Since signal polarization varies with the type of object, no set position of the receiving antenna is correct for all returning signals). (For radar transmissions, a received signal is actually a wave reflected from an object. The change is usually small at low frequencies, but quite drastic at high frequencies. Over long distances, polarization changes. Therefore, transmitting and receiving antennas are oriented alike, especially if they are separated by short distances. In general, the polarization of a wave does not change over short distances. Since the electric field is parallel to the axis of the dipole, the antenna is in the plane of polarization.Ī horizontally placed antenna produces a horizontally polarized wave, and a vertically placed antenna produces a vertically polarized wave. 2-2), and if the E lines are vertical, the wave is said to be vertically polarized. That is, if the electric lines of force (E lines) are horizontal, the wave is said to be horizontally polarized (fig. The direction of polarization is the direction of the electric vector. Most electromagnetic fields in space are said to be linearly polarized.
The radiation field is made up of magnetic and electric lines of force that are always at right angles to each other. At a considerable distance, the wavefront can be considered as a plane surface at right angles to the direction of propagation.įigure 2-2-Horizontal and vertical polarization. The farther from the antenna the wave is, the less curved it appears. Usually, all points on the wavefront are an equal distance from the antenna. Within this wavefront, all energy is in phase.
A small section of this sphere is called a wavefront, positioned perpendicular to the direction of the radiation field (fig. The gain of an antenna is constant, whether the antenna is used for transmitting or receiving.Įnergy from an antenna is radiated in the form of an expanding sphere. The ratio between the amount of energy propagated in these directions and the energy that would be propagated if the antenna were not directional is known as antenna GAIN. That is, they propagate more energy in certain directions than in others. The elements can be considered as a group of antennas fed from a common source.Īs we mentioned earlier, some antennas are highly directional. The elements can be arranged so the radiated energy is focused in one direction.
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