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Receiving power Via radio transmission

  1. Nov 26, 2003 #1
    i was reading an article about the new RFID tags that could replace ATM cards. these tags respond to query via a radio tranmission. the tag then uses the power that was in the initial radio transmission to reply back to the system. how are the tags using the power from the transmission? can any one point me to a site or a book that has that information in it.

    here is a link to the article.

    "They listen for a radio query and respond by transmitting a unique ID code..." "Most RFID tags have no batteries. They use the power from the initial radio signal to transmit their response."

  2. jcsd
  3. Nov 26, 2003 #2


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    All antennae rebroadcast some power. Usually attempts are made to minimise reflections. In these ID devices, filters are added to dampen some reflected frequencies, but not others. By examining the spectrum of reflected frequencies, you can identify the device.

  4. Nov 26, 2003 #3
    As always, first take "RFID", take Google. Paste. Stir. Enjoy.
    eg. http://www.aimglobal.org/technologies/rfid/

    Radio is alternating field. To generate one, takes energy. That energy is in that field, in form of "alternations". Antenna in RFID is resonant with frequency of radio signal, and selects the radio signal of interest, which produces AC voltage on antenna terminals. Then the energy in the field is extracted by means of converting AC into DC. Chips inside RFID are so damn low-power that its enough energy in radio signal to "light em up" for the duration of being within range. They cumulate some energy as "chargers" to then create short bursts of transmission back to the reader.

    Data is all digital, I don't believe RFID relies on any reflections or spectrum analysis. Its more from cryptography side, chips contain some key or code that is communicated.
  5. Nov 26, 2003 #4


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    Hmm, I'll admit to answering off-the-cuff, so I am probably wrong. I assumed it would be more energy-efficient to use reflections. Maybe what you can gain in spectral purity more than makes up for lost energy efficiency.

  6. Nov 28, 2003 #5
    So how does a receiving device convert radio waves to power. i mean,i know that radio is AC but what is the device called that goes between the antenna and the device that converts it to AC power?

  7. Nov 29, 2003 #6
    AC already IS power. What you probably mean is how they convert it to usable DC. I don't know how, it must be some fancy stuff, because radio AC is so damn faint. But normally, every electronic device in your house uses socalled diode rectifier. for eg.
    It works well when voltages are at least in volts range. So it won't fit to radio directly, although AM-radios basically use diodes to convert radio signal to audio signal. But anyway, principle is about the same, you rectify AC into DC and draw from it.
  8. Dec 6, 2003 #7
    I keep thinking of one word, Zenor diode. Remember crystal radio sets that don't use batteries, just the power received via antenna. I am sure that is how they do it but not positive, no I am positive it is going to be negative that is the only thing I am positive about. Any way I believe the information is stored in a re-write able dongle That sends the code to back the device dude.
  9. Dec 26, 2003 #8
    Re: Raido Frequency Power

    The antenna inside fo the card recieves the RF energies by means of induction, which in turn drives a small LC tank, which then drives a bridge rectifier in order to convert the ac to dc for digital use in its chip sets.

    It takes more power to transmit than it does to recieve due to the modulation (a lot of power loss occures during modulation). Therefore, the card itself would need to be in close proximity to the high powered tranmitting station that it is recieving rf energy from, in order to induce enough power for the card itself to transmit a high enough powered digital encoded transmision back out from itself to the atm's reciever.
  10. Jan 7, 2004 #9
    Some RFID systems don't really use radio waves at all, instead they use high frequency transformers with widely separated coils. The "transmitter" is the primary coil, and the "receiver" is the secondary. The tag can receive power just as any transformer coil does: add a diode and a filter capacitor to get DC output. Once the computers on each end are active, they can decide to switch roles, and the secondary coil can become the primary for a moment.

    What's the difference between transformers versus radio antennas? Easy: if the antennas are far closer than 1/2 wavelength and each antenna is far smaller than 1/2 wavelength, then it's a transformer, not a radio system. Frequency is irrelevant; you might be using so called "radio frequencies", yet the system still acts just like a transformer, and no radio waves are emitted.

    For example, if an RFID tag uses 100 kilohertz for power & communication, then it's just a transformer, not a real radio system. The half-wavelength at 100KHz is 1.5 kilometers! To become a genuine radio system the control pod and the RFID tag would have to be several kilometers apart, and their antennas would have to be hundreds of meters wide.

    On the other hand, if the RFID system used UHF frequencies or cellphone frequencies (microwave), then the wavelengths involved are a few centimeters, and in that case any antennas really do behave as antennas and not as primaries/secondaries of transformers. They really do send out radio waves, and if there is no receiver in the way, the waves just fly off to infinity.
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