Stacked antenna

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The use of multiple antennas is called stacking or ganging. The antennas must be identical.

Dipoles are commonly stacked horizontally (collinearly), vertically (broadside), and in echelon (end-fire).
[img]http://www.hdtvprimer.com/ANTENNAS/stacked.gif[/img]
When dipoles are stacked horizontally, the horizontal beam-width becomes very narrow. This is because they do not add in-phase for directions not straight ahead. Similarly, when stacked vertically, the vertical beam-width becomes narrower.

The same principles apply when stacking whole antennas, not just dipoles. See