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What is a Doublet?
  Doublet is the original name for the ½ wave dipole antenna.
 Where the `Marconi' antenna DEPENDS on the `reflection' of THE one quarter wavelength long radiating element in `the counterpoise' creating a `mirror image' for the antenna to be complete, the dipole needs no counterpoise, it is a complete antenna without any reflector or ground.  It is two `one quarter wavelength long radiating elements' normally facing in opposite directions, with the "bases" near each other, which is where the pair are fed. They can take the form of an "L", or be bent in various ways.  These two quarter wave elements mounted base to base have an impedance twice that of one quarter wave element: (2 X 36 = 72 ohms).  This makes the dipole well suited to be directly fed with RG/11 (1 kw +) or RG/59 (250 w) or RG/6 (? about 500 w) (72-75 ohms) cables. (A balancing scheme or device can be used to prevent feed line radiation) Typically made of a single strand of wire, it can be made of multiple strands, a fan or cage antenna, or tubing.  (pipe, (increasing bandwidth) The dipole is the most common single band antenna used by HAMs world wide.  Although typically the dipole is mounted horizontally between two supports such as between two trees (or a tree and a building) it can be mounted from one support with one end up and one end down, a "sloper," or with the center mounted high in a tree or from a pole or tower, with the ends low, near the ground. (an inverted "V")  This tends to lower the feed point impedance somewhat.  It has been reported: "very acute angles will result in a good match to 50 ohm cable."  The dipole normally has a radiation pattern perpendicular to the wire, but the inverted "V" antenna exhibits a mixture of horizontal and vertical radiation.  Dipoles can be mounted vertically producing an omnidirectional (NON directional), vertically polarized signal.  Dipoles can be feed in the center, an end or any place between.  The feed point impedance varies greatly with feed point location, the impedance rises as the feed point moves from the center toward an end.  If a dipole is fed at it's end with a quarter wavelength long matching stub, the result is called a "J" antenna, because of it's physical appearance.  Feeding the ends (at about 10,000 ohms) two of these can be connected end to end with a quarter wavelength long matching stub between them, producing a (collinear) one wave length antenna.  We would think the resultant gain would be twice one dipole, (3 db) but because the antennae (centers) are not one wavelength apart, a theoretical maximum of 1.8 db gain results and the normal bi-directional figure "8" pattern is turned into a four lobed (leafed) pattern.  The height of a horizontal dipole, above the reflective surface on or in the ground below, (because of reflected energy) greatly effects the `launch' angle (toward the ionosphere between the transmit site and over the horizon) determines where the signal will come down.  (skip distance)
  Although the Marconi antenna was developed first, the dipole is the "basic" antenna and it is the basis of many antenna types (of course the folded dipole which is used with most U/VHF repeaters) the familiar beam (Yaggi), cubical quad. and almost all wire antennas, in fact most FM & TV station antenna types were developed from the dipole.
Ron KA4INM [email protected]