down load a text version
of this file: doublet.txt
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]