Tuesday, July 14, 2015

Wi-Fi Backscatter


I have heard over the years, of radio experimenters, hams, and hackers that converted radio waves into electrical power. The concept is not complicated. You receive radio waves and instead of consuming power to modulate that wave and extract a signal, just convert it directly back into DC power. Tesla actually described this in principle over a century ago.

The problem from the get go is that this is a hugely lossy process. In the whole system you have loss to heat conversion, wave dissipation, AC to DC loss, etc etc. This is not a net neutral exchange. But in a broadcast most of the power is wasted. If you imagine a room mostly filled with radio waves, imagine how much of that wave if actually being received by your wifi devices: the laptop, tablet, printer, cell phone, maybe your Roku?  Btu the signals fill whole rooms and serve no devices. That's all loss. If you recover some of that energy, (without losing more in the process) then that's gravy.

A rectenna is a rectifying antenna. It;s most common application is to convert microwave energy into direct current. the designs are usually simple: a dipole antenna (or array of dipole antennas) with a diode connected across the dipole elements. The diode rectifies the AC current induced in the antenna by the microwaves, to produce DC power, which powers a load connected across the diode. Schottky diodes are usually used because they have the lowest voltage drop and highest speed and therefore have the lowest power loss.

It was patented by William C. Brown at Raytheon back in 1964, though development had been ongoing since at least 1958.  Back in the 1970s the development of rectennas inspired scifi writers to imagine solar satellites beaming energy down to earth. The utopian solar power dreams never came to fruition, though the technology ended up in RFID systems. So here we see the same idea being re-purposed with wifi backscatter. Wi-Fi backscatter uses radio frequency signals as a power source and reuses existing Wi-Fi infrastructure to provide Internet connectivity to battery-free devices. It has some advantages over solar satellites in being small and cheap and also very narrow band which makes the antenna much more efficient. More here and here.