In the following discussion on radiation patterns, the term DIPOLE is used to mean the basic half-wave antenna. The term DOUBLET is used to indicate an antenna that is very short compared with the wavelength of the operating frequency.
Physically, it has the same shape as the dipole.
RADIATION PATTERNS OF A DOUBLET
The doublet is the simplest form of a practical antenna. Its radiation-pattern can be plotted like the radiation-pattern of the flashlight (see the figure below).
Polar-coordinate graph for anisotropic radiator.
The next figure below shows the development of vertical and horizontal patterns for a doublet. This in NOT a picture of the radiation, but three-dimensional views of the pattern itself. In three views the pattern resembles a doughnut. From the dimensions in these views, two types of polar-coordinate patterns can be drawn, horizontal and vertical.
The HORIZONTAL PATTERN view A is derived from the solid pattern view C by slicing it horizontally. This produces view B, which is converted to the polar coordinates seen in view A. The horizontal pattern illustrates that the radiation is constant in any direction along the horizontal plane.
Development vertical and horizontal-patterns.
A VERTICAL PATTERN view E is obtained from the drawing of the vertical plane view D of the radiation pattern view C. The radiation-pattern view C is sliced in half along a vertical plane through the antenna. This produces the vertical plane pattern in view D. Note how the vertical plane in view D of the radiation pattern differs from the horizontal plane in view B.
The vertical pattern view E exhibits two lobes and two nulls. The difference between the two patterns is caused by two facts: (1) no radiation is emitted from the ends of the doublet; and (2) maximum radiation comes from the doublet in a direction perpendicular to the antenna axis. This type of radiation-pattern is both NONDIRECTIONAL (in a horizontal plane) and DIRECTIONAL (in a vertical plane).
From a practical viewpoint, the doublet antenna can be mounted either vertically or horizontally. The doublet shown in the figure above is mounted vertically, and the radiated energy spreads out about the antenna in every direction in the horizontal plane. Since ordinarily the horizontal plane is the useful plane, this arrangement is termed NONDIRECTIONAL.
The directional characteristics of the antenna
in other planes is ignored. If the doublet were mounted horizontally, it
would have the effect of turning the pattern on edge, reversing
the patterns given in the figure above. The antenna would then be
directional in the horizontal plane. The terms "directional" and
"nondirectional" are used for convenience in describing specific
radiation patterns. A complete description always involves a figure in
three dimensions, as in the radiation pattern of the figure shown above.
RADIATION-PATTERN OF A DIPOLE
The pattern of a dipole (see the figure below) is similar to that of the doublet (the figure above). Increasing the length of the doublet to 1/2 wavelength has the effect of flattening out the pattern. The pattern in the horizontal plane of a dipole is a larger circle than that of the doublet. The vertical pattern lobes are no longer circular. They are flattened out and the radiation intensity is greater.
Radiation-pattern of a dipole.