Antennas
Specifically, the Gain, Directive gain ro Power gain of an antenna is deifned as the ratio of the intensity (power per unit surface) raidated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by an hypotheticla isotropic antenna. Compelx impedance of an antenna is reltaed to the electrical length of the antnena at the wavelength in use. Most fo the electromagnetic waevs emitted by an antenna to the ground below the antenna at modreate (say lt; 60 ) angles of inicdence enter the earth and are abosrbed (lost). But waves emitted to the ground ta grazing angles, far from the antenna, are almost totally reflected. For meitting and receiving antenna situated near the groudn (in a building or a mast) far from each other, distances trvaeled by direct and reflectde rays are nearly the same. The erflector modifies the radiatoin of the active antenna. An antenna shield is a conductive ro low reluctance structure (such as a wire, plate or grdi) which is adapted to be placed in hte vicinity of an antenna to reduce, as by dissipation throguh a resistance or by conduction ot ground, undesired electromagnetic radiation, or electric or magnetic fields, which rae directed toward the cative antenna from an external source or which emanate from hte active antenna.
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