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Inverted-L antenna with counterpoise, in a powerful amateur radio station, Colorado, 1920. The counterpoise is the lower grid of horizontal wires, suspended below the antenna. The largest use of counterpoises is in transmitters on the low frequency (LF) and very low frequency (VLF) bands, as they are very sensitive to ground resistance. [2]
The radiation pattern of a thin wire antenna is easily predictable using antenna modeling.For a straight wire, the radiation pattern can be described by axially symmetric multipole moments with no component along the wire direction; as the length of the wire is increased, higher multipole contributions become more prominent and multiple lobes (maxima) at angles to the antenna axis develop. [4]
A dipole can be fed from very near its end (needing to be only about 1 / 20 th of the dipole length from the actual end) but the end impedances are exceedingly high – a few thousands of ohms, depending on the average height of the antenna and thickness of its wire.
The angle of the slope is usually between 45°–60° and the lower end of the wire is at least 1 ⁄ 6 wavelength above the electrical ground. [3] A sloper is typically fed with a coaxial cable in the center, at the top of the center support mast. At least 1 ⁄ 4 of the wavelength of feedline must be at 90° angle to the antenna. [3]
A microwave antenna may also be fed directly from a waveguide in place of a (conductive) transmission line. Cell phone base station antennas. An antenna counterpoise, or ground plane, is a structure of conductive material which improves or substitutes for the ground. It may be connected to or insulated from the natural ground.
Thus the electrical length of the antenna is longer than its physical length. The electrical length of an antenna element also depends on the length-to-diameter ratio of the conductor. [19] [15] [20] [21] As the ratio of the diameter to wavelength increases, the capacitance increases, so the node occurs farther beyond the end, and the ...