Skull + Key fob = Extended range. Why?

Discussion in 'Wireless & RF Design' started by wayneh, Oct 11, 2016.

  1. wayneh

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    It may seem silly but I'm pretty convinced that holding your car's key fob remote to your chin extends the range maybe 30%. I know I'm not alone in this.

    What I can't figure out is, why? If anything you might predict that tissue would absorb some of the energy and reduce range, not increase it.
     
  2. SLK001

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    Increased height? I don't know what the freq of these things are, but I imagine that they are in the ISM range of bands (1-3GHz). Also, a key dangling from the fob might provide an additional parasitic antenna element.
     
  3. nsaspook

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  4. GopherT

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    Remember the old rabbit-ear antennas? Holding the antenna improved reception. A big bag of salt water is an antenna.
     
  5. nsaspook

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    or a ground plane/RF reflector.
     
  6. wayneh

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    I guess the problem is that I don't understand antennas in general. A wire held up on a tower I get. How is a nearly grounded bag of salt water useful?
     
  7. ci139

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    i first remembered the water being diamagnetic also the net scan revealed something interesting
    - which on a second thought may not be the GO for EM waves as µ.air ~ µ.water
    - the bones are where the red blod-cells are formed ? concentration of Fe+ ? in U arc form ±U∓( ((•)) )RKF -- just a random hypothesis
     
  8. BR-549

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    Counterpoise.
     
  9. wayneh

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  10. nsaspook

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    IMO it's less capacitance coupling into a 'real' antenna and more of a 'image' antenna using the body/water as a ground plane.
    https://www.ece.nus.edu.sg/stfpage/...e Notes/Supplementary Notes/Image Theorem.pdf

    [​IMG]
     
  11. ci139

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    the loops in receiver of FM broadcast antenna that select/prefer the certain station are typically at the size of 30cm/1foot×10cm/4" . . . for about 100MHz ((personal experience atemping to receive FM in bad spot × atm.-c conditions × distant/weak station))
     
  12. wayneh

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    So this says the RF energy is reflected, and therefore a bit more of it shows up at the receiver? That makes some sense. The last video I linked seemed to make sense until I thought longer about it. He essentially said the water (or your head) absorbs and re-radiates the energy, but that doesn't explain to me why more energy arrives at the receiver.

    If the reflection theory is correct, the range should be reduced if you face away from your car? I'll have to try that. I'll watch that water jug experiment again.
     
  13. nsaspook

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    The 'radiation resistance' of your typical Key Fob closed-loop antenna is very low. This limits the actual power that's transmitted to a low level due to the large mismatch with the near field EM impedance near it. The low antenna resistance is usually RF matched internally to the transmitter chip output impedance to deliver max power but most of it is wasted in the wire resistance instead of being radiated into space to the far-field. Any sizable ground-plane affects the radiation pattern, near field EM impedance and efficiency of these small antennas.

    http://pdfserv.maximintegrated.com/en/an/AN3621.pdf
     
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