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RF near-field vs far-field

Discussion in 'Electronic Basics' started by Bob Masta, Oct 30, 2004.

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  1. Bob Masta

    Bob Masta Guest

    The lastest issue of EDN (Oct 14, 2004) has an article
    on RFID that includes a line that has me puzzled:
    "With all other things being equal, high-frequency RFIDs
    have longer range than their low-frequency counterparts,
    fundamentally because near-field effects don't degrade
    high-frequency RFIDs' signals. If a tag is less than one
    wavelength away from a reader, the signal decays with the
    cube of the distance; beyond one wavelength the signal
    decays with the square of the distance."

    I don't recall any "cube of the distance" stuff from my
    school days, but I haven't done any RF since then.
    It sounds counter-intuitive to me, and certainly not the
    way sound waves behave in the near-field. ( Where
    sound wave fronts are nearly parallel, the decay is greatly
    reduced.) Can somebody explain why RF should be
    different? Or is the article wrong?

    Thanks!


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  2. Hi,
    In spherical coordinates the E-field has three components. Two
    'near-field' parts decreasing as r^3 and r^2 and the radiated
    'far-field' one as plain 'r'. The H-field on the other hand has
    only two which decrease as r^2 and 'r'. Both of the near E-field
    components are out of phase with the H-field and so do not
    radiate any useful power. They are not proportional to the H-
    field either which dominates close in.

    The above simply drops out of the mathematics because of an
    expansion of terms but I've never been able to get my head round
    the 'one current many fields' aspect of it. I once did a half-
    credit course on E/M theory but can remember only about five
    percent of it now. Maybe I'll dig out the books and have another
    go.

    As for sound waves, they really are another kettle of fish
    being longitudinal and not transverse waves as the E/M variety
    are.


    Cheers - Joe
     
  3. jgreimer

    jgreimer Guest

    The near field effect is widely misunderstood and there are conflicting
    explanations about what it is or how it behaves. Some sources even say the
    near field strength decreases close to the antenna.

    At first it seems that a field decaying according to the inverse cube of the
    distance suggests the existence of a fourth spatial dimension. Other fields
    obey the inverse square law because in 3 space as the distance is doubled
    the surface area of a field quadruples. Fortunately there's another
    explanation.

    Just as an inductor conducting a signal of some frequency has an alternating
    magnetic or H field around it, so does an antenna. Most of that field
    collapses back into the inductor every half cycle but some of it radiates
    away. With an antenna it is similar except that we are normally interested
    in the part of the field that radiates. If another antenna is brought close
    to it, it picks up not only the radiated signal, but also part of the
    induction field, the same way bringing a second coil close to an active one
    creates a transformer.

    However, when two antennas are close to each other, their physical size
    becomes important and it is possible for the second antenna to pick up
    signals from different parts of the radiating antenna in way that the fields
    partially or completely cancel. So a field strength reading close to a
    radiating antenna may indicate either a higher or lower field strength than
    actual.

    jgreimer
     
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