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Re: 2.4 GHz Cordless Phones





Gary \"Joe\" Mayfield wrote:

> Honest to God, my sources inform me the original microwave tests were on
> cooking a hot dog.  2.4GHz won out because it is about the resonant
> frequency of a hot dog (made it cook faster).

Both the 915 MHz and 2450 MHz frequency are allocated for unlicensed ISM
(Industrial, Scientific and Medical) use. At 915, has been used for both
diathermy units (for chiropractors to "cook" stiff joints) and for laminating
plywood.

The 2450 MHz is used for domestic cooking for several technical reasons
including
  - the 2.45 GHz frequency was allocated for "garbage" uses, and
  - reliable, low-cost (although spectrally impure) high power microwave
oscillators
    (magentrons)  are easy to make and
  - resonant cavities a couple of wavelengths in size for 13 cm  (5 inch) are a
convenient
    size for countertop use (915 MHz is 33 cm, so a corresponding cavity would
be about
    2' in size).

At either frequency, the "ovens" cook by "flexing" the the ~120 degree
"boomerang" bond in the H=O=H
molecule (aka H2O, aka water). The resulting friction is what does the heating.
The 2.4 GHz frequency bends the molecules ~3 times faster at 2.4 GHz than at 915
MHz. The coupling of energy for heating works best on objects that are about a
(wavelength)*(velocity factor of meat or potatos)  in size.

[It is interesting to note that much of the worry about ~850 MHz cell phones on
humans comes from the fact that the "meat" in your skull is a pretty good match
to ~35 cm RF -- Are you slowly cooking your brains?]

In this thread there were questions about the microwave spectral purity of
microwave oven magnetrons. Its been years since I did them, but measurements I
made showed that ovens have an assymetric spectrum. They cut  off rapidly on the
low-side (i.e. below ~2425 MHz) but have quite a bit of energy on  the high
frequency side, not tailing off until above ~2500 MHz. This was good news for
2.4 GHz amateur activity and even better news to Deep Space and Radio Astronomy
facilities at 2.2-2.3 GHz. I recall that the lunch room at the 70M dish at
Goldstone used to have a microwave oven for the staff to heat their meals; the
total system temperature of the 70M dish at 2.2 GHz (including the effects of
microwave ovens plus the normal 3K background, atmosphere, feed spillover, LNA
noise, etc) is in the 10K-20K range.

73, Tom



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