RF near-field vs far-field

[Bob Masta]

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

 

[Joe McElvenney]

Hi,

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?

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

 

[jgreimer]

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