Inverse D squared v/s Inverse D sixth power ?

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

see

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

Auracomm's technical claims seem a trifle suspect to me. Have you tried actually asking them directly?

"Michael Gray"

The magnetic field said to be " quasi static " ??

Is that a bar magnet dangling on a string ??

........... Phil

I read it carefully. From what I could determine through their haze of BS, they were at one point claiming that the field was static, and then elsewhere shyly admitting that it was moving.

They can't have it both ways.

It's dangling on *something*! Quite possibly credulity.

The system that is described in your reference is based on a magnetic coupling. This means that the antenne is inductive in nature; a loop antenne. The receiver antenne is also a loop antenne and the two actually form a transformer where the two windings are very loosly coupled.

In such systems(125kHz and 13.56Mhz) the radius of the antenne is small in respect to the wavelenght of the transmitted signal. So the electrical field can not be very strong since that can only achieve maximum levels if the electrical length of the antenne is a quarter wave or more.

So in the near field (distance< wave length/2pi ) the energy in the electrical field will be smaller then the energy in the magnetic field. But the greater the distance the more magnetic energy is transferred to electrical energy. In the far field the energy in the electrical field is identical to that in the magnetic field and the two are 90 degrees out of phase.

Bottom line; there is no shielding of the electrical field. The antenne is simply much less efficient for electrical fields.

In general it is very difficult to determine what the amount of energy in a field will be if you are within the 'near field area'. This is because this depends very much on the antenna properties and its surroundings.

It applies to these special cases of using this loop antenna that is small in side in respect to the wavelenght. But even then it depends on the dimensions and shape of the antenne.

Have a look at the web sites of

Regards,

John