Friday, 6 February 2009

How Much Signal Can They Harvest?

Here is a power-harvesting idea that is, I will say, a bit dodgy.  In their hurry to perfect this, the engineers in question have actually crossed the line into criminal activity.  Why do I say this?  Well, read on:

In the late 60's to early 70's, one clever guy realised that radio waves are energy being pumped around the sky for free.  In those days there were pretty much only AM transmitters or TV transmitters.  TV was too high frequency to work with, so this guy worked with a local AM radio transmitter's frequency.  He worked out the dimensions of his yard, and how many turns around his yard would couple to the frequency of the transmitter, to the tune of a couple of wavelengths.

This was just ordinary fence wire, but on insulators, and with a clever and ingenious series of contacts at his front gate so that if you closed the gate, there was the circuit, if you opened it, the circuit was open and not likely to electrocute you.

He then hooked up his tuned coil antenna to his house's lighting, using fluorescent tubes wherever he could.  Result?  His house was lighted (for him) for free!  The radio waves were well and truly enough at the size of coil and the number of turns that he had, to light everything beautifully.

But like everything else, there's a price to pay.  That energy had to come from somewhere, and that somewhere was the radio transmitter.  The amount of power drawn by the guy's loop was enough to seriously reduce the signal strength everywhere else, with all that power going to his lights (and the inefficiency of running gear designed for 50Hz at some ten times higher frequency) the radio station suddenly found that most of its listeners couldn't get a signal any more.

A bit of driving around with a signal strength meter soon established the direction in which the signal took its biggest dive, and the chap was summonsed and actually charged with the theft of electricity and ordered to cease and desist.

What I'm saying is - these devices may be only miniscule consumers of the radiated energy - but if you consider that many companies are just looking for technology to power and deploy sensor networks of many tens of thousands of devices - and there are hundreds of manufacturers each with a different sensor network in mind - this drain will slowly and imperceptibly reduce the range of the transmitters these devices are "feeding" from, as more and more devices are deployed.  The companies transmitting will either have to increase transmission power, or else issue "cease and desist" orders for each of those devices or the operators of them.

In effect, these engineers are repeating that 50 year old theft of electricity scheme, just not with one huge drain but rather with a lot of tiny drains.  We can already see how much many small drains in the form of "vampire power" cost us on our electric bill - now imagine tens of thousands of them attached to a radio or TV station's power bill...

And this will not only affect TV stations and RFID readers - it's not explicitly stated in that article, but the technology will just as happily harvest energy from your WiFi access point, from your portable phone and your mobile phone, your wireless-enabled laptop or PDA - in short, from anything that radiates radio waves in the right frequency range.  And that's of course not the worst of it.  Given that these devices are already tuned to your frequency, as it were, they can also forward your transmission itself using swarm network technology, and allow someone at quite a distance from you to eavesdrop on those signals, store them, and decode/decrypt them...

So this is, on the face of it, quite clever and useful technology.  But it has a very serious downside that could cause extremely bad repercussions to systems around them, and is technically illegal.



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