University of Texas researcher Andrea Alù, best known for research into a Harry Potter-style “invisibility cloak,” appears to have found a way to enhance radio antennae by disguising them from the very signals they send out.
Alù and his colleagues say they have used deception to thwart “reciprocity,” a phenomenon that causes a radio signal to be disrupted when it bounces back or because another signal on the same frequency hits the antenna during transmission.
Reciprocity can limit the quality and range of transmissions. For example, Alù said, it interferes with cellphones in crowded places.
In such situations, “There will be a lot of signals coming back to the antenna … (that) fundamentally limit the quality of transmission,” said Alù, an Italian-born engineering professor who specializes in metamaterials, or materials engineered to have properties not found in nature.
Alù’s research into invisibility involves manipulating the behavior of light waves. Similarly, the antenna work of Alù and his postdoctoral fellows Yakir Hadad and Jason Soric manipulates radio waves. Their work will likely take years to become widely used commercially. They envision it eventually making many antennas — including those in cell phones and other wireless devices — more powerful and efficient.
To understand how it could help make radios more powerful, start with this concept: A home has snow piled up against the door. If the people living in the home want to get outside, they have to open the door. When they do, some snow could fall inward and block some of them from getting out.
Reciprocity basically works that way, although there is a bit more to it. When a radio sends out a signal, it opens itself up to receiving signals on that frequency. Radio transmissions often bounce back by hitting obstacles such as clouds, mountains or buildings. Sometimes there are other signals traveling on that frequency. When a radio has “opened the door,” it also opens the door for waves on that frequency to enter. That dynamic can disrupt the outgoing waves. This disruption diminishes the quality of the signal.
In some situations, reciprocity is particularly likely.
“Think of a stadium full of people trying to transmit and receive” through cell phones, Alù said.
There are workarounds, he said, such as “isolators” behind the transmitter that can limit the effects of reciprocity. But those technological add-ons tend to be bulky and expensive. Reciprocity also creates a particular difficulty: The better an antenna transmits in a specific direction, the more likely it is to receive a signal, as well. The better it works, the more reciprocity occurs.
Alù and his colleagues have worked on various solutions. The newest one foils reciprocity by basically confusing the incoming signal, according to a peer-reviewed paper to be published in the journal Proceedings of the National Academy of Sciences this month.
Alù and his colleagues paired the outgoing signal with a second one, which is effectively “modulating” (constantly rotating through frequencies). Unlike the radio signal that is being sent out, the second one moves up and down the antenna itself, sort of like a snake coiled around the antenna. A reciprocal signal coming back to the antenna essentially encounters a different one than the one that was transmitting. Reciprocity appears to be disrupted.
Or, put another way, the snow is essentially blocked from even getting to the door. And the signals, Alù said, can make it out.