Horizontal layer that propagates electromagnetic radiation
For other uses, see Duct (disambiguation).
In telecommunications, an atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals (and light rays) are guided or ducted, tend to follow the curvature of the Earth, and experience less attenuation in the ducts than they would if the ducts were not present. The duct acts as an atmospheric dielectric waveguide and limits the spread of the wavefront to only the horizontal dimension.[1]
Atmospheric ducting is a mode of propagation of electromagnetic radiation, usually in the lower layers of Earth’s atmosphere, where the waves are bent by atmospheric refraction.[2] In over-the-horizon radar, ducting causes part of the radiated and target-reflection energy of a radar system to be guided over distances far greater than the normal radar range. It also causes long-distance propagation of radio signals in bands that would normally be limited to line of sight.
Normally radio "ground waves" propagate along the surface as creeping waves. That is, they are only diffracted around the curvature of the earth. This is one reason that early long-distance radio communication used long wavelengths. The best known exception is that HF (3–30 MHz.) waves are reflected by the ionosphere.
The reduced refractive index due to lower densities at the higher altitudes in the Earth's atmosphere bends the signals back toward the Earth. Signals in a higher refractive index layer, i.e., duct, tend to remain in that layer because of the reflection and refraction encountered at the boundary with a lower refractive index material. In some weather conditions, such as inversion layers, density changes so rapidly that waves are guided around the curvature of the earth at constant altitude.
Phenomena of atmospheric optics related to atmospheric ducting include the green flash, Fata Morgana, superior mirage, mock mirage of astronomical objects and the Novaya Zemlya effect.[1]
^ abThomas, Michael Frederic (2006). Optical propagation in linear media: atmospheric gases and particles, solid state components, and water. Oxford [Oxfordshire]: Oxford University Press. pp. 327–328. ISBN 0-19-509161-2.
^Navy Supplement to the DOD Dictionary of Military and Associated Terms(PDF). Department Of The Navy. August 2006. pp. 2–25. NTRP 1-02.[permanent dead link]
In telecommunications, an atmosphericduct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that...
Earth–ionosphere waveguide, a type of atmosphericduct Surface duct, a sound propagation phenomenon at sea Duct bank, a set of electrical conduits or...
propagation near the Earth's surface has multiple causes, including atmosphericducting, ionospheric reflection and refraction, and reflection from water...
temperatures in a steep thermal inversion where an atmosphericduct has formed. A thermal inversion is an atmospheric condition where warmer air exists in a well-defined...
The rays will bend and form arcs. An observer needs to be within an atmosphericduct to be able to see a Fata Morgana. Fata Morgana mirages may be observed...
airbreathing jet engine (or ducted jet engine) is a jet engine in which the exhaust gas which supplies jet propulsion is atmospheric air, which is taken in...
control. Atmosphericducting where distant thunder or other sounds are propagated across long distances due to travel through distinct atmospheric layers...
that is atmospherically powered. Lightweight trains ride on rails mounted on an elevated hollow concrete box girder that forms the air duct. Each car...
layers of different temperatures in a steep thermal inversion where an atmosphericduct has formed. The Italian term Fata Morgana is derived from the name...
Official UFO investigations in France suggest: As is well known, atmosphericducting is the explanation for certain optical mirages, and in particular...
layers of different temperatures in a steep thermal inversion where an atmosphericduct has formed. The Italian name Fata Morgana is derived from the name...
receiving antenna by two or more paths. Causes of multipath include atmosphericducting, ionospheric reflection and refraction, and reflection from water...
Furthermore, layers with an inverse trend of temperature or humidity cause atmosphericducting, which bends the beam downward or even traps radio waves so that...
refraction (such as through the foliage of a tree) and atmospheric effects such as atmosphericducting and ionospheric reflection. Since the various paths...
turbine engine which achieves mechanical energy from combustion, and a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards...
An atmospheric diving suit (ADS) is a small one-person articulated submersible which resembles a suit of armour, with elaborate pressure joints to allow...
green from a fraction of a second to a couple of seconds. In addition to atmospheric refraction and lensing, another primary cause of the Green Flash may...
organ (VNO, or Jacobson's organ) located above the roof of the mouth via a duct which exits just behind the front teeth of the animal. The word originates...
conditioned space. The fluid flow rate in a duct can then be estimated from: Volume flow rate (cubic feet per minute) = duct area (square feet) × flow velocity (feet per minute)...
GEPAN/SEPRA, the official UFO investigation in France, As is well known, atmosphericducting is the explanation for certain optical mirages, and in particular...
an additional fan at the front of the engine, which accelerates air in a duct bypassing the core gas turbine engine. Turbofans are the dominant engine...