18-12-2012, 12:11 PM
Antenna & wave propagation
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Antenna Arrays
Antennas with a given radiation pattern may be arranged in such a way that the pattern yield a different radiation pattern. Like:
Linear array
Circular array
Planar array {rectangular array}
Conformal array {aircraft skin}
Pattern multiplication theorem
Array pattern = array element pattern
array factor [AF].
Array element pattern - the pattern of the individual array element. Array factor - a function dependent only on the geometry of the array
and the excitation (amplitude, phase) of the elements.
Antenna arrays types:
Broadside array
End fire array
Collinear array
Parasitic array
Broadside array is one in which a number of identical parallel antennas are set up along a line drawn perpendicular to their respective axis as shown in figure. In broadside array individual elements are equally spaced along a line and each element is fed with current of equal magnitude.
End fire array An end-fire array looks similar to a broadside array. The currents in the elements of the end-fire array, however, are usually 180 degrees out of phase with each other as indicated by the arrows.
Collinear array is usually mounted vertically, in order to increase overall gain and directivity in the horizontal direction.
Parasitic arrays represent another method of achieving high antenna gains. A parasitic array consists of one or more parasitic elements placed in parallel with each other and, in most cases, at the same line-of-sight level. The parasitic element is fed inductively by radiated energy coming from the driven element connected to the transmitter.