A phased antenna system consists of two or more active antenna elements, arranged so the electromagnetic fields effectively add in somedirections and cancel in other directions. This produces enhanced transmission andreception in the directions where the fields add, and reduces the strength of radiated andreceived signals in the directions where the fields cancel.
Many AM (amplitude-modulation) broadcast stations use sets of two, three, or fourphased vertical antennas. This results in a directional pattern that optimizescoverage, so the station can reach the greatest possible number of listeners in itsdesignated area. The nulls in the pattern reduce or eliminate interference withother AM broadcast stations.
In two-way radio communications, several vertical dipole antenna can be placed end-to-endand fed in phases. This is known as a collinear antenna and is a specializedtype of phased array. At low elevation angles, the radiation and response areenhanced relative to a single vertical dipole. This gain occurs at the expense ofradiation and response at higher elevation angles, increasing the range of communicationfor surface-to-surface communications.