Re: Non-Radiative Evanescent Waves are back in the news... (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 10 Jun 2007 18:22:18 -0700 (PDT)
From: William Beaty <billb@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Non-Radiative Evanescent Waves are back in the news... (fwd)

On Fri, 8 Jun 2007, Tesla list wrote:

>    Can someone explain to me why the word 'Evanescent' is used?

"Evanescent fields" is a slight distortion.  Those authors are using terms
from wave mechanics to describe an air-core RF transformer.

But since their transformer is smaller than a conventional 1/4-wave
antenna, yet much larger than a conventional inductor, they do have some
justification for saying "evanescent."  The evanescent part, that's the
part which doesn't radiate as EM waves.

You're aware that evanescent fields in optics are no different than the e-
and b-fields surrounding a capacitor or inductor?  They're the nearfield
part of EM, the non-radiating part.  When you break the circuit of an
inductor, one part of the field collapses back inwards, and another part
is radiated as EM waves.  Here's an excellent animation of a dipole
antenna from an EM course:

   http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/light/DipoleRadiation/DipoleRadiation.htm

The 2nd of those three animations shows the "evanescent" fields which
move in and out but don't break loose and fly off as EM radiation.



> I'm an
> optics guy, and an evanescent field is hardly seen a few meters away from
> the source, let alone within a few microns of the surface.

Not a "few meters," but a few fractions of one wavelength.  The division
between "evanescent fields" and "EM radiation" takes place at between 1/4
and 1/2 wavelength from the emitter (or reflector.)  For 500nM light
waves, the evanescent field is only significant within a few hundred nM of
the surface.  For 50 meter EM waves, the evanescent field is significant
within a few tens of meters.

> Forgive me if I
> don't understand. My background is (heavily) in optics, so I'm not well
> versed in RF.




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