Antennas often need to scan in circles across the sky. For example, radar arrays on an air traffic control tower will rotate to broadcast their signals in all directions.
The non-stop movement of a large, spinning object requires a lot of time and mechanical energy, which is why engineers have long sought methods for improving radar beam reorientation, such as being able to scan from a stationary position.
Progress in this direction has been slow and existing technology is too expensive to be widely used. However, electrical engineers at the University of Wisconsin-Madison are developing a new strategy to create antennas that can spin their beams in circles while the devices themselves stand still.
The research is supported by a $1.1-million grant from the US Office of Naval Research and presents engineering students at the university with great opportunities to explore their education in radar, defense and communications fields.
“In defense situations, you need to detect incoming objects or see where you are going very quickly,” said John Booske, a UW–Madison electrical and computer engineering professor and co-principal investigator of the research project. “The ability of a mechanical rig to move a big, heavy parabolic dish back and forth limits how quickly you can respond to potential threats.”
Improved beam-steering technology would also benefit other long-range detection and communications systems.
“Our approach doesn’t depend on exotic materials that bend the laws of physics,” explained 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.”
Alternatives to Mechanical Motion
One common alternative to the mechanical motion of an antenna system is to use a collection of flat planes made up of miniature transmitters, each of which emit fractions of an overall signal. Every fraction is varied, so that it all adds up to a single linear beam.
This type of antenna is called a phase-varied array. They are also able to modulate the direction of the overall beam by altering the electronic properties of each individual signal source.
However, packing multiple small-scale antennas into one surface in this manner adds up to costly devices, severely limiting their usefulness.
But the UWM engineers are showing the ingenuity that university researchers are known for and instead of the classic phased array antenna, they are working to create special reflective surfaces that can achieve the same effect while relying only on a single signal source.
Much like how car headlights concentrate light from a single bulb into a forward beam using curved reflectors, these flat reflective arrays focus microwave signals into directed columns. However, unlike mirrored dishes, these devices can vary the direction of the reflected beams by tuning the individual elements on the surface.
This tuning is complicated, however. Eventually the team realized that they did not need to control each element, but could instead harness small-scale mechanical motion within the antenna by making tiny adjustments to a single component called the ground plane.
“Luckily for us, in order to do beam-steering, we really don’t need to individually tune each element,” said 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.”
These small tilting motions inside an overall flat plane require much less time and mechanical force than spinning an entire reflector dish. To test the feasibility of their approach, the group made a low-cost prototype which successfully provided a proof of concept. Now, the team is working to identify appropriate materials and techniques to improve this concept, making it suitable for real-world applications.
For more information, check out the University of Wisconsin-Madison College of Engineering.