Broadcast Energy Transmitter

Yes. that's a GI Joe reference. (Pop culture reference explained here.) The point is that the idea of sending energy through the air has been done before. It's what Tesla was working on this in his Shoreham Long Island lab starting back in 1905... until JP Morgan cut him off. It's based around a resonant transformer, same as those used in high power radio systems everywhere. More here. To put it bluntly, Tesla was as much a mad man as mad scientist. I've written of him before [Here].

I bring this up because in recent years a few companies have made less crazy, and more believable attempts in this arena.  In 2007 Powercast actually did come demos at CES (Consumer Electronic Association) with some small scale equipment that actually did work. The idea is that they are broadcasting over a small area, and that RF energy instead of just rectifying the signal, it's "harvested" to charge a battery, for example. Now as an engineer I know that small transmitters output bupkis. The RF field around a 1 or 2 watt transmitter is just a couple mV/m. But Powercast isn't imagining this to be a high amperage product, they're just suggesting that they can use it to continually charge a battery via their special receivers. That's at least in the realm of reality. Allow me to quote directly from their own website:

"Powercast is a leading innovator of wireless power technology based on RF energy harvesting. Micro-power can be provided over distance using the same common radio waves that are widely used by many devices including mobile phones, cordless phones, etc. Batteries can be recharged remotely and without wires, or devices can be made completely battery-free. Primary use includes wireless sensors for commercial, industrial, and defense applications."

Their  CEO explained further, "Our solution is, if talk time (on a cell phone) is 5 1/2 hours, by trickle-charging at work, now talk time is 10 hours because the battery never gets to dead..."  It's that word trickle-charging that tells me how this is working. It's not that kooky.  I can imagine a wireless keyboard, a webcam, or just LEDs, running off a system like this, but not a stereo, or a power drill. This is strictly for low amperage devices and batteries.

Another mroe recent one is the Rectenna.The word "Rectenna" starts with the wrong kind of prefix for this crowd. Interestingly both rectum and rectify have related word roots. Rectify  comes from from the old French rectifier meaning  "to make straight."  That comes form the Latin rectificare meaning "make right" from the earlier Latin rectus meaning straight.  The word rectum is from  the Latin intestinum rectum meaning "straight intestine, which again comes from the Latin regere "to straighten."  The word Rectenna is actually a portmanteau of the two words rectifying and antenna. What it does is convert radio waves into electricity (ostensibly.) More here.

My suspicion is that most of these actually operate via induction. There is no actual radio signal, just an electromagnetic field. Induction is the production of voltage in a conductor which is moving through a magnetic field. But the conductor doesn't have to move, Faraday's Law states that a varying electromagnetic field will induce an electric current. In other words, you can vary the field to the same effect and leave the conductor stationary. The theory dates to 1831, and for the record induction is DEFINITELY public domain.  The problem is that to induce a strong (i.e. large) em field you consume much mroe energy than you receive. It's very lossy. But it's all still sci-fi fun. You can watch their dog-and-pony show here: