Super high frequency

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Super high frequency
Frequency range
3 to 30 GHz
Wavelength range
10 to 1 cm
Related bands

Super high frequency (SHF) is the

microwave hyperthermy to treat cancer, and to cook food in microwave ovens
.

Frequencies in the SHF range are often referred to by their

NATO
or EU designations.

Propagation

A variety of parabolic antennas on a communications tower in Australia for point-to-point microwave communication links. Some have white plastic radomes over their apertures to protect against rain.
X-band (8 - 12 GHz) marine radar antenna on a ship
. The rotating bar sweeps a vertical fan-shaped beam of microwaves around the water surface to the horizon, detecting nearby ships and other obstructions

frequency reuse by nearby transmitters. They are also used for communication with spacecraft since the waves are not refracted (bent) when passing through the ionosphere
like lower frequencies.

The

beamwidths possible with high gain antennas and the low atmospheric attenuation as compared with higher frequencies make SHF the main frequencies used in radar
. Attenuation and scattering by moisture in the atmosphere increase with frequency, limiting the use of high SHF frequencies for long range applications.

Small amounts of microwave energy are randomly scattered by water vapor molecules in the

troposcatter communications systems, operating at a few GHz, to communicate beyond the horizon. A powerful microwave beam is aimed just above the horizon; as it passes through the tropopause
some of the microwaves are scattered back to Earth to a receiver beyond the horizon. Distances of 300 km can be achieved. These are mainly used for military communication.

Antennas

Microwaves are often carried by waveguide, such as this example from an air traffic control radar, since other types of cable have large power losses at SHF frequencies.

The

inverted F antenna (PIFA) consisting of a monopole antenna bent in an L shape, fabricated of copper foil on the printed circuit board inside the device. Small sleeve dipoles or quarter-wave monopoles are also used. The patch antenna
is another common type, often integrated into the skin of aircraft.

The wavelengths are also small enough that SHF waves can be focused into narrow beams by

phase shifter
, which allows the array's beam to be steered electronically. The short wavelength requires great mechanical rigidity in large antennas, to ensure that the radio waves arrive at the feed point in phase.

Waveguide

At microwave frequencies, the types of cable (

waveguide must be used. Because of the high cost and maintenance requirements of long waveguide runs, in many microwave antennas the output stage of the transmitter or the RF front end
of the receiver is located at the antenna.

Advantages

SHF frequencies occupy a "sweet spot" in the radio spectrum which is currently being exploited by many new radio services.[3] They are the lowest frequency band where radio waves can be directed in narrow beams by conveniently sized antennas so they do not interfere with nearby transmitters on the same frequency, allowing frequency reuse. On the other hand, they are the highest frequencies which can be used for long distance terrestrial communication; higher frequencies in the EHF (millimeter wave) band are highly absorbed by the atmosphere, limiting practical propagation distances to one kilometer or less. The high frequency gives microwave communication links a very large information-carrying capacity (bandwidth). In recent decades many new solid state sources of microwave energy have been developed, and microwave integrated circuits for the first time allow significant signal processing to be done at these frequencies. Sources of EHF energy are much more limited and in an earlier state of development.

See also

References

  1. ^ Freedman, S. (September 1946). "Two-way radio for everyone" (PDF). Radio News. 36 (3). New York: Ziff-Davis Publications: 25–27. Retrieved March 24, 2014. This article from the beginning of the microwave era predicted the future value of microwaves for point-to-point communication.
  2. .
  3. .

External links