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Researchers at UT Austin have built an antenna that breaks time-reversal symmetry. Part of the device is a conventional antenna, but voltage controlled capacitors embedded in transmission lines on the antenna can be forced into oscillation by a weak oscillating voltage, which apparently breaks the symmetry sufficiently that transmission can be 50 times stronger than reception. Could be useful for broadcast-only beacons, but thought to be potentially even more interesting in other parts of the EM spectrum. Effective photovoltaic cells suffer from being good emitters if they are good receivers thanks to symmetry. Breaking symmetry could make them even more efficient at receiving.
It may help to speed up data transfer for some UDP based applications because UDP is a "connectionless" protocol. In a connectionless communication, the recipient doesn't need to acknowledge each packet it received.
Video streaming, voice over IP, and online gaming are some UDP based applications.
I was looking at cognitive radio and beam shaping a few years ago. That seems a whole lot more useful - for instance, adapting a Wi-Fi hotspot pattern to cover your house but not your neighbor's across the road.
This seems like a lot of work to attenuate the return path. I'd be curious to see if it is less power to deal with the antenna itself in this way or with amplifiers in the transmit and receive paths.
Beacons are broadcast-only by design; the smartphone does the heavy lifting of RSSI. Might also be applications in defense to reduce jamming potential. I can see more applications in a near-far context where maybe the transmit gain was reduced and the receive gain kept up, like in a mesh network where nodes are closer together than the maximum allowable range. It would have to be more efficient than existing alternatives.
Bernard: The original paper by Hadad, Soric, and Alu was published March 29th in the Proceedings of the National academy of science (PNAS). The authors acknowledged that antenna reciprocity is already broken with various nonlinear devices (e.g circulators) but state that "this leads to undesirable signal distortion and a power-dependent response". I agree with the authors that their work may be the first in which antenna reciprocity was broken with a nonlinear device that did not include a magnet. However, they used voltage-dependent capacitors--which are nonlinear and I would anticipate that this would also cause "undesirable signal distortion and a power-dependent response" which is what they are trying to avoid. I would suggest that they measure the signal distortion and power-dependence of the response with their device and compare these effects with conventional devices (e.g circulators) to determine if they have made a practical contribution. Otherwise their contribution is only providing a new means to accomplish what others have done since 1948 (B.D.H. Tellegan).
