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A ground-plane antenna is a variant of the dipole antenna, designed for use with an unbalanced feed line such as coaxial cable. It resembles a coaxial antenna whose lower section consists of straight elements called radials instead of a hollow conducting cylinder. There are two or more radials, each measuring 1/4 wavelength. The main element can be any length, but it must be adjusted to function at and near a specific frequency. This adjustment is done using a tuning coil. The radials are connected to the outer conductor or shield of the feed line cable; the main element is connected to the center conductor.
The main element of a ground-plane antenna is almost always oriented vertically. This results in transmission of, and optimum response to, vertically polarized wireless signals. When the base of the antenna is placed at least 1/4 wavelength above the ground or other conducting surface,the radials behave as a near-perfect ground system for an electromagnetic field, and the antenna is highly efficient. It works equally well in all horizontal directions. Ground-plane antennas are favored at frequencies above approximately 10 MHz where the dimensions are manageable. This type of antenna is especially popular among Citizens Band radio operators for fixed-station use in the class-D band at 27 MHz.
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