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Wideband Receiver Power?

  1. Nov 5, 2009 #1
    http://www.fastcompany.com/blog/ari...okia-developing-radio-wave-powered-cell-phone

    Now, I am sure at least some of you have heard of this technology that Nokia is developing. It creates around 5 mW right now. My question is, how do you make an antenna that small pick up that many frequencies/wavelengths? What if I wanted to build one?

    I apologize if any of this is a dumb question, my EET degree hasn't dabbled much in communications...at least not yet.
     
  2. jcsd
  3. Nov 5, 2009 #2

    berkeman

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    Welcome to the PF. You can make a multi-frequency, fairly broadband antenna with a Yagi with stepped length elements:

    http://images.google.com/images?hl=en&source=hp&q=yagi+antenna&gbv=2&aq=f&oq=&aqi=g1

    Or they may have used some other antenna shape. Maybe even a fractal antenna:

    http://images.google.com/images?hl=en&source=hp&q=fractal+antenna&gbv=2&aq=f&oq=&aqi=

    There are also variations in geometry to help make the antenna less directional.
     
  4. Nov 5, 2009 #3
    Thank you, happy to be here :)


    What would be the theoretically best receiver? To receive as many frequencies as they are trying to, could you put one antenna on front of another to catch both?

    That said, does anyone know how much voltage is induced in a wire when it is struck by EM waves?
     
  5. Nov 5, 2009 #4

    berkeman

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    Good questions. To extract power, the antenna terminal voltage needs to be larger than a Schottky diode drop (about 0.2V for small powers) in peak value. I'll need to check my textbook tomorrow, but it would seem to be a moderately high power that is required in receive to get to that kind of terminal voltages in the 500MHz-fewGHz range.

    And if you are trying to extract power from multiple bands, then yes, it would be optimal to use separate antennas and rectifying circuits, to eliminate phase issues that would cut down some on efficiency. I don't think the loss of efficiency is very large, though, so it could be that you just rectify the terminal output voltage from a broadband antenna.
     
  6. Nov 5, 2009 #5

    vk6kro

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    Much as we all love the Internet it has the problem that anybody can say anything even if there is no basis for it at all.

    IF you took all the radio transmissions reaching your house at any one moment, how much power would there be? Absolutely ZERO unless you live under a transmitting tower for a radio station.

    This is because signals on different frequencies cannot be in phase, so their net amplitude is zero or close to it.

    So, even if you rectified the total mess of signals, even with a perfect diode, you would get nothing.


    These radio signals are incredibly weak too. A "very strong" signal you might hear on a shortwave receiver is probably 50 uVolts at the antenna terminal of the receiver. That is 0.00005 Volts.
    This is because there is an inverse square law that makes radio signals sharply decrease in amplitude as they move away from the source.
    So, even if you tuned this one signal in and rectified it somehow, you would have an extremely small amount of power.

    It takes serious power to work the transmitter of a cell phone. You could certainly get it from solar power but you won't get it from other radio transmissions.

    Nokia is a big company and they have their own website. Would they really announce something like this through a website nobody has ever heard of? I checked their website and they were not announcing any such development at this stage, or probably ever.
     
  7. Nov 6, 2009 #6
    What would a schematic look like, exactly? I've never worked with a Schottky diode before, but my research turns up that they are basically just really efficient diodes that drop around 220 mV. Crystal radios are able to run on their own power without being plugged in so...theoretically if I wished, I could light a low voltage set of LEDs or some other highly efficient lighting?

    They aren't the only ones that published the story. I just liked their description.

    http://www.technologyreview.com/communications/22764/
    http://tech.yahoo.com/blogs/null/143945 [Broken]
    http://news.cnet.com/8301-17938_105-10267006-1.html

    Nokia announced it, and they are making it work somehow.
     
    Last edited by a moderator: May 4, 2017
  8. Nov 6, 2009 #7

    vk6kro

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    You will notice that the C-Net article is actually ridiculing the idea.

    What they seem to be trying is to capture the RFID energy used at the entrance of libraries etc to detect illegal removal of books.

    This is a very localised source of power, but at least it is better than trying to capture power from random shortwave transmissions. And it is a single frequency transmission, so some power is available.

    It would be possible to have inductive loops built into desks so that you could charge a phone by just resting the phone above the loop. This would take a lot of hardware, but it could be done.
     
  9. Nov 6, 2009 #8
    I think in the woman writing the article was trying to make a connection so people understand what radio waves are. Some people just don't know.
     
  10. Nov 6, 2009 #9

    vk6kro

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    There must be a lot more energy available from the room lighting than from random radio pickup from the RFID device.

    Calculators have been using this free energy for years. However a cell phone uses serious power whereas a calculator uses very little.
     
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