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Blocking electro-magnetic waves between two near field antennas

  1. Jun 24, 2013 #1
    Its been a while since I studied physics, so pardon my ignorance.

    I have two near field antennas (NFC). The range of the antennas is about 2-3 cm. I am looking for a way where I can block and unblock the communication between them.

    One of the way that I tried is to place a flat tightly woven wire (shown below) between the two antennas like a sandwich. If I short the two ends of the wire, I was able to block the communication. My assumption is that it is effectively reflecting the signal and thus blocking the signal. Is that what is happening?

    If I un-short the two ends, then, two antennas are able to communicate without any issue.

    However, if the radio signal has good strength, the coil is not doing a good job. Is there any other better solution that I can effectively block/unblock.

    I could just put a sheet of metal in between but I chose the wire solution so that I can block/unblock using a relay controlled via electronics. Can someone suggest a better solution ?


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  2. jcsd
  3. Jun 24, 2013 #2

    Baluncore

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    Because your antennas are very close they are highly coupled. It will not be possible to completely isolate them. All you can do is attempt to reduce or null their coupling. The method you use will be determined by the signal. Are they broadband or narrowband signals. The best method will also be dependent on the direction of energy flow and why you need to isolate them.

    If narrowband you could place a resonant element on their plane of symmetry and adjust it's position so that it cancels the coupling between them by reversing the phase. You could also adjust the antennas relative positions to null the coupling. You might consider synthesising a 4 terminal hybrid coupler such as a circulator or directional coupler that couples their feed lines in such a way as to cancel the antenna coupling in both directions on both transmission lines.

    If broadband signals are involved it will require a conductive screen between them. That screen will have to fully separate them and not have sharp conductive edges. It will need to extend beyond the near-field. Skin effect will require a thick material. The presence of the screen will change the characteristics of the individual antennas.

    Magnetic or conductive objects in the near or far-field will scatter energy and so couple the two antennas. There is no way to magically flip the physical antennas in and out of two isolated universes. An antenna is an EM scatterer in an environment of EM scatterers, so total isolation is never possible.
     
  4. Jun 24, 2013 #3

    berkeman

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    Good post by Baluncore.

    @Manoj7500 -- can you just mechanically insert the shield between the two antennas? Use a solenoid or a stepper motor to flip the shield in and out of position...?
     
  5. Jun 25, 2013 #4

    mfb

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    The air between the antennas does not look like the ideal place to block communications. Can you disconnect one of those antennas instead, or block signal transmission somewhere else in the electronics? The relay would be perfect for that.
     
  6. Jun 25, 2013 #5

    sophiecentaur

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    A few more details would help here. Are these two RF sources in sealed equipment, that you can't get at? Can't you just switch one off? Clearly not, or you wouldn't be asking the question but is there no way to get at the feeder and insert an RF switch (an ordinary relay might not give good enough isolation and you would possibly not want to be driving into an open circuit with your RF source).
    A high level of screening is very hard because the damned stuff sneaks around corners and through metal joints - but how many dB of isolation do you need? That's the nub.
     
  7. Jun 26, 2013 #6

    Baluncore

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    The switched loop you show in your original post will not isolate the antennas since the wavelength will be similar to the diameter of the coil's turns, so multiple resonances will couple the antennas through the loops.

    A physical wall between the two antennas would need to prevent circulating currents or currents on the outer edges of conductors or slots. Maybe a sandwich of conductors and resistive material. The bread would be resistive foam, the butter is slightly smaller copper shims with a fill of bread sized resistive foam, insulator, resistive foam. It is not switchable but it should cripple communication.

    A switched wall is a bit of a challenge. Maybe a grid of fast signal diodes with a controlled DC bias supply. When the diodes conduct they act as a mirror, when reverse biassed they are transparent. You would need an array of small surface mount diodes, maybe about 10 x 10 = 100 of them. The topological challenge is to mesh them so they have no big slots between lines of series diodes. The structure would need to have holes no greater than about an eighth of a wavelength.
    It takes about 1mA to bias a diode to conduct RF. The reverse bias voltage needed to turn the diodes off would be determined by the signal power because you don't want the transmitted signal to turn any diodes on. How do you make the broadband RF equivalent of a Kerr Cell ?

    You could also get sneaky. By placing a simple antenna near the two coupled antennas you could transmit an RF carrier or complex signal that would desensitise the receivers by saturating their front ends. Once an in-band interfering signal is about 12dB above the competition, phase detection will usually fail and so blind the receivers. Phase hopping or direct sequence spread spectrum signals can be difficult to jam. By pretending to be a valid signal, but with 20 dB higher power, you might distract the receiver decoder's attention and have it lock to your dummy transmitter. That is easily switchable.
     
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