🚀 go-pugleaf

In article <1992Apr4.180339.19417@bilver.uucp> alex@bilver.uucp 
(Alex Matulich) writes:
>I agree that microwave ovens do not depend on the resonant frequency of
>water, but there is one point you are missing:  Water *does* have a high
>absorbtion band in the 2 GHz range.
>
>When I was doing radar testing of materials at General Dynamics some years
>back, we did transmission testing of water.  I can't say that the higher
>attenuation we observed at approx. 2 GHz was a "resonant frequency" but it
>was certainly an absorbtion frequency.

You are on the right track, but I believe your memory is somewhat faulty.
Water is a good absorber over an EXTREMELY broad frequency range.  In terms of
general nomenclature, you would not say something has an absorption band in
a frequency range unless the absorption in the frequency band exceeds the
absorption at frequencies above and below the band.  This is not the case with
water below 10 GHz or so.

>The dielectric constant of water is not a real number; it's complex, and
>the real and imaginary parts are frequency dependent (until you get to
>visible wavelengths where they are virtually flat).  Water is therefore
>transparent at some frequencies, lossy at others, and reflective at
>others.

You are correct about dielectric constants.  Von Hippel's various books on
dielectrics give graphs of the complex permittivity of water as a function of
frequency and temperature.  [Permittivity is the correct term - the dielectric
constant, epsilon prime, is the real term, and the loss term, epsilon double-
prime, is the complex term.]  At room temperature, the dielectric constant of
water is essentially flat to above 10 GHz, where it begins to drop off from
its nominal value of around 80.  Likewise the loss factor begins to increase
rapidly above 10 GHz, peaking around the resonant frequency of 22.235 GHz, and
falling off again above this frequency.  It has additional peaks at higher
frequencies.  As a simple approximation, the dielectric constant, loss factor,
and the relative dielectric conductivity can be related by a relaxation model
such as that presented by Debye.  But this in only an approximation...
Maybe it is my warped sense of humor, but I found von Hippel's discussion of
the Mosotti Catastrophe and the Local Field particularly humorous (like
"proving" that water should solidify at high temperatures by spontaneous
polatization). [The Debye equation is based on the Mosotti field!]

Water presents a number of other absorption peaks at higher frequencies such as
200 and 350 GHz.  In the infrared region, water is a STRONG absorber.  At
optical frequencies, of course, the square root of the permittivity - the
refractive index, is generally used.

>About the 2.45 GHz microwave oven frequency:  I understand that frequency
>was chosen by engineers who decided how big an "average-sized" dinner
>would be cooked inside, and chose the frequency as a nice compromise
>between penetration and efficient heating, based on the projected size of
>the "load."  This would explain why bigger, industrial ovens have lower
>frequencies -- 2.45 GHz is too "efficient" for larger masses of food.

The 2.45 GHz frequency is an ISM (Industrial, Scientific, and Medical)
frequency.  It was actually more of a compromise between more absorption at
higher frequencies and the cost of generating significant power at these
frequencies.  Industrial ovens (and I do not mean institutional ovens) usually
use the 915 MHz frequency because it is easier to generate very high power
here than at 2.45 GHz.  I am talking tens to hundreds of kilowatts.

In the United States, commercial microwave ovens are available for 915 +/- 13 
MHz, 2450 +/- 50 MHz, and 5800 +/- 75 MHz, all Industrial, Scientific, and 
Medical Frequencies as assigned by the FCC.  In various European countries, 
other frequencies are used like 433.92 MHz (Austria, Germany, etc.), 896 MHz 
(UK), 2375 MHz (former Soviet bloc countries), and 3390 and 6780 MHz 
(Netherlands).

The size of the load does have an effect, however, and lower frequencies are
used for thicker loads.  This is not just because the "penetration" is deeper.
It is also easier to obtain more uniform fields at lower frequencies leading to
more uniform heating.  Home ovens are far from uniform as you can easily
demonstrate with a plate full or marshmellows!

The dielectric properties of many materials have been published in the Journal
of Microwave Power (not available in our library but probably at Rochester).
A number of graphs of this data have been reprinted in "Industrial Microwave
Heating" by Metaxas and Meredith.  For example, they give e' and e" for leather
as a function of moisture content (this probably applies to fast-food burgers
too  ;-}   ), corn, beef and turkey (at a number of frequencies), and dairy
products as a function of temperature.

To summarize - home microwave ovens operate at the frequency they do because
of a number of factors: engineering, economics, dielectric properties, and
politics - not because of any special resonant absorption phenomena.

					Barry
 -----------------
|  ___  ________  |       Dr. Barry L. Ornitz          WA4VZQ
| |  / /        | |       Eastman Kodak Company
| | / /         | |       Eastman Chemical Company Research Laboratories
| |< < K O D A K| |       Process Instrumentation Research Laboratory
| | \ \         | |       P. O. Box 1972, Building 167B
| |__\ \________| |       Kingsport, TN  37662       615/229-4904
|                 |       INTERNET:   ornitz@kodak.com
 -----------------