What is a Doublet?

  Doublet is the original name for the 1/2 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 ka4inm@qsl.net