Re: microwave, RF heating

[Doug Craigen <dcc@ESCAPE.CA>]

Chuck Britton wrote:
> > Seawater:
> >         conductivity: 4 S/m
> >         skin depth at 60 Hz: 32 m
> >         skin depth at 1 MHz: 0.25 m
> >         skin depth at 1 GHz: not given because at 1 GHz "seawater is
> > not a good
> > conductor" so the model breaks down.
>
> Could this mean that seawater becomes 'reflective' at these frequencies???
>
> or does the 'not a good conductor' phrase rule this out????

It is ruled out.  Reflection and absorption are closely tied together.
A review of dispersion relations, skin depth, and/or plasma frequency in
any text you have handy would be helpful.  (Optics or E&M books are
likely to have such a section.)  For those with Jackson, the section
with the plots for water is 7.5 (a web page showing these was cited by
someone else), but the region between 10^8 and 10^10 Hz happens to be
the only frequency range he doesn't comment on.

Basically, there is a lot of reflection and absorption below the plasma
frequency and the higher the conductivity the higher the plasma
frequency (the plasma frequency being the cutoff between real and
imaginary 'k' - we had this discussion about complex indices of
refraction a while ago).  Metals for example have high conductivity
which places their plasma frequency in the ultraviolet.  Hence, they are
reflective in the visible but transparent soon thereafter.  The lower
the conductivity the lower the plasma frequency.  If seawater is not a
good conductor in the GHz range, that means it has a low plasma
frequency so it is basically transparent in this region as far as
conduction mechanisms are concerned.  Still, a bit of absorption is all
that's required or wanted.

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Doug Craigen
          http://www.dctech.com/physics/