This chapter has presented information on the various types of antennas. The information that follows summarizes the important points of this chapter.
An ANTENNA is a conductor, or system of conductors, that radiates or receives energy in the form of electromagnetic waves.
HERTZ (half-wave) and MARCONI (quarter-wave) are the two basic classifications of antennas.
RECIPROCITY of antennas means that the various properties of the antenna apply equally to transmitting and receiving.
RADIATION RESISTANCE is the amount of resistance which, if inserted in place of the antenna, would consume the same amount of power that is actually radiated by the antenna.
RADIATION PATTERNS can be plotted on a rectangular- or polar-coordinate graph. These patterns are a measurement of the energy leaving an antenna.
An ISOTROPIC RADIATOR radiates energy equally in all directions.
An ANISOTROPIC RADIATOR radiates energy directionally.
A LOBE is the area of a radiation pattern that is covered by radiation.
A NULL is the area of a radiation pattern that has minimum radiation.
ANTENNA LOADING is the method used to change the electrical length of an antenna. This keeps the antenna in resonance with the applied frequency. It is accomplished by inserting a variable inductor or capacitor in series with the antenna.
A HALF-WAVE ANTENNA (Hertz) consists of two lengths of rod or tubing, each a quarter-wave long at a certain frequency, which radiates a doughnut pattern.
The GROUND SCREEN and the COUNTERPOISE are used to reduce losses caused by the ground in the immediate vicinity of the antenna. The ground screen is buried below the surface of the earth. The counterpoise is installed above the ground.
The FOLDED DIPOLE consists of a dipole radiator, which is connected in parallel at its ends to a half-wave radiator.
AN ARRAY is a combination of half-wave elements operating together as a single antenna. It provides more gain and greater directivity than single element antennas.
A DRIVEN ARRAY derives its power directly from the source.
A PARASITIC ARRAY derives its power by coupling the energy from other elements of the antenna.
The BIDIRECTIONAL ARRAY radiates energy equally in two opposing directions.
The UNIDIRECTIONAL ARRAY radiates energy efficiently in a single direction.
The COLLINEAR ARRAY has elements in a straight line. Maximum radiation occurs at right angles to this line.
The BROADSIDE ARRAY has elements parallel and in the same plane. Maximum radiation develops in the plane at right angles to the plane of the elements.
The END-FIRE ARRAY has elements parallel to each other and in the same plane. Maximum radiation occurs along the axis of the array.
MATCHING STUBS are used between elements to maintain current in the proper phase.
The GAIN OF A COLLINEAR ANTENNA is greatest when the elements are spaced from 0.4 to 0.5 wavelength apart or when the number of elements is increased.
The OPTIMUM GAIN OF A BROADSIDE ARRAY is obtained when the elements are spaced 0.65 wavelength apart.
A PARASITIC ARRAY consists of one or more parasitic elements with a driven element. The amount of power gain and directivity depends on the lengths of the parasitic elements and the spacing between them.
MULTIELEMENT ARRAYS, such as the YAGI, have a narrow Frequency response as well as a narrow beamwidth.
A V ANTENNA is a bidirectional antenna consisting of two horizontal, long wires arranged to form a V.
The RHOMBIC ANTENNA uses four conductors joined to form a rhombus shape. This antenna has a wide frequency range, is easy to construct and maintain, and is noncritical as far as operation and adjustment are concerned.
The TURNSTILE ANTENNA consists of two horizontal, half-wire antennas mounted at right angles to each other.