MadSci Network: Physics

Re: Why aren't people who work with broadcasting cooked?

Area: Physics
Posted By: Kevin Reed, Engineer, None,
Date: Sat Nov 1 22:46:34 1997
Area of science: Physics
ID: 878272737.Ph

The term "Microwaves" covers a whole band of radiation that has a wavelengths of around a micro-meter. Just like there are different colors of visible light, each color with its own wavelength, there are different "colors" of microwaves, and the microwaves used for cooking are a different color from the microwaves used for cellular phones or broadcasting.

The microwaves in a microwave oven have a wavelength that matches the size of water molecules. When radiation of this wavelength passes by a water molecule, the water molecule vibrates back and forth in sync with the wave. This vibration transfers energy to surrounding molecules in the form of heat; when enough molecules vibrate, enough heat to cook food is released.

The microwaves used for broadcasting, radar and satellite TV are the wrong size to interact with water molecules, and they just pass right through people as though they weren’t even there.

Incidentally, the microwave oven was invented by accident. A scientist doing research to improve radar systems after World War II discovered that a microwave transmitter he was working with melted a chocolate bar in his coat pocket.

Admin note:
David R. Shelby adds the following:

There are Fixed Satellite frequencies in the 38.6 GHz to 275 GHz band. Standard Superhigh Frequencies (SHF) are in the Microwave band as well. At and above 300 GHz, you get into Infrared Radiation frequencies. The microwaves in a microwave oven DO NOT have a wavelength that matches the size of water molecules. Microwave ovens operate in an FCC allowed band from 2.37 to 2.52 GHz. Broadcasters work with many frequencies above and below that band.

The microwaves used for broadcasting, radar and satellite uplinks, are NOT the wrong size to interact with water molecules, as THEY WOULD if the INTENSITY of the field were strong enough. Microwaves used for broadcasting, i.e.: STL (Studio Transmitter Link) and satellite uplinks have a much lower power (less intensity) than that which is found inside of a microwave oven. Microwave frequencies do not corner the market on heating. I used to climb broadcast towers for maintenance, and have been exposed to the effects of EMR, from FM and TV antenna bays, and it heats up everything on your body which is made up of metal quite quickly, and can cause burns just by being in the field near the antenna bays.

Body tissues that are subjected to very high levels of RF energy may suffer serious heat damage. These effects depend upon the frequency of the energy, the power density of the RF field that strikes the body, and even on factors such as the polarization of the wave.

At frequencies near the body's natural resonant frequency, RF energy is absorbed more efficiently, and maximum heating occurs. In adults, this frequency usually is about 35 MHz if the person is grounded, and about 70 MHz if the person's body is insulated from the ground. Also, body parts may be resonant; the adult head, for example is resonant around 400 MHz, while a baby's smaller head resonates near 700 MHz. Body size thus determines the frequency at which most RF energy is absorbed. As the frequency is increased above resonance, less RF heating generally occurs. However, additional longitudinal resonance's occur at about 1 GHz near the body surface.

David R. Shelby

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