"Chuck" wrote in message
...
Mike Kaliski wrote:
That would be why returns on 3 Cm marine radar are wiped out in heavy
rain
then. I can vouch for the fact that heavy rain and spray can result in
100%
obscuration of ship radar targets.
Aircraft also use radar to detect storms. The severity of the weather
ahead
is determined by analysis of the probable size of the water droplets in
the
cloud, which affects the level of the radar return.
While water molecules may have an inherent resonance at 1 THz, it is
generally accepted by the scientific community that microwave ovens work
by
exciting water molecules, amongst others, in food. I can't recall ever
cooking anything in a microwave that wasn't wet or at least damp. Well I
have tested some bits of plastic for suitability for use at RF, but that
wasn't for eating - and the wife was out!!!
I am happy to accept that there is no direct correlation between the
resonant frequency of a water molecule at 1 THz and a microwave oven
magnetron operating in the Ghz range. However, microwave ovens will heat
even distilled water quite effectively and heating is defined as an
increase
in vibrational energy of molecules. So however the process actually
works,
some principle of resonance must exist to allow the excitation of the
water
molecules to occur. Either that, or microwave ovens actually work by
heating
the food with infra red radiation derived using waste heat from the
magnetron and we have all been fooled. :-)
Increased molecular motion certainly
accompanies increased temperature, but
depending on the water's salt content,
the skin depth of water at microwave
oven frequencies is 0.5cm to 4.7cm. So I
guess it is an absorption phenomenon,
rather than resonance. Or, infrared
radiation from the maagnetron! ;-)
This information, BTW, comes from Ron
Schmitt's "Electromagnetics Explained".
Chuck
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Chuck
When I learned about radar it was common practice to break the waveguide and
stick your hand over the transmitter end to see if you could feel any
warmth! If your hand got hot, the magnetron was firing up okay. If you were
of a nervous disposition, then a neon bulb was a sissy substitute for a
hand.
The marine radar sets on my first ship were all Korean War vintage with a 10
Cm set with a standard range of 96 miles - well over the horizon. The
navigators claimed they could spot mountain tops at over 200 miles.
The equipment was all valves and worked for an average of 3 days before each
failure. Usually a 2 to 4 hour fault finding job each time. The most
spectacular failure occured when the 10 Cm radar continued to work
fautlessly through a severe storm before finally packing up when the sun
came out. Eventually the waveguide running up the mast to the scanner was
found to contain over 30 vertical feet of water! The scanner horn had been
holed by a piece of railing torn off the bow in the storm. The metal railing
had flown up 60 feet in the air before hitting the scanner horn and holing
it. Clearing the water and patching the scanner horn with duct tape restored
operation until we docked.
I know that the 10 Cm radar appeared not to have any water heating effects
because the water that poured out of the waveguide was freezing and the
radar had been continuously switched to transmit until shortly before
looking for the fault.
So there is definitely something significant about the 3 Cm wavelengths used
in microwave ovens and interaction with water molecules at that frequency.
Mike G0ULI