Collinear antenna array

An antenna mast with four colinear directional arrays.

In telecommunication, a collinear (or co-linear) antenna array is an array of dipole antennas mounted in such a manner that every element of each antenna is in an extension, with respect to its long axis, of its counterparts in the other antennas in the array.

Note: A colinear array is usually mounted vertically, in order to increase overall gain and directivity in the horizontal direction. When stacking dipole antennas in such a fashion, doubling their number will, with proper phasing, produce a 3 dB increase in directive gain.

Source: from Federal Standard 1037C