An antenna array is a group of multiple radiating elements whose signals are combined in such a way the overall radiation pattern is reinforced in certain desired direction and suppressed in other undesired directions. The combined radiation pattern of the system can be controlled by means of five parameters:
- The geometrical distribution of the unitary elements (i.e., linear, circular, rectangular, spherical, etc.).
- The relative displacement between the elements.
- The excitation amplitude of the individual elements.
- The excitation phase of the individual elements.
- The relative pattern of the individual elements.
Let us consider the simple case of an antenna array formed by two identical isotropic radiators. The total electric field ET radiated by the two elements assuming no coupling between elements is proportional to
| ||(Eq. 2)|
|Fig. 1: Geometry of a two-element antenna array (a) and far-field approximation (b).|
With regard to Eq. (3), it can be seen that the total field of the array can be expressed as the field radiated by a single element multiplied by a term that is widely referred as array factor.
|Fig. 2: Radiation pattern of two omnidirectional elements: (a) separated λ/2, (b) separated λ, and (c) separated λ/4 and β=-π/2.|
N-element linear array
The geometry of a N-element equispaced linear array is shown in Fig. 3. Let us assume identical amplitudes and that each succeeding element has a progressive phase term β relative to the preceding one. The array factor in this case can be calculated as
|Fig. 3: Geometry of an N-element linear antenna array.|