the laws of physics,” says Nader Behdad, principal investigator on the project and UW–Madison professor of electrical and computer engineering. “We’ve found a practical way to achieve beam steering that the antennas field has largely overlooked for many years.”

Amin Momeni (l) illuminates the antenna-testing chamber while Nader Behdad installs a phased-array antenna. Image credit: Stephanie Precourt. Rather than building a phased array from numerous individual antennas, the team instead is working to create special reflective surfaces that achieve the same effect, but rely on a single signal source.

Much like the way the curved reflector in a car’s headlamp concentrates light emanating spherically outward from a single bulb into a forward beam, these flat arrays focus microwave signals into directed columns by altering the electronic properties of individual elements on their surfaces. But unlike mirrored dishes, these devices can vary the direction of the reflected beams by tuning individual elements on the surface.

Achieving that tuning, however, is no easy task. Behdad tried numerous complicated approaches to modulate every component before realizing he did not need to control each element one by one. Instead the team harnessed small-scale mechanical motion within the entire antenna itself by making tiny adjustments to one large component, called the ground plane, that sits below the entire structure.

“Luckily for us, in order to do beam steering, we really don’t need to individually tune each element,” says Behdad. “All we need to do is create a gradient, and we can do that by simply tilting the ground plane on one corner a little bit down and the other a little bit up.”

To test the feasibility of this approach, the group made a low-cost prototype, which successfully provided proof of concept of electromagnetic principles. Now, the team is working to identify appropriate materials and techniques to improve the concept, making it suitable for real-world applications.