Re: Inverse D squared v/s Inverse D sixth power ?

Graeme Zimmer wrote:

Hi,  (I posted this in sci.physics.electromag but didn't get a sqeak).

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, but I also understand that if you have just an alternating magnetic
field, then the electric field is promptly re-created (and vice versa) (in the Far Field ?).

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


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









If one creates an alternating magnetic field, it is impossible to "shield" the electric field; it will automatically be created (even in near-field).
Vice-versa also.
Just look at the E-M equations...
.

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