often involves a number of parallel dipole antennas with a certain spacing. An antenna's linear polarization is generally along the direction (as viewed from the receiving location) of the antenna's currents when such a direction can be defined. When a transmission line is used in between the antenna and the transmitter (or receiver) one generally would like an antenna system whose impedance is resistive and near the characteristic impedance of that transmission line in order to minimize the standing wave ratio (SWR) and. Contrary to an ideal (lossless) series-resonant circuit, a finite resistance remains (corresponding to the relatively small voltage at the feed-point) due to the antenna's radiation resistance as well as any actual electrical losses. At higher frequencies (UHF, microwaves) trading off performance to obtain a smaller physical size is usually not required. The refractor alters the spatial characteristics of the wave on one side relative to the other side. A particular antenna design will present a particular feedpoint impedance. Many important antenna types have a uniform radiation pattern in the horizontal plane, but send little energy upward or downward. In some applications where the receiving antenna must work in any position, as in mobile phones, the base station antennas use mixed polarization, such as linear polarization at an angle (with both vertical and horizontal components) or circular polarization. Array edit Array antennas consist of multiple antennas working as a single antenna. This is distinct from the antenna's directionality.
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Highly directional antennas need to be significantly larger than the wavelength. Beamwidth and polarization can be actively reconfigurable. Rather than just using a thick tube, there are similar techniques used to the same effect such as replacing thin wire elements with cages to simulate a thicker element. Even when the antenna system has a vertical orientation, such as an array of horizontal dipole antennas, the polarization is in the horizontal direction corresponding to the current flow. Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. Extremely narrow bandwidth, requires large loading coil and low resistance counterpoise ground. In Quad antennas, the loops which are not connected to the feedline act as helpers for the driven loop that is connected to the feedline; they are spaced and tuned so that they absorb and re-radiate signal power from the main loop beneficially for the.