Importance of arrays; end fire vs broadside

An antenna array is a group of similar antennas arranged in a regular geometry and fed with controlled amplitudes and phases so that their fields add in the desired directions.

Importance of antenna array

• Gives high directivity and gain that one small element cannot give.
• Narrow, steerable beam: the beam direction can be changed electronically by changing the phase (phased arrays, radar).
• The pattern can be shaped: low side lobes, nulls towards interference.
• Higher total power handling, since the power is shared by many elements.
• Better reliability: failure of one element does not stop operation.

For a uniform linear array of N elements, spacing d, progressive phase δ:

ψ = βd cosθ + δ
AF = sin(Nψ/2) / [N sin(ψ/2)]
Maximum when ψ = 0

Broadside array

• All elements are fed in phase (δ = 0).
• ψ = 0 at θ = 90°, so maximum radiation is perpendicular to the array axis.
• The pattern is a disk-like beam around the axis (narrow in the plane containing the axis).

End-fire array

• Progressive phase δ = −βd, i.e. each element lags the previous one by the phase delay over spacing d.
• ψ = βd(cosθ − 1) = 0 at θ = 0°, so maximum radiation is along the array axis.

 Broadside                End-fire
      ^ ^ ^ ^                 
      | | | |             o o o o o  ====>
   o--o--o--o--o          max along axis
   max perpendicular

Difference

The end-fire spacing is kept below λ/2: with d = λ/2 and δ = −βd, a second equal main lobe appears at θ = 180°, and the directivity drops to about that of a broadside array.