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Microwave Radiation Definition
by
Anne Marie
Helmenstine, Ph.D.
Microwave radiation is a type of electromagnetic
radiation.
The prefix "micro-"
in microwaves doesn't mean microwaves have micrometer wavelengths, but rather
that microwaves have very small wavelengths compared with traditional radio
waves (1 mm to 100,000 km wavelengths).
In the electromagnetic spectrum, microwaves fall between
infrared radiation and radio waves.
Frequencies
Microwave radiation has a frequency between
300 MHz and 300 GHz (1 GHz to 100 GHz in radio engineering) or a wavelength ranging
from 0.1 cm to 100 cm.
The range includes the SHF (super high frequency), UHF (ultra
high frequency) and EHF (extremely high frequency or millimeter waves) radio
bands.
While lower frequency radio waves can follow the contours of the
Earth and bounce off layers in the atmosphere, microwaves only travel
line-of-sight, typically limited to 30-40 miles on the Earth's surface.
Another important property of microwave radiation is that it's
absorbed by moisture. A phenomenon called rain fade occurs
at the high end of the microwave band.
Past 100 GHz, other gases in the atmosphere absorb the energy,
making air opaque in the microwave range, although transparent in the visible and
infrared region.
Band Designations
Because microwave radiation encompasses such a broad
wavelength/frequency range, it is subdivided into IEEE, NATO, EU or other radar
band designations:
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Uses
Microwaves are used primarily for communications, include analog
and digital voice, data, and video transmissions.
They are also used for radar (RAdio Detection and Ranging) for
weather tracking, radar speed guns, and air traffic control.
Radio telescopes use
large dish antennas to determine distances, map surfaces, and study radio
signatures from planets, nebulas, stars, and galaxies.
Microwaves are used to transmit thermal energy to heat food and
other materials.
Sources
Cosmic microwave background
radiation is a natural source of microwaves. The radiation is
studied to help scientists understand the Big Bang.
Stars, including the Sun, are natural microwave sources.
Under the right conditions, atoms and molecules can emit
microwaves.
Man-made sources of microwaves include microwave ovens, masers,
circuits, communication transmission towers, and radar.
Either solid state devices or special vacuum tubes may be used
to produce microwaves.
Examples of solid-state devices include masers (essentially
lasers where the light is in the microwave range), Gunn diodes, field-effect
transistors, and IMPATT diodes.
The vacuum tube generators use electromagnetic fields to
direct electrons in
a density-modulated mode, where groups of electrons pass through the device
rather than a stream. These devices include the klystron, gyrotron, and
magnetron.
Health Effects
Microwave radiation is called "radiation"
because it radiates outward and not because it's either radioactive or ionizing
in nature.
Low levels of microwave radiation are not known to produce
adverse health effects. However, some studies indicate long-term exposure may
act as a carcinogen.
Microwave exposure can cause cataracts, as dielectric heating
denatures proteins in the eye's lens, turning it milky.
While all tissues are susceptible to heating, the eye is
particularly vulnerable because it doesn't have blood vessels to modulate
temperature.
Microwave radiation is associated with the microwave auditory effect, in which microwave exposure
produces buzzing sounds and clicks. This is caused by thermal expansion within
the inner ear.
Microwave burns can occur in deeper tissue — not just on the
surface — because microwaves are more readily absorbed by tissue that contains
a lot of water.
However, lower levels of exposure produce heat without burns.
This effect may be used for a variety of purposes.
The United States military uses millimeter waves to repel
targeted persons with uncomfortable heat.
As another example, in 1955, James Lovelock reanimated frozen
rats using microwave diathermy.
Anne
Marie Helmenstine, Ph.D.
Introduction
Ph.D. in
biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge
National Laboratory.
Science
educator with experience teaching chemistry, biology, astronomy, and
physics at the high school, college, and graduate levels.
ThoughtCo
and About Education chemistry expert since 2001.
Widely-published
graphic artist, responsible for printable periodic tables and other
illustrations used in science.
Experience
Anne
Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education
since 2001, and other sciences since 2013. She taught chemistry, biology,
astronomy, and physics at the high school, college, and graduate levels.
She has worked as a research scientist and also abstracting and indexing
diverse scientific literature for the Department of Energy.
In
addition to her work as a science writer, Dr. Helmenstine currently serves as a
scientific consultant, specializing in problems requiring an interdisciplinary
approach. Previously, she worked as a research scientist and college
professor.
Education
Dr.
Helmenstine holds a Ph.D. in biomedical sciences from the University of
Tennessee at Knoxville and a B.A. in physics and mathematics with a minor
in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine
developed ultra-sensitive chemical detection and medical diagnostic tests.
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and Dotdash
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