Encyclopedia of Earth and Physical Sciences - Vol. 7

By Joyce Tavolacci | Go to book overview

MICROWAVES
Microwaves are radio waves of very
short wavelength that are vital in many
modern technologies

Cell phones are microwave
transmitting and receiving
devices. They send and
receive microwave signals
to large antennae in the
local area.

CONNECTIONS
Microwave energy
called COSMIC
RADIATION
was
left behind after the
huge explosion (BIG
BANG)
that is thought
to have created the
UNIVERSE.
Microwave
SPECTROSCOPY is
used to study molecular
structure.

Microwaves constitute a hand of the electromagnetic spectrum beginning at the short end of the radio waves region and extending nearly to the infrared region. Like all electromagnetic radiation, microwaves travel at the speed of light, 186,000 miles per second (300,000 km/s). They have wavelengths of between about 12 inches (30 cm) and

inches (1 mm), equivalent to frequencies from 1 gigahertz (GHz, or a billion hertz) to 300 GHz. (Visible light has wavelengths in millionths of inches |nanometers], and radio signals are carried by wavelengths yards long; see MEASUREMENT.)


Microwave communications

Most microwave frequencies penetrate the atmosphere with little scattering and are easily focused. Like radio waves, they can be used to convey information and so are excellent for communications. However, they are not reflected by the upper atmosphere (see IONOSPHERE), as are longer radio waves, so microwave transmitters and receivers must be in line of sight of each other.

A microwave transmitter includes a generator, a modulator, and an amplifier. Signals are sent through a hollow tube, or wave guide, to a dish-shaped antenna, which focuses the signal into a beam. A receiving antenna directs the signal through a wave guide to components that either retransmit it or translate it via communications equipment.

Microwaves can carry hundreds of times more information than the longer radio wavelengths used in broadcasting, and they can deliver radio, television, and telephone communications at high rates. Since the 1960s communications systems increasingly have used satellites to relay microwave transmissions instead of ground-based stations for long range communications. Space agencies control interplanetary probes with microwave signals, and the probes send back pictures and other information.


Radar and other uses

Radar (radio detection and ranging) was developed by British scientists during World War II to warn of approaching German bombers. Since then, radar has been installed on aircraft, ships, and spacecraft, as well as in ground stations to aid in navigation. It allows the craft to detect other craft. It reveals ground formations at night or in bad weather and tracks storms. A radar transmitter antenna sends out powerful microwave radiation in narrow beams, usually as pulses a few microseconds long. When the beam strikes an object, part of it is reflected toward the transmitter as an echo. A receiving antenna converts the echo to an image on a screen. Computerized radar systems can reveal the speed of objects as well as their position, elevation, and course.

Microwave ovens have a generator, called a magnetron, that scatters radiation in the oven compartment. A magnetron is a small tube with a large potential difference between the cathode and the anode. The cathode emits microwaves. The microwaves heat the inside of the food, because the molecules, especially water molecules, vibrate as they absorb the radiation and heat is an increase in molecular vibration. The molecules in paper, glass, and ceramic containers have a rigid molecular structure; they do not absorb the microwaves and so do not heat. Foil or metal cannot be used in microwave ovens because it interferes with the radiation.


Generation of microwaves

Microwave technology depends upon devices that generate and amplify microwaves. The microwaves are generated by devices that produce rapidly oscillating electron currents. These devices include klystrons and magnetrons. Klystrons are special beam tubes that use an axial magnetic field to hold the beam together; magnetrons are similar tubes that have a cross-magnetic field, and the electrons spiral from the cathode to the anode.

The maser (microwave amplification by stimulated emission of radiation) operates by exciting

CORE FACTS
Most microwaves pass easily through the atmosphere
because of their short wavelengths. They are not
reflected by the upper atmospheric layers.
Microwave relays, either ground-based stations or
satellites, can send telecommunication signals efficiently
over long distances.
Radar bounces microwaves off objects to detect their
distance, speed, and elevation.

-952-

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