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Mariner 1

Mariner 1

Mariner 1 was the first spacecraft of the Mariner program. Intended to fly by Venus, it failed during launch on July 22, 1962. A hardware failure in an antenna caused the booster to lose contact with guidance systems on the ground, so an onboard computer assumed control. However, that computer's software contained a bug. The error had occurred when an equation was being transcribed by hand in the specification for the guidance program. The writer missed the superscript bar in

\bar\dot_n

(the nth smoothed value of the time derivative of a radius). Without the smoothing function indicated by the bar, the program treated normal minor variations of velocity as if they were serious, causing spurious corrections that sent the rocket off course. It was then destroyed by the range safety officer. (It is sometimes claimed that the bug consisted a period typed in place of a comma, causing a FORTRAN statement of the form "DO 17 I = 1, 10" to be interpreted as "DO17I = 1.10" (an assignment to a variable called DO17I), since space characters are not significant in that language. There was in fact such a bug in a NASA orbit computation program at about this time, but it was a program for Project Mercury, not Mariner, and this bug was discovered before serious consequences occurred.) The probe's mission was later completed by Mariner 2.

Detailed Description

This was to be the first Mariner mission. It was intended to perform a Venus flyby. The vehicle was destroyed by the Range Safety Officer 293 seconds after launch at 09:26:16 UT when it veered off course. The booster had performed satisfactorily until an unscheduled yaw-lift (northeast) maneuver was detected by the range safety officer. Faulty application of the guidance commands made steering impossible and were directing the spacecraft towards a crash, possibly in the North Atlantic shipping lanes or in an inhabited area. The destruct command was sent 6 seconds before separation, after which the launch vehicle could not have been destroyed. The radio transponder continued to transmit signals for 64 seconds after the destruct command had been sent. The failure was apparently caused by a combination of two factors. Improper operation of the Atlas airborne beacon equipment resulted in a loss of the rate signal from the vehicle for a prolonged period. The airborne beacon used for obtaining rate data was inoperative for four periods ranging from 1.5 to 61 seconds in duration. Additionally, the Mariner 1 Post Flight Review Board determined that the omission of a hyphen in coded computer instructions in the data-editing program allowed transmission of incorrect guidance signals to the spacecraft. During the periods the airborne beacon was inoperative the omission of the hyphen in the data-editing program caused the computer to incorrectly accept the sweep frequency of the ground receiver as it sought the vehicle beacon signal and combined this data with the tracking data sent to the remaining guidance computation. This caused the computer to swing automatically into a series of unnecessary course corrections with erroneous steering commands which finally threw the spacecraft off course.

Spacecraft and Subsystems

The Mariner 1 spacecraft was identical to Mariner 2, launched 27 August 1962. Mariner 1 consisted of a hexagonal base, 1.04 meters across and 0.36 meters thick, which contained six magnesium chassis housing the electronics for the science experiments, communications, data encoding, computing, timing, and attitude control, and the power control, battery, and battery charger, as well as the attitude control gas bottles and the rocket engine. On top of the base was a tall pyramid-shaped mast on which the science experiments were mounted which brought the total height of the spacecraft to 3.66 meters. Attached to either side of the base were rectangular solar panel wings with a total span of 5.05 meters and width of 0.76 meters. Attached by an arm to one side of the base and extending below the spacecraft was a large directional dish antenna. The Mariner 1 power system consisted of the two solar cell wings, one 183 cm by 76 cm and the other 152 cm by 76 cm (with a 31 cm dacron extension (a solar sail) to balance the solar pressure on the panels) which powered the craft directly or recharged a 1000 Watt-hour sealed silver-zinc cell battery, which was to be used before the panels were deployed, when the panels were not illuminated by the Sun, and when loads were heavy. A power-switching and booster regulator device controlled the power flow. Communications consisted of a 3 Watt transmitter capable of continuous telemetry operation, the large high gain directional dish antenna, a cylindrical omnidirectional antenna at the top of the instrument mast, and two command antennas, one on the end of either solar panel, which received instructions for midcourse maneuvers and other functions. Propulsion for midcourse maneuvers was supplied by a monopropellant (anhydrous hydrazine) 225 N retro-rocket. The hydrazine was ignited using nitrogen tetroxide and aluminum oxide pellets, and thrust direction was controlled by four jet vanes situated below the thrust chamber. Attitude control with a 1 degree pointing error was maintained by a system of nitrogen gas jets. The Sun and Earth were used as references for attitude stabilization. Overall timing and control was performed by a digital Central Computer and Sequencer. Thermal control was achieved through the use of passive reflecting and absorbing surfaces, thermal shields, and movable louvers. The scientific experiments were mounted on the instrument mast and base. A magnetometer was attached to the top of the mast below the omnidirectional antenna. Particle detectors were mounted halfway up the mast, along with the cosmic ray detector. A cosmic dust detector and solar plasma spectrometer detector were attached to the top edges of the spacecraft base. A microwave radiometer and an infrared radiometer and the radiometer reference horns were rigidly mounted to a 48 cm diameter parabolic radiometer antenna mounted near the bottom of the mast. Total research, development, launch, and support costs for the Mariner series of spacecraft (Mariners 1 through 10) was approximately $554 million.

Mariner program

The Mariner program was a series of unmanned interplanetary probes designed to investigate Mars, Venus and Mercury. The program included a number of firsts, including the first planetary flyby, the first planetary orbiter, and the first gravity assist. Of the ten vehicles in the Mariner series, seven were successful and three were lost. The planned Mariner 11 and 12 vehicles evolved into Voyager 1 and Voyager 2 of the Voyager program.

Mariners 1 and 2

Mariner 1, intended to fly by Venus, failed shortly after launch. Mariner 2 was built as a backup to Mariner 1. It was launched on August 27, 1962, sending it on a 3½-month flight to Venus. The mission was a success and Mariner 2 became the first spacecraft to fly by another planet.
- Mission: Venus flyby
- Mass: 203 kg (446 lb)
- Sensors: microwave and infrared radiometers, cosmic dust, solar plasma and high-energy radiation, magnetic fields

Mariners 3 and 4

Mariner 3 and Mariner 4 were Mars flyby missions. Mariner 3 was lost when the launch vehicle's nose fairing failed to jettison. Its sister ship, Mariner 4, launched on November 28, 1964, the first successful flyby of the planet Mars and gave the first glimpse of Mars at close range.
- Mission: Mars flyby
- Mass: 261 kg (575 lb)
- Sensors: camera with digital tape recorder (about 20 pictures), cosmic dust, solar plasma, trapped radiation, cosmic rays, magnetic fields, radio occultation and celestial mechanics

Mariner 5

The Mariner 5 spacecraft was launched to Venus on June 14, 1967 and arrived in the vicinity of the planet in October 1967. It carried a complement of experiments to probe Venus' atmosphere with radio waves, scan its brightness in ultraviolet light, and sample the solar particles and magnetic field fluctuations above the planet.
- Mission: Venus flyby
- Mass: 245 kg (540 lb)
- Sensors: ultraviolet photometer, cosmic dust, solar plasma, trapped radiation, cosmic rays, magnetic fields, radio occultation and celestial mechanics

Mariners 6 and 7

Mariners 6 and 7 were identical teammates in a two-spacecraft mission to Mars. Mariner 6 was launched on February 24, 1969, followed by Mariner 7 on March 27, 1969. They flew over the equator and southern hemisphere of the planet Mars.
- Mission: Mars flybys
- Mass 413 kg (908 lb)
- Sensors: wide- and narrow-angle cameras with digital tape recorder, infrared spectrometer and radiometer, ultraviolet spectrometer, radio occultation and celestial mechanics.

Mariners 8 and 9

Mariner 8 and Mariner 9 were identical sister craft designed to map the Martian surface simultaneously, but Mariner 8 was lost in a launch vehicle failure. Its identical sister craft, Mariner 9, was launched in May 1971 and became the first artificial satellite of Mars. It entered Martian orbit in November 1971 and began photographing the surface and analyzing the atmosphere with its infrared and ultraviolet instruments.
- Mission: orbit Mars
- Mass 998 kg (2,200 lb)
- Sensors: wide- and narrow-angle cameras with digital tape recorder, infrared spectrometer and radiometer, ultraviolet spectrometer, radio occultation and celestial mechanics

Mariner 10

The Mariner 10 spacecraft launched on November 3, 1973 and was the first to use a gravity assist trajectory, accelerating as it entered the gravitational influence of Venus, then being flung by the planet's gravity onto a slightly different course to reach Mercury. It was also the first spacecraft to encounter two planets at close range, and the first (and so far only) spacecraft to photograph Mercury in closeup.
- Mission: Venus and Mercury flybys
- Mass: 433 kg (952 lb)
- Sensors: twin narrow-angle cameras with digital tape recorder, ultraviolet spectrometer, infrared radiometer, solar plasma, charged particles, magnetic fields, radio occultation and celestial mechanics Category:Mars missions Category:Mercury spacecraft Category:Venus spacecraft Category:NASA programs ja:マリナー計画

Venus (Planet)

Venus, the second planet from the Sun, is named after the Roman goddess Venus. A terrestrial planet, it is sometimes called Earth's "sister planet", as the two are very similar in size and bulk composition. Although all planets' orbits are elliptical, Venus's orbit is the closest to circular, with an eccentricity of less than 1%. As Venus is closer to the Sun than the Earth, it always appears in roughly the same direction from Earth as the Sun (the greatest elongation is 47.8°), so on Earth it can usually only be seen a few hours before sunrise or a few hours after sunset. However, when at its brightest, Venus may be seen during the daytime, making it one of only two heavenly bodies that can be seen both day and night (the other being the Moon). It is sometimes referred to as the "Morning Star" or the "Evening Star", and when it is visible in dark skies it is by far the brightest star-like object in the sky. The cycle between one maximum elongation and the next lasts 584 days. After these 584 days Venus is visible in a position 72 degrees away from the previous one. Since 5
- 584 = 2920, which is equivalent to 8
- 365 Venus returns to the same point in the sky every 8 years (minus two leap days). This was known as the Sothis cycle in ancient Egypt, and was familiar to the Maya as well. Another association is with the Moon, because 2920 days equal almost exactly 99 lunations (29.5
- 99 = 2920.5). Venus has a very slow retrograde rotation, meaning that, unlike with most planets, the direction of rotation (around its axis) is the opposite of its orbital rotation (around the Sun). The very slow rotation means that the distinction between the Sidereal day (rotation relative to the stars) and the Solar day (relative to the Sun) is very significant. Solar day The pentagram has long been associated with the planet Venus and the worship of the goddess Venus, or her equivalent. It is most likely to have originated from the observations of prehistoric astronomers. When viewed from Earth, the successive conjunctions of Venus plot the points of a pentagram around the Sun every eight years, returning to its starting point after a forty year cycle. Venus was known to ancient Babylonians around 1600 BC, and to the Mayan civilization (the Mayans developed a religious calendar based on Venus's motion) and must have been known long before in prehistoric times, given that it is the third brightest object in the sky after the Sun and Moon. The Maasai people in Africa named the planet Kileken, and have a myth about it called "The Orphan Boy." The Morning Star was called the Bearer of Light ("phōsphoros" or "eōsphoros" in Greek and "Lucifer" in Latin, a term later used of the fallen angel cast out of heaven, see Isaiah 14:12). To the Jews it was known as Noga ("shining") and it was used in rabbinic literature as a symbol of beauty and purity Isaiah Its symbol is the sign also used in biology for the female sex, a stylized representation of the goddess Venus's hand mirror: a circle with a small cross underneath (Unicode: ♀). The Venus symbol also represents femininity, and in ancient alchemy stood for copper. Alchemists constructed the symbol from a circle (representing spirit) above a cross (representing matter). The association with sex and femininity is supposed to relate to the period of 266 days between the conjunction and maximum elongation of Venus, which corresponds more or less to the length of human pregnancy. The adjective Venusian is commonly used for Venus, but it is etymologically incorrect. The true adjective coming from Latin, Venereal, is avoided because of its modern association with sexually transmitted diseases. Some astronomers use Cytherean, which comes from Cythera. Other less common adjectives include Venerean, Venerian, and Veneran. The Chinese, Korean, Japanese and Vietnamese cultures refer to the planet as the metal star, 金星, based on the Five Elements.

Physical characteristics

Atmosphere

Venus has an atmosphere consisting mainly of carbon dioxide and a small amount of nitrogen, with a pressure at the surface about 90 times that of Earth (a pressure equivalent to a depth of 1 kilometer under Earth's oceans); its atmosphere is also roughly 90 times more massive than ours. This enormously CO₂-rich atmosphere results in a strong greenhouse effect that raises the surface temperature more than 400 °C (750 °F) above what it would be otherwise, causing temperatures at the surface to reach extremes as great as 500 °C (930 °F) in low elevation regions near the planet's equator. This makes Venus's surface hotter than Mercury's, even though Venus is nearly twice as distant from the Sun and only receives 25% of the solar irradiance (2613.9 W/m² in the upper atmosphere, and just 1071.1 W/m² at the surface). Owing to the thermal inertia and convection of its dense atmosphere, the temperature does not vary significantly between the night and day sides of Venus despite its extremely slow rotation of less than one rotation per Venusian year, meaning that, at the equator, Venus' surface rotates at a mere 6.5 km/h (4 mph). Upper atmosphere winds circling the planet approximately every 4 days help distribute the heat to other areas on the surface. The solar irradiance is so much lower at the surface of Venus because the planet's thick cloud cover reflects the majority of the sunlight back into space. This prevents most of the sunlight from ever heating the surface. Venus's bolometric albedo is approximately 60%, and its visual light albedo is even greater. Thus, despite being closer to the Sun than Earth, the surface of Venus is not as well heated and even less well lit by the Sun. In the absence of any greenhouse effect, the temperature at the surface of Venus would be quite similar to Earth. A common conceptual misunderstanding regarding Venus is the mistaken belief that its thick cloud cover traps heat, as the opposite is actually true. The cloud cover keeps the planet much cooler than it would be otherwise. The immense quantity of CO₂ in the atmosphere is what traps the heat by the greenhouse mechanism. There are strong 300 km/h (200 mph) winds at the cloud tops, but winds at the surface are very slow, no more than a few miles per hour. However, owing to the high density of the atmosphere at Venus's surface, even such slow winds exert a significant amount of force against obstructions. The clouds are mainly composed of sulfur dioxide and sulfuric acid droplets and cover the planet completely, obscuring any surface details from the human eye. The temperature at the tops of these clouds is approximately −45 °C (−50 °F). The mean surface temperature of Venus, as given by NASA, is 464 °C (864 °F). The minimal value of the temperature, listed in the table, refers to cloud tops —the surface temperature is never below 400 °C (750 °F). (This makes the surface temperature hot enough to melt lead.) The atmosphere also contains hydrogen sulfide (H₂S) and carbonyl sulfide (SCO). Hydrogen sulfide reacts with sulfur dioxide, which implies that some process must be creating these components. It is unclear how the carbonyl sulfide could be formed--it is often a sign of biological activity. Some have suggested that microbes exist in the clouds (which also contain droplets of water), and produce these components from water, carbon monoxide and sulfur dioxide. [http://www.newscientist.com/article/mg17523621.800.html New Scientist, Sept. 28, 2002, p. 16]

Surface features

sulfur dioxide Venus has slow retrograde rotation, meaning it rotates from east to west, instead of west to east as most of the other major planets do. (Pluto and Uranus also have retrograde rotation, though Uranus's axis, tilted at 97.86 degrees, almost lies in its orbital plane.) It is not known why Venus is different in this manner, although it may be the result of a collision with a very large asteroid at some time in the distant past. If the Sun could be seen from Venus' surface, it would appear to rise and set in a 116.75 day cycle (Venus' synodic rotation period), and a Venusian year would thus last 1.92 Venusian "days". In addition to this unusual retrograde rotation, the periods of Venus' rotation and of its orbit are synchronized in such a way that it always presents the same face toward Earth when the two planets are at their closest approach (5.001 Venusian days between each inferior conjunction). This may simply be a coincidence, but there is some speculation that this may be the result of tidal locking, with tidal forces affecting Venus' rotation whenever the planets get close enough together —although the tides raised by Earth on Venus are vanishingly small. Venus has two major continent-like highlands on its surface, rising over vast plains. The northern highland is named Ishtar Terra and has Venus's highest mountains, named the Maxwell Montes (roughly 2 km taller than Mount Everest) after James Clerk Maxwell, which surround the plateau Lakshmi Planum. Ishtar Terra is about the size of Australia. In the southern hemisphere is the larger Aphrodite Terra, about the size of South America. Between these highlands are a number of broad depressions, including Atalanta Planitia, Guinevere Planitia, and Lavinia Planitia. With only the exception of Maxwell Montes, all surface features on Venus are named after real or mythological females. Venus' thick atmosphere causes meteors to decelerate as they fall toward the surface, and even large meteors will strike the surface at too low a speed to form an impact crater if they have less than a certain threshold kinetic energy. Because of this, no impact crater smaller than about 3 km (2 mi) in diameter can form. Nearly 90% of Venus's surface appears to consist of recently (in the geological sense) solidified basaltic lava, with very few meteorite craters. The oldest features present on Venus seem to be only around 800 million years old, with most of the terrain being considerably younger (though still not less than several hundred million years for the most part). This suggests that Venus underwent a major resurfacing event in the not too distant geological past. The interior of Venus is probably similar to that of Earth: an iron core about 3000 km in radius, with a molten rocky mantle making up the majority of the planet. Recent results from the Magellan gravity data indicate that Venus's crust is stronger and thicker than had previously been assumed. It is theorized that Venus does not have mobile plate tectonics as Earth does, but instead undergoes massive volcanic upwellings at regular intervals that inundate its surface with fresh lava. Other recent findings suggest that Venus is still volcanically active in isolated geological hotspots. Venus's intrinsic magnetic field has been found very weak compared to other planets in the solar system. This may be due to its slow rotation being insufficient to drive an internal dynamo of liquid iron. As a result, solar wind strikes Venus's upper atmosphere without mediation. It is thought that Venus originally had as much water as Earth, but that under the Sun's assault water vapor in the upper atmosphere was split into hydrogen and oxygen, with the hydrogen escaping into space owing to its low molecular mass; the ratio of hydrogen to deuterium (a heavier isotope of hydrogen which doesn't escape as quickly) in Venus's atmosphere seems to support this theory. Molecular oxygen is thought to have combined with atoms in the crust (large amounts of oxygen, however, remain in the atmosphere in the form of carbon dioxide). Because of their dryness, Venus's rocks are much harder than Earth's, which leads to steeper mountains, cliffs and other features.

Venus' moon

Venus was once thought to possess a moon, named Neith after the chief goddess of Sais, Egypt (whose veil no mortal raised), first observed by Giovanni Domenico Cassini in 1672. German astronomers called the moon Kleinchen (literally "tiny"), and sporadic sightings by astronomers continued until 1892. These sightings have since been discredited, and are thought to have been either spurious internal reflections, mostly faint stars that happened to be in the right place at the right time, or maybe even asteroids passing by the planet. Venus is now known to be moonless.

Observations and explorations of Venus

Venus has been observed several times within the past 4000 years by a number of people, including the Greeks.

Appearance

Cultural references

:See also Venus in fiction Until it was penetrated by probes, Venus's opaque cloud layer gave science fiction writers free rein in imagining the planet's surface, and they frequently imagined it to be Earthlike. There are some religious sects who believe that Hell may be located on Venus. Its extremely high surface temperature and impenetrable cloud cover cause people to believe that the fires of Hell burn on the surface, obscured from our earthly view. Conversely, other sects consider Venus to be some form of paradise or an advanced secret base for angels/aliens to operate from.
- In Olaf Stapledon's epic Last and First Men (1930), Venus is an oceanic idyll where humans evolve the power of flight.
- In the mythology of Middle-earth (1937), by J. R. R. Tolkien, Venus is the Star of Eärendil. The star was created when Eärendil the Mariner was set in the sky on his ship, with a Silmaril bound to his brow. In fact, Tolkien chose the name directly from the ancient Old English word for the planet Venus.
- In H. P. Lovecraft's Cthulhu Mythos (1928–), there are mentions of the 'Lords of Venus', and conflicting indications that the Serpent People originated there.
- Edgar Rice Burroughs wrote a series of five books on Venus, featuring hero Carson Napier, who discovers that Venus (or Amtor, as it is known by the Venusians) is a world of sky-high trees, warring kingdoms and princesses in need of rescue. [http://www.tarzan.com/worlds/amtor.html]
- The H. P. Lovecraft and Kenneth Sterling short story 'In the Walls of Eryx' (1939), takes place on Venus, but is not considered part of the Cthulhu Mythos.
- The second book of the Space Trilogy (1938–1945) by C.S. Lewis, Perelandra 1943) takes place on Venus (called by the natives Perelandra), the site of a second garden of Eden.
- In the military science fiction classic Clash by Night (1943) by Henry Kuttner (writing as Lawrence O'Donnell) and C. L. Moore, underwater city-states hire mercenary companies and their battleships to fight their wars on the surface.
- Venus was the home planet of the Mekon, arch-enemy of the 1950s comic book hero Dan Dare.
- Many science-fiction movies and serials of the '50s and '60s, such as Abbott and Costello Go to Mars and Space Patrol, have used Venus' namesake goddess and her domain to contrive planetary populations of nubile women welcoming (or attacking) all-male astronaut crews.
- In the Noon Universe created by the Soviet science fiction writers Boris and Arkady Strugatsky, Venus is depicted as a extremely harsh planet covered by strange flora and fauna but also very rich in minerals and heavy metals. The novel The Land of Crimson Clouds (Strana Bagrovykh Tuch in the original) describes the first successful manned mission to Venus, although a full-scaled colonization of the planet was not initiated until much later (in 2119; see Noon: 22nd Century).
- Venus is the location of several Starfleet Academy training facilities and terraforming stations in the fictional Star Trek universe (1966–).
- In Jacqueline Susann's Yargo (1979), Venus is inhabited by bees that are as big as horses.
- Venus is briefly mentioned in Arthur C. Clarke's 3001: The Final Odyssey (1997).
- A presumably terraformed Venus was the setting of one episode of the anime Cowboy Bebop (1998). In the show, Venus was revealed to be an arid but habitable world. Much of the population lived in floating cities in the sky. In the cartoon Exosquad, terraformed Venus was portrayed as one of the three habitable planets in the solar system (the others being Earth and Mars).
- In the Japanese anime series, Bishoujo Senshi Sailor Moon (1992), Sailor Venus is a soldier representing the planet of the same name. In mythology, Venus is the Roman goddess of love (Aphrodite in Greek), therefore, Sailor Venus's attacks and weapons (e.g. Venus Love Me Chain and Venus Love and Beauty Shock) represent the idea of love and femininity. Her image colours are gold and orange--similar to the colour of the planet. Also, on her forehead is the planet's symbol.
- A more scientifically accurate depiction of the planet is offered in Ben Bova's novel Venus (2000, ISBN 031287216X)-

References

- Arnett, Bill (2005). [http://www.nineplanets.org/venus.html Venus]. Retrieved March 27, 2005.
- European Space Agency (2005). [http://www.esa.int/SPECIALS/Venus_Express/ Venus Express overview]. Retrieved March 27, 2005.
- Grayzeck, Ed (2004). [http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html Venus Fact Sheet]. NASA. Retrieved March 27, 2005.
- Grieger, Bjoern (2004). [http://www.space-vision.biz/product.venuslandscape.de.html Picture “Real Venus”]. Retrieved March 27, 2005.
- The Maya Astronomy Page (2002). [http://www.michielb.nl/maya/venus.html Venus]. Retrieved March 27, 2005.
- Mitchell, Don P. (2004). [http://www.mentallandscape.com/V_Venus.htm The Soviet Exploration of Venus]. Retrieved March 27, 2005.
- Rosenthal, David. (2003). [http://www.ridgecrest.ca.us/~n6tst/maya/newpage.html THE SOUTHERNMOST RISE OF VENUS AT UXMAL, 1997]. Retrieved March 27, 2005.
- Vienna University of Technology (2004). [http://www.vias.org/spacetrip/venus_dimensionalviews.html Venus Three-Dimensional Views]. Retrieved March 27, 2005.
- [http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1996JBAA..106...16M]
- [http://www.ibiblio.org//e-notes/VRML/Globe/Globe.htm 3D VRML Venus globe]
- [http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html Venus Fact Sheet]

Pentagram

- http://www.mikecrowson.co.uk/pentagram.html
- http://www.symbols.com/encyclopedia/29/2914.html
- http://www.hyperflight.com/venus-five-pointed-star.htm
- [http://www.run4space.com/viewforum.php?f=8 Venus Forum]
- ko:금성 ms:Zuhrah ja:金星 simple:Venus (planet) th:ดาวศุกร์

July 22

22 July is the 203rd day (204th in leap years) of the year in the Gregorian Calendar, with 162 days remaining.

Events

- 1298 - Battle of Falkirk - Edward I (Longshanks) of England and his longbowmen defeat William Wallace and his scottish schiltrons outside the town.
- 1499 - Battle of Dornach - The Swiss decisively defeat the Imperial army of Emperor Maximilian I.
- 1587 - Colony of Roanoke: A second group of English settlers arrive on Roanoke Island off of North Carolina to re-establish the deserted colony.
- 1793 - Alexander Mackenzie reaches the Pacific Ocean becoming the first Euro-American to complete a transcontinental crossing north of Mexico.
- 1796 - Surveyors of the Connecticut Land Company name an area in Ohio "Cleveland" after Gen. Moses Cleaveland, the superintendent of the surveying party.
- 1805 - Napoleonic Wars: War of the Third Coalition - inconclusive battle of Cape Finisterre fought between a combined French and Spanish fleets under Admiral Pierre-Charles Villeneuve of Spain and a British fleet under Admiral Robert Calder.
- 1812 - Napoleonic Wars: Peninsular War - Battle of Salamanca - British forces led by Arthur Wellesley (later the Duke of Wellington) defeat French troops near Salamanca, Spain.
- 1864 - American Civil War: Battle of Atlanta - Outside of Atlanta, Georgia, Confederate General John Bell Hood leads an unsuccessful attack on Union troops under General William T. Sherman on Bald Hill.
- 1908 - Albert Fisher establishes the Fisher Body Company to manufacture carriage and automobile bodies.
- 1916 - In San Francisco, California, a bomb explodes on Market Street during a Preparedness Day parade killing 10 and injuring 40.
- 1933 - Wiley Post becomes first person to fly solo around the world traveling 15,596 miles in 7 days, 18 hours and 45 minutes.
- 1934 - Outside Chicago's Biograph Theatre, "Public Enemy No. 1" John Dillinger is mortally wounded by FBI agents.
- 1937 - New Deal: The United States Senate votes down President Franklin D. Roosevelt's proposal to add more justices to the Supreme Court of the United States.
- 1942 - The United States government begins compulsory civilian gasoline rationing due to the wartime demands.
- 1942 - Holocaust: The systematic deportation of Jews from the Warsaw Ghetto begins.
- 1943 - Allied forces capture the Italian city of Palermo.
- 1944 - The Polish Committee of National Liberation publishes its manifesto, starting the period of Communist rule in Poland
- 1946 - King David Hotel bombing: Irgun bombs King David Hotel in Jerusalem, headquarters of the British civil and military administration, killing 90.
- 1962 - Mariner program: Mariner 1 spacecraft flies erratically several minutes after launch and has to be destroyed.
- 1977 - Chinese leader Deng Xiaoping is restored to power.
- 1991 - Serial killer Jeffrey Dahmer is arrested after the remains of 11 men and boys are found in his Milwaukee apartment.
- 1992 - Near Medellín, Colombian drug lord Pablo Escobar escapes from his luxury prison fearing extradition to the United States.
- 1997 - The second Blue Water Bridge opens between Port Huron, Michigan and Sarnia, Ontario.
- 2002 - Israel assasinates Salah Shahade, the Commander-in-Chief of Hamas's military arm, Ezzedeen-al-qassam Brigades, along with 14 civilians.
- 2003 - Members of 101st Airborne of the United States, aided by Special Forces, attack a compound in Iraq, killing Saddam Hussein's sons Uday and Qusay, along with Mustapha Hussein, Qusay's 14-year old son, and a bodyguard.
- 2005 - A man is shot dead by police as the hunt begins for the London Bombers. See 7 July 2005 London bombings and 21 July 2005 London bombings
- 2005 - Microsoft releases the final name for its next-gen operating system, Longhorn. The name will be "Windows Vista".

Births

- 1210 - Joan of England, queen of Alexander II of Scotland (d. 1238)
- 1478 - King Philip I of Castile (d. 1506)
- 1510 - Alessandro de' Medici, Duke of Florence (d. 1537)
- 1519 - Pope Innocent IX (d. 1591)
- 1535 - Katarina Stenbock, queen of Gustav I of Sweden (d. 1621)
- 1559 - Lawrence of Brindisi, Italian monk (d. 1619)
- 1621 - Anthony Ashley-Cooper, 1st Earl of Shaftesbury, English politician (d. 1683)
- 1711 - Georg Wilhelm Richmann, Russian physicist (d. 1753)
- 1713 - Jacques-Germain Soufflot, French architect (d. 1780)
- 1733 - Mikhail Shcherbatov, Russian philosopher and writer (d. 1790)
- 1784 - Friedrich Bessel, German mathematician and astronomer (d. 1846)
- 1822 - Gregor Mendel, Austrian geneticist (d. 1884)
- 1844 - William Archibald Spooner, English priest and scholar (d. 1930)
- 1859 - Emma Lazarus, American poet (d. 1887)
- 1882 - Edward Hopper, American painter (d. 1967)
- 1887 - Gustav Ludwig Hertz, German physicist, Nobel Prize laureate (d. 1975)
- 1888 - Selman Waksman, Ukrainian-born biochemist, recipient of the Nobel Prize in Physiology or Medicine (d. 1973)
- 1893 - James Whale, English film director (d. 1957)
- 1898 - Stephen Vincent Benét, American author (d. 1943)
- 1898 - Alexander Calder, American artist (d. 1976)
- 1905 - Doc Cramer, baseball player (d. 1990)
- 1908 - Amy Vanderbilt, American author on etiquette (d. 1974)
- 1913 - Gorni Kramer, Italian bandleader and songwriter
- 1916 - Marcel Cerdan, French boxer (d. 1949)
- 1921 - William Roth, U.S. Senator (d. 2003)
- 1923 - Bob Dole, U.S. Senator from Kansas and Presidential candidate
- 1923 - Mukesh, Indian singer (d. 1976)
- 1924 - Margaret Whiting, singer
- 1928 - Orson Bean, American film actor
- 1932 - Oscar De la Renta, Dominican-born fashion designer
- 1934 - Louise Fletcher, American actress
- 1936 - Tom Robbins, American author
- 1939 - Terence Stamp, English actor
- 1940 - Alex Trebek, Canadian-born game show host
- 1941 - George Clinton, American musician
- 1941 - Ron Turcotte, Canadian jockey
- 1943 - Kay Bailey Hutchison, U.S. Senator from Texas
- 1944 - Estelle Bennett, American singer (Ronettes)
- 1944 - Rick Davies, British musician (Supertramp)
- 1944 - Sparky Lyle, baseball player
- 1946 - Mireille Mathieu, French singer
- 1946 - Stephen M. Wolownik, Russian musician (d. 2000)
- 1947 - Albert Brooks, American comedian
- 1947 - Danny Glover, American actor
- 1947 - Don Henley, American drummer, singer, and songwriter (Eagles)
- 1948 - S.E. Hinton, American author
- 1948 - Otto Waalkes, German comedian
- 1949 - Alan Menken, American composer
- 1949 - Lasse Virén, Finnish athlete
- 1954 - Lonette McKee, American actress
- 1954 - Al Di Meola, American guitarist
- 1955 - Willem Dafoe, American actor
- 1961 - Keith Sweat, American singer
- 1964 - Patrick Labyorteaux, American actor
- 1964 - John Leguizamo, Colombian actor
- 1964 - David Spade, American actor, comedian, and producer
- 1965 - Shawn Michaels, American professional wrestler
- 1966 - Tim Brown, American football player
- 1971 - Kristine Lilly, American soccer player
- 1972 - Keyshawn Johnson, American football player
- 1973 - Mike Sweeney, baseball player
- 1973 - Rufus Wainwright, American singer and songwriter
- 1974 - Daddy Kev, American record producer and engineer
- 1974 - Franka Potente, German actress
- 1978 - Dennis Rommedahl, Danish footballer
- 1980 - Scott Dixon, New Zealand race car driver
- 1980 - Dirk Kuyt, Dutch football striker
- 1983 - Shelby Belle, Canadian actress

Deaths

- 1362 - Louis of Durazzo, Italian soldier (poisoned) (b. 1324)
- 1387 - Franz Ackerman, Flemish statesman (b. 1330)
- 1461 - King Charles VII of France (b. 1403)
- 1525 - Richard Wingfield, English diplomat
- 1619 - Lawrence of Brindisi, Italian monk (b. 1559)
- 1645 - Gaspar de Guzmán y Pimentel, Count-Duke of Olivares, Spanish statesman (b. 1587)
- 1676 - Pope Clement X (b. 1590)
- 1713 - Jacques-Germain Soufflot, French architect (d. 1780)
- 1734 - Peter King, 1st Baron King, Lord Chancellor of England
- 1789 - Joseph-François Foulon, French administrator (executed) (b. 1715)
- 1802 - Marie François Xavier Bichat, French anatomist (b. 1771)
- 1832 - Emperor Napoleon II of France (b. 1811)
- 1852 - Auguste Marmont, French marshal (b. 1774)
- 1904 - Wilson Barrett, English actor (b. 1846)
- 1908 - William Randal Cremer, English politician and pacifist, recipient of the Nobel Peace Prize (b. 1828)
- 1916 - James Whitcomb Riley, American author and poet (b. 1849)
- 1922 - Jokichi Takamine, Japanese chemist (b. 1854)
- 1932 - Errico Malatesta, Italian anarchist (b. 1853)
- 1932 - Florenz Ziegfeld, theatrical producer (b. 1867)
- 1934 - John Dillinger, American bank robber (shot) (b. 1903)
- 1950 - William Lyon Mackenzie King, tenth Prime Minister of Canada (b. 1874)
- 1958 - Mikhail Zoshchenko, Russian writer (b. 1895)
- 1967 - Carl Sandburg, American poet (b. 1878)
- 1974 - Wayne Morse, U.S. Senator from Oregon (b. 1900)
- 1979 - Sándor Kocsis, Hungarian footballer (b. 1929)
- 1989 - Martti Talvela, Finnish bass (b. 1935)
- 1990 - Manuel Puig, Argentinian writer (b. 1932)
- 1998 - Hermann Prey, German bass-baritone (b. 1929)
- 2000 - Eric Christmas, British actor (b. 1916)
- 2003 - Qusay Hussein, son of Saddam Hussein (b. 1966)
- 2003 - Uday Hussein, son of Saddam Hussein (b. 1964)
- 2003 - Wahome Muthahi, Kenyan humourist
- 2004 - Sacha Distel, French singer (b. 1933)
- 2004 - George Kidd, Canadian diplomat (b. 1917)
- 2005 - Jean Charles de Menezes, Brazilian electrician (shot) (b. 1978)

Holidays and observances

- Saint Mary Magdalene
- Swaziland - Birthday of former King Sobhuza II
- Pi Approximation Day
- Ratcatcher's Day. See:The Pied Piper of Hamelin.

External links

- [http://news.bbc.co.uk/onthisday/hi/dates/stories/july/22 BBC: On This Day] ---- 21 July - 23 July - 22 June - 22 August -- listing of all days ko:7월 22일 ms:22 Julai ja:7月22日 simple:July 22 th:22 กรกฎาคม

Antenna (radio)

] Most simply, an antenna (U.S.) or aerial (UK) is an electronic component designed to transmit or receive radio waves. The words "antenna" and "aerial" are used throughout this article with precisely the same meaning. More specifically, an antenna is an arrangement of conductors designed to radiate (transmit) an electromagnetic field in response to an applied alternating electromotive force (EMF) and the associated alternating electric current. Alternatively, if an antenna is placed into an electromagnetic field, that field will induce an alternating current upon the antenna, and EMF between its terminals. See radio frequency induction.

Overview

There are two fundamental types of antennas. The first type is omni and the second type is directional. Omni type of antennas function in all possible directions whereas directional type of antennas work only in a single direction,i.e, "Line of Sight(LOS)". The first type couples to the electric field of an electromagnetic wave, and usually consists of a length of wire in which an electric charge moves back and forth (electric dipole). The second type couples to the magnetic field of an electromagnetic wave, and is usually a coil or loop of wire (magnetic dipole). By adding additional conducting rods or coils (called elements) and varying their length, spacing, and orientation, an antenna with specific desired properties can be created, such as a Yagi-Uda Antenna (often abbreviated to "Yagi"). Typically, antennas are designed to operate in a relatively narrow frequency range. The design criteria for receiving and transmitting antennas differ slightly, but generally an antenna can receive and transmit equally as well. This property is called reciprocity. The vast majority of antennas are simple vertical rods a quarter of a wavelength long. Such antennas are simple in construction, usually inexpensive, and both radiate in and receive from all horizontal directions (omnidirectional). One limitation of this antenna is that it does not radiate or receive in the direction in which the rod points. This region is called the antenna blind cone or null. Antennas have practical use for the transmission and reception of radio frequency signals (radio, TV, etc.), which can travel over great distances at the speed of light, and pass through nonconducting walls (although often there is a variable signal reduction depending on the type of wall, and natural rock can be very defective to radio signals).

Antenna effectiveness

Antennas may be omni and directional. There are several critical parameters that affect an antenna's performance and can be adjusted during the design process. These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties.

Resonant frequency

The resonant frequency is related to the electrical length of the antenna. This is usually the physical length of the wire multiplied by the ratio of the speed of wave propagation in the wire. Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on that resonant frequency. However, the other properties of the antenna (especially radiation pattern and impedance) change with frequency, so the antenna's resonant frequency may merely be close to the center frequency of these other more important properties. Antennas can be made resonant on harmonic frequencies and with lengths that are fractions of the target frequency. Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. The most commonly known type of wide band aerial is the logarithmic or log aerial but its gain is usually much lower than that of a specific or narrower band aerial.

Impedance

Impedance is similar to refractive index in optics. As the electric wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance. At each interface, some fraction of the wave's energy will reflect back to the source, forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface will reduce SWR and maximize power transfer through each part of the antenna system. Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.

Gain

capacitor An antenna has gain if it radiates more strongly in one direction than in another. Gain is measured by comparing an antenna to a model antenna, typically the isotropic antenna which radiates equally in all directions. Often a dipole is also used as a practical reference as the isotropic source cannot be realised in practice, but it has 2.1 dB gain over an isotropic source. Most practical antennas radiate more than the isotropic antenna in some directions and less in others. Gain is inherently directional; the gain of an antenna is usually measured in the direction which it radiates best. Gain is one-dimensional. Gain does not mean that the antenna radiates more power than is fed to it, merely that it distributes the power more strongly in some directions than in others. Aperture, and radiation pattern are closely related to gain. Aperture is the shape of the "beam" cross section in the direction of highest gain, and is two-dimensional. (Sometimes aperture is expressed as the radius of the circle that approximates this cross section or the angle of the cone.) Radiation pattern is the three-dimensional plot of the gain, but usually only the two-dimensional horizontal and vertical cross sections of the radiation pattern are considered. Antennas with high gain typically show side lobes in the radiation pattern. Side lobes are peaks in gain other than the main lobe (the "beam"). Side lobes detract from the antenna quality whenever the system is being used to determine the direction of a signal, as in radar systems.

Efficiency

Efficiency is the ratio of power actually radiated to the power put into the antenna terminals. A dummy load may have a SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates none, showing that SWR alone is not an effective measure of an antenna's efficiency. Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance.

Bandwidth

The bandwidth of an antenna is the range of frequencies over which it is effective, usually centered around the resonant frequency. The bandwidth of an antenna may be increased by several techniques, including using thicker wires, replacing wires with cages to simulate a thicker wire, tapering antenna components (like in a feed horn), and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna. Small antennas are usually preferred for convenience, but there is a fundamental limit relating bandwidth, size and efficiency. Of the parameters above, SWR is most easily measured. Impedance can be measured with specialized equipment, as it relates to the complex SWR. Measuring radiation pattern requires a sophisticated setup including significant clear space (enough to get into the antenna's far field) or an anechoic chamber designed for antenna measurements, careful study of experiment geometry, and specialised measurement equipment such as robots that rotate the antenna during the measurements. Bandwidth depends on the overall effectiveness of the antenna, so all of these parameters must be understood to understand bandwidth. However, typically bandwidth is measured by only looking at SWR, i.e., by finding the frequency range over which the SWR is less than a given value. Bandwidth over which an antenna exhibits a particular radiation pattern might also be considered.

Polarization

The polarization of an antenna or orientation of the radio wave is determined by the electric field or E-plane. The ionosphere changes the polarization of signals unpredictably, so for signals which will be reflected by the ionosphere, polarization is not crucial. However, for line-of-sight communications, it can make a tremendous difference in signal quality to have the transmitter and receiver using the same polarization. Polarizations commonly considered are linear, such as vertical and horizontal, and circular, which is divided into right-hand and left-hand circular.

Transmission and receiving

All of these parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving antenna, due to reciprocity. Impedance, however, is not applied in an obvious way; for impedance, the impedance at the load (where the power is consumed) is most critical. For a transmitting antenna, this is the antenna itself. For a receiving antenna, this is at the (radio) receiver rather than at the antenna. Antennas used for transmission have a maximum power rating, beyond which heating, arcing or sparking may occur in the components, which may cause them to be damaged or destroyed. Raising this maximum power rating usually requires larger and heavier components, which may require larger and heavier supporting structures. Of course, this is only a concern for transmitting antennas; the power received by an antenna rarely exceeds the microwatt range. If an antenna is to be used for reception at very low frequencies (below about ten megahertz), its noise rejection capabilities become important. At such frequencies, signals are reflected very effectively by the ionosphere; however, at these frequencies there are many forms of natural radio noise, including the noise produced by lightning. Successfully rejecting these forms of noise is an important antenna feature. For example, a small coil of wire with many turns is more able to reject such noise than a vertical antenna. However, the vertical will radiate much more effectively on transmit, where extraneous signals are not a concern.

Theoretical antenna types

- A dielectric resonator is a variation on the conventional antenna in which an insulator with a large dielectric constant is used to modify the electromagnetic field. It is claimed that the dielectric contains the antenna's near field and therefore prevents it from interfering with other nearby antennas or circuits, making it suitable for miniature equipment such as mobile phones.
- A feedhorn is an antenna system that handles the incoming waveform from the dish to the focal point. It usually comprises of a series of rings with decreasing radius in order to drive the signal to the polarizer.
- An isotropic radiator is an antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator). This antenna type is purely theoretical and is not achievable in real life.

Practical antenna models

There are many variations of antennas, but here are a few common models. More can be found in :Category:Radio frequency antenna types.
- The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Variations of the dipole include the folded dipole and the whip antenna which is really just half of a dipole using a ground plane as the image of the second half. The dipole antenna is usually a multiple of a half wavelength long. For this reason, the dipole antenna is sometimes referred to as the half-wave antennna. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. The popular J-pole antenna is a variation of the half dipole with a built in quarter wave transmission line impedance matching section.
- The yagi-Uda antenna is a directional variation of the dipole with parasitic elements added with functionality similar to adding a reflector and lenses (directors) to focus a filament lightbulb.
- The groundplane antenna takes the form of a driven vertical element 1/4 wave long in the center of a grounded plane 1/2 wave in diameter. The end of the vertical element nearest the ground plane is connected to the radio, and the far end is in hanging in free space. The ground plane can take the form of the natural Earth surface, or a network of wires and ground rods, or a solid metal sheet, or four wires arranged as two crossed dipoles and centrally connected to ground.
- The (large) loop antenna is similar to a dipole, except that the ends of the dipole are connected to form a circle, triangle (delta loop antenna) or square. Typically a loop is a multiple of a half or full wavelength in circumference. A circular loop gets higher gain (about 10%) than the other forms of large loop antenna, as gain of this antenna is directly proportional to the area enclosed by the loop, but circles can be hard to support in a flexible wire, making squares and triangles much more popular. Large loop antennas are more immune to localized noise partly due to lack of a need for a groundplane. The large loop has its strongest signal in the plane of the loop, and nulls in the axis perpendicular to the plane of the loop.
- The small loop antenna, also called the magnetic loop antenna is a loop of wire (in other words, both ends of the wire connect to the radio) less than a wavelength in circumference. Typically, the circumference is less than 1/10 for a receiving loop, and less than 1/4 for a transmitting loop. Unlike nearly all other antennas in this list, this antenna detects the magnetic component of the electromagnetic wave. As such, it is less sensitive to near field electric noise when properly shielded. The receiving aperture can be greatly increased by bringing the loop into resonance with a tuning capacitor. Due to the small size of the loop, the radiation pattern is 90 degrees from that of the large loop. The radiation pattern is perpendicular to the plane of the loop, with sharp nulls in the plane of the loop.
- The electrically short antenna is an open-end wire far less than 1/4 wavelength in length - in other words only one end of the antenna is connected to the radio, and the other end is hanging free in space. Unlike nearly all other antennas in this list, this antenna detects only the electric field of the wave instead of the electromagnetic field - think of the free end of the wire as measuring the voltage of that point in space, as opposed to measuring both the voltage and the magnetic field. Its receiving aperture can be greatly increased by increasing the voltage; by adding an inductor or resonator tuned to resonance with the signals of interest. Electrically short antennas are typically used where operating wavelength is large and space is limited, e.g. for mobile transceivers operating at long wavelengths.
- The microstrip antenna consists of a patch of metalization on a ground plane. These are low profile, light weight antennas, most suitable for aerospace and mobile applications. Because of their low power handling capability, these antennas can be used in low-power transmitting and receiving applications. Microstrip antennas are the most commonly used antennas in mobile communications, satellite links, W-LAN and so on because circuit functions can be directly integrated to the microstrip antenna to form compact tranceivers and spatial power combiners.
- The quad antenna is an array of square loops that vary in size. The quad is related to the loop in exactly the same way the yagi is related to the dipole. Typically, the quad needs fewer elements to get the same gain as a yagi. Variations of the quad include the delta loop antenna which uses a triangle instead of a square, requiring fewer supports for large wavelength antennas.
- The random wire antenna is simply a very long (greater than one wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency.
- The Beverage antenna is a form of directional long-wire antenna which uses a resistive termination at one end and feed from the other.
- The helical antenna is a directional antenna suited for receiving signals that are either circular polarized or randomly polarized. These are usually used with satellites, and are frequently used for the driven element on a dish.
- The Phased array antenna is a group of independently fed active elements in which the relative phases of the respective signals feeding the elements are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. In plain language, this is a directional antenna that can be aimed without moving any parts.
- Synthetic aperture radar uses a series of observations separated in time and space to simulate a very large antenna. More generally, interferometry allows the combining of signals from several radio receivers or a single moving receiver.
- A trailing wire antenna is used by submarines when submerged. These antennas are designed to pick up transmissions in the low frequency (LF) and very low frequency (VLF) ranges.
- An evolved antenna refers to an antenna fully or substantially designed using a computer algorithm based on Darwinian evolution.

External links

- [http://hamradio.co.in/tcvr/antena.php Antenna] Antena for Ham / Amateur Radio
- [http://www.maxstream.net/helpdesk/article-27 dBi vs. dBd] How to measure antenna gain
- [http://www.radio-electronics.com/info/antennas/index.php Radio-Electronics.Com] Further information regarding antennas
- [http://www.dxzone.com/catalog/Antennas/ Antenna Plans] Over 400 amateur radio antenna plans and documents from [http://www.dxzone.com dxzone.com]
- [http://www.vias.org/simulations/simusoft_twoaerials.html Learning by Simulations] Interactive simulation of two coupled antennas
- [http://www.n0hr.com/total NØHR.com Best Ham Radio Links] Ham radio antenna sites sorted by band, design, and homebrew vs. commercial antenna products.
- Category:Amateur radio Category:Electrical components Category:Radio electronics ms:Antena ja:空中線

Computer bug

A software bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from working as intended, or produces an incorrect result. Bugs arise from mistakes and errors, made by people, in either a program's source code or its design. It is said that there are bugs in all useful computer programs, but well-written programs contain relatively few bugs, and these bugs typically do not prevent the program from performing its task. A program that contains a large number of bugs, and/or bugs that seriously interfere with its functionality, is said to be buggy. Reports about bugs in a program are referred to as bug reports, also called PRs (problem reports), trouble reports, CRs (change requests), and so forth. Bugs can have a wide variety of effects, with varying levels of inconvenience to the user of the program. Some bugs have only a subtle effect on the program's functionality, and may thus lie undetected for a long time. More serious bugs may cause the program to crash or freeze. Other bugs lead to security problems; for example, a common type of bug called a buffer overflow may allow a malicious user to execute other programs that are normally not allowed to run. The results of bugs may be extremely serious. A bug in the code controlling the Therac-25 radiation therapy machine was directly responsible for patient deaths and in 1996, the European Space Agency's US$1 billion prototype Ariane 5 rocket was destroyed less than a minute after launch, due to a bug in the on-board guidance computer.

Etymology

Usage of the term "bug" to describe inexplicable defects has been a part of engineering jargon for many decades; it may have originally been used in hardware engineering to describe mechanical malfunctions. For instance, Edison wrote the following words in a letter to an associate in 1878:

It has been just so in all of my inventions. The first step is an intuition, and comes with a burst, then difficulties arise—this thing gives out and [it is] then that "Bugs"—as such little faults and difficulties are called—show themselves and months of intense watching, study and labor are requisite before commercial success or failure is certainly reached.
:Source: Edison to Puskas, 13 November 1878, Edison papers, Edison National Laboratory, U.S. National Park Service, West Orange, N.J., cited in Thomas P. Hughes, American Genesis: A History of the American Genius for Invention, Penguin Books, 1989, on page 75.

category:ISBN needed Problems with radar electronics during World War II were referred to as bugs (or glitches), and there is additional evidence that the usage dates back much earlier. World War II The invention of the term is often erroneously attributed to Grace Hopper, who publicized the cause of a malfunction in an early electromechanical computer. A typical version of the story is given by this quote: :In 1946, when Hopper was released from active duty, she joined the Harvard Faculty at the Computation Laboratory where she continued her work on the Mark II and Mark III. Operators traced an error in the Mark II to a moth trapped in a relay, coining the term bug. This bug was carefully removed and taped to the log book September 9th 1945. Hopper recounted the cause to be an actual insect stuck between the contacts of a relay in the logic mechanisms of the device. [http://ei.cs.vt.edu/~history/Hopper.Danis.html] Hopper was not actually the one who found the insect, as she readily acknowledged. And the date was September 9th of 1947, not of 1945 [http://www.ticam.utexas.edu/~organism/bug.html]. The operators who did find it were familiar with the engineering term and, amused, kept the insect with the notation "First actual case of bug being found." Hopper loved to recount the story. [http://www.jamesshuggins.com/h/tek1/first_computer_bug.htm] While it is certain that the Mark II operators did not coin the term "bug", it has been suggested that they did coin the related term "debug".

Preventing bugs

It can be psychologically difficult for some engineers to accept that their design contains bugs. They may hide behind euphemisms like "issues" or "unplanned features". This is also true of corporate software where a fix for a bug is often called "a reliability enhancement". Bugs are a consequence of the nature of the programming task. Some bugs arise from simple oversights made when computer programmers write source code carelessly or transcribe data incorrectly. Many off-by-one errors fall into this category. Other bugs arise from unintended interactions between different parts of a computer program. This happens because computer programs are often complex, often having been programmed by several different people over a great length of time, so that programmers are unable to mentally keep track of every possible way in which different parts can interact (the so-called hrair limit). Many race condition bugs fall into this category. The computer software industry has put a great deal of effort into finding methods for preventing programmers from inadvertently introducing bugs while writing software. These include:
- Programming techniques. Bugs often create inconsistencies in the internal data of a running program. Programs can be written to check the consistency of their own internal data while running. If an inconsistency is encountered, the program can immediately halt, so that the bug can be located and fixed. Alternatively, the program can simply inform the user, attempt to correct the inconsistency, and continue running.
- Development methodologies. There are several schemes for managing programmer activity, so that fewer bugs are produced. Many of these fall under the discipline of software engineering (which addresses software design issues as well.) For example, formal program specifications are used to state the exact behavior of programs, so that design bugs can be eliminated.
- Programming language support. Programming languages often include features which help programmers deal with bugs, such as exception handling. In addition, many recently-invented languages have deliberately excluded features which can easily lead to bugs. For example, the Java programming language does not support pointer arithmetic.

Debugging

Main article: Debugging Finding and fixing bugs, or "debugging", has always been a major part of computer programming. Maurice Wilkes, an early computing pioneer, described his realization in the late 1940s that much of the rest of his life would be spent finding mistakes in his own programs. As computer programs grow more complex, bugs become more common and difficult to fix. Often programmers spend more time and effort finding and fixing bugs than writing new code. Usually, the most difficult part of debugging is locating the erroneous part of the source code. Once the mistake is found, correcting it is usually easy. Programs known as debuggers exist to help programmers locate bugs. However, even with the aid of a debugger, locating bugs is something of an art. Typically, the first step in locating a bug is finding a way to reproduce it easily. Once the bug is reproduced, the programmer can use a debugger or some other tool to monitor the execution of the program in the faulty region, and (eventually) find the problem. However, it is not always easy to reproduce bugs. Some bugs are triggered by inputs to the program which may be difficult for the programmer to re-create. One cause of the Therac-25 radiation machine deaths was a bug that occurred only when the machine operator very rapidly entered a treatment plan; it took days of practice to become able to do this, so the bug did not manifest in testing or when the manufacturer attempted to duplicate it. Other bugs may disappear when the program is run with a debugger; these are heisenbugs (humorously named after the Heisenberg uncertainty principle.) Debugging is still a tedious task requiring considerable manpower. Since the 1990s, particularly following the Ariane 5 Flight 501 disaster, there has been a renewed interest in the development of effective automated aids to debugging. For instance, methods of static analysis by abstract interpretation have already made significant achievements, while still remaining much of a work in progress.

Famous computer bugs

The following is a list of famous computer bugs:

Space exploration

- NASA Mariner 1 went off-course during launch, due to a missing 'bar' in its FORTRAN software (July 22, 1962).[http://www.faqs.org/faqs/space/probe/]
- NASA Apollo 11 landing problem (July 20, 1969).
- NASA Voyager 2 (January 25, 1986).
- Phobos 1 lost (September 10, 1988).
- ESA Ariane 5 Flight 501 self-destruction 40 seconds after takeoff (June 4, 1996).
- NASA Mars Climate Orbiter destroyed due to incorrect orbit insertion (September 23, 1999).
- Mars Polar Lander lost (December 3, 1999).
- NASA Mars Rover freezes due to too many open files in flash memory (January 21, 2004).

Medical

- The Therac-25 accidents (1985-1987), quite possibly the most serious computer-related failure ever in terms of human life lost.

Computing

- Pentium FDIV bug, resulting in inaccuracies in certain floating point division (FDIV) operations.
- Pentium F0 bug, causing the processor to stop under certain instructions.
- The year 2000 problem, popularly known as the "Y2K bug", spawned fears of worldwide economic collapse and an industry of consultants providing last-minute fixes.

Telecommunications

- AT&T long distance network crash (January 15, 1990).

Military

- The MIM-104 Patriot bug, which resulted in the deaths of 28 Americans in Dharan, Saudi Arabia (February 25, 1991).
- Chinook crash on Mull of Kintyre

Video games

- The Missingno. and Glitch City bugs, found in the Pokémon series
- The Minus world in NES version of Super Mario Brothers

Modern bugs and security holes

Traditionally bugs are fixed before a new release. In the first decade of the twenty-first century, as software becomes more complex, sometimes software is released with unknown bugs. Such bugs may just prevent the user from operating the software properly, but often they also produce:
- Operating System instability: Some of these bugs will cause the operating system to crash.
- Windows will display what is known as the "Blue screen of death," a stop message that lets you know that an error has occurred in either your computer's hardware or software.
- the Linux kernel has a similar message called "kernel panic." This is displayed when an unstable version of the Linux kernel or a buggy driver is used and an error occurs.
- Though these messages do occur, modern operating systems using the latest Linux kernel and Windows NT kernel (Windows 2000/2003/XP) are known to be able to run without a restart for months and even years.
- Application instability: Many applications will crash because of unknown bugs. Usually when an application crashes, the system is still running.
- Security vulnerabilities or security holes:
- Many computer systems are able to be infected by viruses. Viruses exploit known vulnerabilities in the system.
- Although all operating systems are vulnerable to viruses, most virus writers only target (wrote viruses for) operating systems with large userbases, so to maximize the virus distribution and damages caused by the virus. In general, all unverified software might have bugs. To find more about the number of known vulnerabilities a particular software may have at this moment, [http://secunia.com/advisories/ you can search for security bugs on the Secunia web page].

Common types of computer bugs

- Divide by zero
- Infinite loops
- Arithmetic overflow or underflow
- Exceeding array bounds
- Using an uninitialized variable
- Accessing memory not owned (Access violation)
- Memory leak or Handle leak
- Stack overflow or underflow
- Buffer overflow
- Deadlock
- Off by one error
- Race hazard
- Loss of precision in type conversion

External links

- [http://www5.in.tum.de/~huckle/bugse.html Collection of Software Bugs] (Thomas Huckle, TU München)
- [http://www.rvs.uni-bielefeld.de/publications/Incidents/Main.html Computer-Related Incidents with Commercial Aircraft] (Peter B. Ladkin et al., Universität Bielefeld)
- [http://courses.cs.vt.edu/~cs3604/lib/Therac_25/Therac_1.html An Investigation of the Therac-25 Accidents] (Nancy Leveson, University of Washington and Clark S. Turner, University of California at Irvine)
- [http://www.ccnr.org/fatal_dose.html Fatal Dose: Radiation Deaths linked to AECL Computer Errors] (Barbara Wade Rose, Canadian Coalition for Nuclear Responsibility)
- [http://www.cs.tau.ac.il/~nachumd/verify/horror.html Software Horror Stories] (Nachum Dershowitz)
- [http://www.history.navy.mil/photos/images/h96000/h96566kc.htm Picture of the "first computer bug"] The error of this term is elaborated above. (Naval Historical Center)
- [http://americanhistory.si.edu/collections/object.cfm?key=35&objkey=30 Page from 1947 log book with "first actual case of bug being found" (moth)] (National Museum of American History)
- [http://www.chiark.greenend.org.uk/~sgtatham/bugs.html How to Report Bugs Effectively] (Simon G. Tatham)
- [http://www.stickyminds.com/r.asp?F=DART_5898 Bug Tracking Basics: A beginner’s guide to reporting and tracking defects] (Mitch Allen)
- [http://wired.com/news/technology/bugs/0,2924,69355,00.html History's Worst Software Bugs] Category:Programming bugs ja:バグ th:จุดบกพร่อง (คอมพิวเตอร์)

Full stop

A full stop or period, also called a full point, is the punctuation mark commonly placed at the end of several different types of sentences in English and several other languages. A period consists of a small dot placed at the end of a line of text, thus: "." (sans quotes). The term full stop is less common in the United States and Canada, but is generally differentiated from period in contexts where both might be used: a full stop is specifically a delimiting piece of punctuation that represents the end a sentence. When a distinction is made, a period is then any appropriately sized and placed dot in English language text, including use in abbreviations (such as U.K.) and at the ends of sentences, but excluding certain special uses of dots at the bottom of a line of text, such as ellipses. The term full stop is also used, vernacularly, to terminate a phrase or thought with finality and emphasis, as in "I told him I was leaving him, full stop." The term period is used in the same sense in North America but also to some extent in the UK, having fallen less completely out of use in this context than as a general reference to the punctuation mark.

Abbreviations

The period is also used after abbreviations, such as Mrs. & Ms. If the abbreviation is ending a declaratory sentence a second period is not needed (e.g. My name is Phil Simpson Jr.), but in the case of an interogative or exclamatory sentence a question or exclamation mark is needed. In British English, "Dr" and "Mr" do not need a period, as they include both the first and last letter of the abbreviation; but in American English, these are written "Dr." and "Mr." In this use, the period is also occasionally known as the suspension mark.

Decimal point

The same glyph is very often used, rather than a mid-line point, as a decimal point (or dot) in English-speaking countries. For example: :3.14159 For more on this use see Decimal separator.

Spacing after full stop

In typewritten texts and other documents printed in uniform-width fonts, there is a convention among lay writers that two spaces are placed after the full stop (along with the other sentence enders: question mark and exclamation mark), as opposed to the single space used after other punctuation symbols. In modern American English typographical