[ILUG] OT: internet access/MMDS

[Paul Curtayne]

Actually, not quite correct, Colm

2.45 GHz is *not* a resonant frequency for H20 molecules. It would be
useless if it did, as the microwaves would all be absorbed by the H2O
molecules on the surface. Instead a frequency a bit away from the
resonant frequency is chosen so the water molecules only *partially*
absorb the radiation, so that it penetrates some distance.

Note: a resonance is one *sharp* frequency which is absorbed *much*
more strongly than frequencies above and below this.

Also, and here we get technical, watch out:

There is not really any such thing as a resonance for liquid water
molecules.

The resonances we see quoted in text books (900 MHZ for example are
for one isolated H2) molecule, ie a gas. (An isolated molecule is
effectively a gas as a definition of a gas is actually that molecules
are far away enough from each other not to interact with each other)

In liquid, the distance between molecules is *way* less, and
resonances get smeared out into broad bands of frequencies that are
well absorbed (technical note, this is due to overlapping of wave
functions and the quantum mechanical requirement that no two particles
can have exactly the same state. This gets far more smeared again in
solids as individual molecules are even closer and more meshed up)

So, yes microwave ovens work usually at 2.45 GHz, not because it's a
resonance, but because it's in a range of frequencies that are
moderately well absorbed.

2.1 or 2.8 GHz would work pretty much just as well, but manufacturers
are allocated this exact frequency to be theirs so that it won't
interfere with other equipment.


<further note>

Basically, any molecule will be sensitive to absorbing radiation at
many different frequencies, though they tend to be grouped.

e.g.

Frequencies of the order of 1-10 gigahertz make it rotate.

Freqs around 10 to the 5 or 6 gigahertz make it vibrate (equals
infra-red very roughly)

Radiation around 10 to the 7 or 8 or 9-ish gigahertz make the
electrons in the component atoms get excited and absorb and then
re -emit radiation (equals visible and UV)

So if you could tune through the spectrum, you would see greatly
increased absorption of EM radiation around these frequencies, but for
*completely* different reasons.

This is so simplified! Rotation and vibration resonances can happen
together for example. A molecule with 2 or 3 or 4 atoms etc starts
getting so complicated, and is far from exhaustively studied.


Paul



>
>
> > 2 Watts, actually, while talking or searching for a network. Also,
the
> > resonant (sp?) frequency of the water molecule is around 900 MHz.
MW
> > ovens work that way.
>
> Nope, uw ovens use a frequency of ~2.45GHz, which is the resonant
> frequency of water.  Microwaves of different wavelengths (such as
900,
> 1800 and 1900 MHz, as used in GSM cellphones) don't have nearly the
same
> dramatic effect on water - in fact, water is pretty transparent to
most
> frequencies in these ranges, *except* for a tight spike of
> high-absorbency around the 2.45GHz mark.  Most of the "radiation"
from
> your cellphone goes straight through your head with virtually no
> effects.
>
>              Colm