Tuesday, August 9, 2011

Phased Array

A phased arry antenna is composed of lots of radiating elements each with a phase shifter.Beams are formed by shifting the phase of the signal emitted from each radiating element,to provide constructive/destructive interference so as to steer the beams in the desired direction.

The figure shows the interference of two one above the other lying in-phasely radiant antenna elements. The main beam direction is centric.The figure shows the interference of two one above the other lying antenna elements radiating with a different phase shift. The lower antenna element radiates with a phase shift of 15 degrees earlier as the upper antenna element. The main beam direction is steared up.

-Antenna elements fed with same phase                        -Fed with different phase shift

There are two basic types of smart antennas. As shown in Fig below the first type is the phased array or multibeam antenna, which consists of either a number of fixed beams with one beam turned on towards the desired signal or a single beam (formed by phase adjustment only) that is steered toward the desired signal. The other type is the adaptive antenna array as shown in Fig.  which is an array of multiple antenna elements, with the received signals weighted and combined to maximize the desired signal to interference plus noise power ratio. This essentially puts a main beam in the direction of the desired signal and nulls in the direction of the interference.
A smart antenna is therefore a phased or adaptive array that adjusts to the environment. That is, for the adaptive array, the beam pattern changes as the desired user and the interference move; and for the phased array the beam is steered or different beams are selected as the desired user moves.


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