RF near-field vs far-field

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?


Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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Bob Masta
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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

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Joe McElvenney

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.


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