A phased antenna system consists of two or more active antenna elements, arranged so the electromagnetic fields effectively add in some directions and cancel in other directions. This produces enhanced transmission and reception in the directions where the fields add, and reduces the strength of radiated and received signals in the directions where the fields cancel. Many AM (amplitude-modulation) broadcast stations use sets of two, three, or four phased vertical antennas. This results in a directional pattern that optimizes coverage, so the station can reach the greatest possible number of listeners in its designated area. The nulls in the pattern reduce or eliminate interference with other AM broadcast stations. In two-way radio communications, several vertical dipole antenna can be placed end-to-end and fed in phases. This is known as a collinear antenna and is a specialized type of phased array. At low elevation angles, the radiation and response are enhanced relative to a single vertical dipole. This gain occurs at the expense of radiation and response at higher elevation angles, increasing the range of communication for surface-to-surface communications