Inverse D squared v/s Inverse D sixth power ?

Hi,

For Electromagnetic Radiation, most text books show the power density
(in watts per square meter) of a wavefront is inversely proportional to the
square of the distance from the source.

e.g. 1/r2

However recent discussions of Near Field Communications (NFC) indicate a
1/r6 relationship (inverse Sixth power).

see http://www.radio-electronics.com/info/wireless/nfc/nfc_overview.php
and http://www.auracomm.com/site/content/roll_off.asp

Now I understand that this is in the special case where only a magnetic
field is present (the electric field having been suppressed with shielded
antenna) (or vice versa).

The title suggests that this 1/r6 relationship only occurs in the Near
Field, and I also understand that if you have just an alternating magnetic
field, then the electric field is promptly re-created (and vice versa)
(Far Field ?).

So can anyone elaborate on just where and when this Inverse Sixth Power
relationship applies please?


Thanks ........... Graeme Zimmer








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