back to article Boffins harvest TV, mobile signals for BATTERY-FREE comms

Radio boffins from the University of Washington have created tags and readers which reflect and feed off ambient radio frequency energy for communications - without needing a power source. The team calls the technology "ambient backscatter" and reckons it could connect up the much-heralded Internet of Things without either …


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  1. Frederic Bloggs

    And is illegal in the UK

    As case law has (more than once) defined this as "stealing electricity". As in the celebrated case of a a farmer using fluorescent tubes with some wire attached to the ends to light his cowshed. He was in the near field of a some large (IIRC BBC) transmitter.

    1. Anonymous Coward
      Anonymous Coward

      Re: And is illegal in the UK

      So all Crystal sets are also illegal then?


      It needs no battery or power source and runs on the power received from radio waves by a long wire antenna.

      Back on topic. When reading the post I kept thinking of Crystal Sets. I made my own radio in the 1960's. So what is really new about this tech? Re-inventing the wheel a little maybe?

      1. Mage Silver badge

        Re: And is illegal in the UK

        Actually the article was quite clear it's not new. The "new" bit is that one unit detects the signal variation by the other unit connecting its aerial and thus reducing local signal. Laws of Thermodynamics. No free lunch.

        If you have an appliance making interference on only one frequency and it can't be shielded, then a series tuned L C circuit with its inductor coil well coupled will significantly reduce the radiated interference.

        1. frank ly

          Re: And is illegal in the UK

          The near field of a transmitter contains stored energy that forms part of the operation of the transmitter and tapping into this certainly constitutes 'stealing'. When the radiated field has been 'launched' and you tap some power from it, you are not affecting the transmitter in any way. If you created a large enough signal shadow to affect people who wanted to receive the signal, I'm not sure what the legal situation would be.

          1. Anonymous Coward
            Anonymous Coward

            Re: And is illegal in the UK

            "If you created a large enough signal shadow to affect people who wanted to receive the signal, I'm not sure what the legal situation would be."

            of course, you'd have to find a legal team that understand physics first, so we'll be safe for a few years yet. - although I suppose various patent attorneys in lots of cases have shown that understanding basic science is not a requirement for issuing legally binding orders / patents.

          2. Loyal Commenter Silver badge

            Re: And is illegal in the UK

            I did hear one anecdote, years back, where BBC technicians were trying to track down the reason for a large radio shadow on one transmitter. This turned out to be someone who lived close to the mast using the power received through a large aerial in his attic to heat his domestic hot water. I believe the gentleman in question was prosecuted for causing radio interference, although the story may very well be apocryphal.

      2. Charles Manning

        The Mystery Box Crystal Set

        I too cut my teeth on crystal sets in the 1960s. Built my first one before I could even read properly. The hardest part was counting up to 45 when winding the main coil.

        A bit later I built a thing called a Mystery Box. It was basically a crystal set that was wired to a uA meter instead of headphones. This made the meter needle move with no battery or other power. Confused the hell out of many people.

        In the early 1970s I remember writing a science "paper" on using crystal sets to harvest power to run things. Being a bit overconfident about the Future Of Science, I predicted it would power cars some day and the science teacher (English + History major) thought I was a genius. It was a few years later that I understood things a bit better.

    2. LazyLazyman

      Re: And is illegal in the UK

      If you can find the name or more details of that case I would love to read why. I assume there must be more too it than "stealing electricity", presumably something to do with it causing interference. Unfortunately all I can find when searching for the obvious terms is lots of psudo science bollacks about the health effects of radio waves and florescent lights (WiFi gives me migraines type of stuff).

    3. Mage Silver badge

      Re: And is illegal in the UK

      Depends on the scale. Microwatts is not stealing as a larger aerial in strong signal area might almost achieve it. Milliwatts or more is questionable and difficult. Actual Watts is harder still unless you are near a LW, SW, MW mast and is certainly theft.

    4. peter_dtm

      Re: And is illegal in the UK

      1 Crystal sets - are legal for receiving broadcast stations - as they are nothing other than a 'tuned reciever' doing what they are meant to to do and do not leach (meaningful amounts of) power from the air - check out how the crystal detector works to see why.

      2 Using an array of aerials and devices to obtain electric current from radio transmissions is illegal - it is all there in the various Radio Acts & ITU treaties

      You are only licenced to recieve broadcast radio signals (yes; the government 'holds' your receiving licence for you; even today; even in this day and age) - note that it does not matter WHAT is being broadcast (AM; FM TV DAB etc). Use of radio recieving apparatus for any other purpose (unless explicitly licenced eg CB PMR HAM) is illegal.

      Using radio signals to generate electrical power is theft. In the '60s a guy near Rugby Radio was done for the following :

      Illegal unlicenced reception of radio signals - confiscation of ALL radio recieivers/equipemnt and associated bits and pieces - which incuded lights and a couple of low power items he had connected up -

      theft of electricity from the GPO; MoD (Navy) BBC (domestic & world service) - massive fine plus repay for the estimated amount of electricity stolen (some 5 years woth). Oh & the assumption was he was using the maximum amount of calculated power generated 24x7 i- it was only a few 100 watts - but it did add up !

      Defrauding the MEB (Midlands Electricity board) of the money he SHOULD have paid for the electricity stollen - for which he was fined AND had to pay back the estimated amount

      and of course he also ended up with a criminal record

      Leaching USEFULL amounts of power from a radio signal (ambient radio environment) causes holes in the radiation pattern

      Oh - placing a fluorescent tube WITH NOTHING attached such that it emits visible light in a near field situation is NOT theft - but nigh on impossible more than a few meters away from a (very high power) transmitting aerial

    5. Jess--

      Re: And is illegal in the UK

      That would probably have been somewhere around borough hill in Daventry when the bbc world service used to transmit from there.

      it used to be hell to avoid listening to it for about 5 miles in any direction at 6pm when they cranked the transmitters up to full for the news

      stereo amplifier (no tuner) check

      gas cooker check

      false teeth check

      dogs chain check

      barbed wire fence check

      BT Telephone check

      You used to hear the news in english (with about 4 other languages in the background) on any / all of the above, especially if you lived on the southbrook estate (parts of which were well within 500m of some of the masts)

      given the field strength in that area I would think that a nice resonant length of wire would easily light a tube or 10

    6. hplasm

      Re: And is illegal in the UK

      There was a case* of someone in a house under a powerline using a coil in the loft to get free power. It caused a noticeable effect in the Power Co monitoring systems, and also in an overhead thundercloud that blew his house up before TPTB could tack him down...

      * May be a bit Urban Legendy...

      However the Power Co systems are quite able to spot unexpected drain and standing wave anomalies etc, so who knows?

    7. Anonymous Coward
      Anonymous Coward

      Re: And is illegal in the UK

      Just in my office alone there must be hundreds of bits wire or metal that are having tiny currents induced in them from a myriad of RF external/internal sources. And not only that, the entire building will be shadowing lots of RF transmissions from various sources.

      So if this is really 'stealing electricity' it is already going on at an epic scale already - and we've all been guilty since radio was invented. Same goes for RF shadowing.

    8. RobHib

      Re: And is illegal in the UK

      "As in the celebrated case of a a farmer using fluorescent tubes with some wire attached to the ends to light his cowshed. He was in the near field of a some large (IIRC BBC) transmitter."

      You're right but I was told during a lecture that it was in the U.S. and it was signals from the ultra long wave submarine service (in the 10s of kHz). Perhaps, that's just another case.

      Anyway, the net result was that it's considered stealing electricity (by longstanding case law). Seriously, this is a potential problem:

      (a) with enough RX antennas absorbing energy, the effective service area will be reduced, and;

      (b) increasing the TX power to overcome the increased 'absorption' will lead to excessive and necessary power levels, which, in the extreme, will increase the radio spectrum noise floor. (Increasing noise floor in spectrum management is already a significant issue.)

      This getting-power-for-nothing idea has been around for some considerable time. Except for induction charging batteries (a la electric toothbrushes etc.) it's not been very effective. Always guaranteeing sufficient Volts/m to power devices is a problem as signal strength can fluctuate wildly. Whilst signal strength fluctuations are unlikely to cause problems to the the RF link (with AGC, limiting etc.), that cannot be said for devices which have to absorb power from surrounding RF to work.

      These devices aren't absorbing low frequency stuff, rather UHF (from the antennas). UHF, of course, is much more prone to nodes and anti-nodes thus more unpredictable/unreliable than the ultra low freq. case to which I referred.

  2. Mage Silver badge

    Never be practical.

    These kinds of gadgets almost go back to 1921. You need a strong signal. Perhaps it would work at 2m range for 15% of the population.

    Clue, what % per country can run TV without freezing or pixelation on a set top aerial?

    Harvesting the local user's Mobile modem, phone or Home Plug transmission will work better than receiving outdoor transmission. Actually some plug top PSUs /Chargers (Wall Warts I think to North Americans) have so much interference they are almost usable as "wireless" chargers at close range. Maybe the cable with plug is for the "charging mat" :-D

    1. Charles Manning

      Re: Never be practical.

      Works fine though if you live near those lovely old transmitters that belted out many hundreds of kW of AM if any of them still exist.

  3. JimmyPage Silver badge

    Legality ...

    IANAL but I am sure the word missing in all of the discussion was "authorised". It's authorisation which defines the legal/illegal nature of the act.

    Taking electricity you have paid for ? Authorised.

    Receiving radio waves intended for entertainment broadcast ? Authorised.

    Inducting electricity without payment[1] ? Not authorised - jail time.

    So on the face of it, receiving radio waves to *power* equipment is outside of the implied license granted by the broadcaster that their transmission was only intended to be used for reception in playing a show. However, that said, laws should be practical, and trying to prosecute - especially when any quantifiable losses are likely to be measure in nanopounds, doesn't really seem to be a happening idea.

    [1]A classic example of just because you *can* doesn't mean you *should*.

    1. gerryg

      Re: Legality ...

      IANAL either - but the offence "abstracting electricity" originated in the Theft Act 1968 in an attempt to control phone phreaking (for those of us who remember that)

      Whether capturing excess radio transmission is an offence under the Theft Act, Telecommunications Act or Communications Act is probably one for the Courts.

      However the most close analogy would seem to be "piggybacking" and according to this

      yes it can get you into trouble - but "jail time?"

      These RFID devices will use microwatts of power - how much RF power gets grounded by a tree?

      (I can see the plot for new cop show: "keep your hands where we can see them and take the tree out of the ground, slowly, now")

      See Wireless World article of some vintage "Tree Antennas, TV & Wireless World, UK , 1985" for work in using trees as aerials in rural India (and then I found this on using an oak tree as an antenna)

  4. Phil O'Sophical Silver badge

    Old news

    Georgia tech were doing this two years ago, I read a story about it in some IT rag.

    Oh yes, it was here:

  5. Anonymous Coward
    Anonymous Coward

    Knowing the frequency or scanning

    That sounds good, as long as every device knows the frequency of a suitable transmission nearby, or can scan for available transmissions

    Or if it has some kind of _very_ broadband antenna, although I don't know if such exist yet or would be scalable to reasonable size.

  6. hplasm


    What happens when broadcast TV and radio goes away... WiFi and Cell Towers won't power much.

    1. Charles 9

      Re: Also-

      Radio probably won't be leaving anytime soon. As long as we need something to distract ourselves during our drives, radios will always find a use. As for television, they're compacting but not going away anytime soon. The BBC still has its mandate, and as long as the commercial networks still attract viewers and ad revenues, they can keep on kicking.

      There are also, IIRC, various utility radio frequencies that remain in use for both military and civilian applications. For example, there is WWV in Colorado, USA: the official channels of the NIST. They transmit constant time signals at several frequencies (generally low ones so as to cover the entire continental US plus parts of Canada and Mexico) that can be picked up (increasingly by some consumer clocks) for calibrating internal time.

  7. adnim

    Can I charge rent

    for the transmissions that pass through and thus occupy space in my house?

  8. Mike Banahan

    Illegal in the UK

    I'd like to see some references to that story about the farmer and the fluorescent tubes, just to prove that it's not urban myth (though a good one). I heard something very similar when on a tour of the LW transmitter at Droitwich, at least as far as the leeching-the-power bit went. Given that the transmitter there delivers 500kw into the antenna (what I was told on the tour and also what Wikipedia claims), even though the antenna efficiency is probably lamentable, there will still be a pretty strong field in the locality.

    I can quite believe that a few hundred yards away you can wire up a fluorescent and get some dim light off it, but I'm sceptical about the stealing electricity claim until someone can quote a case number or a reference to the judgement.

    It's actually REALLY impressive to see a full-length fluorescent alight in the presence of a strong RF field. A friend of mine who (motto: "it's rude to be weak") liked to run well up to the legal limit on HF frequencies from his car would do it as a party trick in pub car parks. A few hundred watts into a short car vertical antenna will keep a full-length tube glowing albeit not at full brightness (once it's struck) up to about 10 metres from the vehicle.

    1. Tanuki

      Re: Illegal in the UK

      I'd be most interested in a proper legal cite of anyone who's been found guilty of "absorbing RF for lighting/power purposes".

      Strapping a long fluorescent tube to a VHF antenna and shoving some RF up it is a very good way of demonstrating the concept of voltage-maxima. You can do the same to demonstrate standing-waves on open wire feeders.

      Similarly, when at uni I lived near to the Blaenplwyf TV/radio transmitter, and a torch-bulb connected across the feedpoint of a dipole cut to Radio 2 FM would glow dimly when you got the polarisation right.

      There used to be a car-park adjacent to a certain London airport which had a rotating radar-antenna perched on the roof. When operated at full power the fluorescent tubes on the floor below would glow as the antenna rotated.

      Oh, the fun you can have with RF.

    2. Phil O'Sophical Silver badge

      Re: Illegal in the UK

      just to prove that it's not urban myth

      Rural myth, surely?

    3. dorsetknob

      Re: Illegal in the UK

      Its No urban myth Not unless i got a set of fairy wings on my back.

      as a child back in the late 60s we used to visit relatives who lived adjacent to the national grid pylon network

      we played with our cousins on the heath that these pylons crossed

      my uncle's house had the power lines Cross directly overhead in the kitchen they had a twin 5ft florescent kitchen light fitted day or night it was always glowing even when switched off turn it on and it shone at full strength turn it off and the light dropped down to a just turned on threshold ( ie powered by the home owners mains = 1200 lumen turned of and powered by field effect or RF magnetic field = 200 lumen)

      as kids we did not really notice this until my cousins said that's nothing watch this he then held up an old unbroken florescent tube by one end as by magic it glowed no wires just like a JEDI Sword

      I Nearly Sh*t myself thinking power was going to jump from the pylon to the florescent tube like Lightning and electrocute my cousin.

      when this did not happen like all kids we also had to have a go ( 8 yrs old and this was like REAL MAGIC ).

    4. dorsetknob

      Re: Illegal in the UK

      Fu**ing Firefox ver 23 crashed while i was replying

      cant be bothered typing my response again (was over 200 words )

      Short response

      URBAN MYTH no its not not unless i grown a pair of fairy wings as kids in the late 60s we kids played under pylons with flouresent tubes that glowed in the dark

      try it yourself wave a flouresent tube through the air under a national grid power pylon and it will GLOW LIKE A JEDI SWORD

  9. ted frater

    Were off grid, tho have 2 sets of 33,000 3 phase overhead some 50 yds away overhead.

    My son built a 10ft high darrius wind turbine for his A level cdt. Won the Siemens prize that year at Poole Grammar.

    We thought to put 10 ft long copper by 50 turns on each blade and set it up near the power lines just to see if the cutting of the powerline field would give us a voltage across the coils when the wind was spinning the turbine. Never got round to it. We did think it was stealing power from the SEB.. Also considered wether the coils would draw power from the earths magnetic field. Never found out.

    1. Anonymous Coward
      Anonymous Coward

      Your son sounds really intelligent...

      I hope that he knows to write his name with a capital letter.

  10. John Smith 19 Gold badge

    Of course that depends if you *want* an "internet of things"

    Sometimes being clever is not really enough.

  11. Anonymous Coward
    Anonymous Coward


    This technology has the feel of a 'game changer' to me. How do I invest?

  12. Anonymous Coward
    Anonymous Coward

    Replacing POE with POWiFi ?

    Anyone tried tweaking WiFi router to generate enough power to drive a WiFi IP camera ?

    1. Mage Silver badge

      Re: Replacing POE with POWiFi ?

      Maybe if you used the Microwave Oven's Magnetron. Over say 2m distance you'd need much more WiFi power than the power brick supplies even if it was 100% turned into RF. You'd get cataracts if you looked closely at the new high power WiFi aerial too.

      1. Anonymous Coward
        Anonymous Coward

        Re: Replacing POE with POWiFi ?

        Cataracts are one thing, that microwave radiation doesn't really cause.

        What it will do however is permanently fry the actual receptors in the retina, which is much worse as it will not heal and cannot be corrected surgically.

  13. At0micAndy

    something for nothing, someone always complains

    I used to work for the CEGB and I remember a complaint that came in from a a woman who had a new Fluorescent Light fitted in her kitchen that would not turn off, even when the mains to the house was isolated. When we investigated we noted that she had part of the National Supergrid going right over her bungalow. Local area board replaced her tube light fitting with a standard bulb fitting. It amused me to see how the story changed over the years, at the last iteration I heard she had hundreds of tubes all over her house getting free light. All rubbish of course. Just one woman complaining, dealt with. Next.

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