Collinear antenna array
In telecommunications, a collinear (or co-linear) antenna array is an array of dipole antennas mounted in such a manner that the corresponding elements of each antenna are parallel and collinear, that is they are located along a common line or axis.
Collinear arrays of dipoles are high gain omnidirectional antennas. A dipole has an omnidirectional radiation pattern, it radiates equal radio power in all azimuthal directions perpendicular to the antenna, with the signal strength dropping to zero on the antenna axis. A collinear array of dipoles has a higher gain, radiating more of its power in azimuthal directions and less toward the axes. A collinear array is usually mounted vertically, in order to increase overall gain and directivity in the horizontal direction and reduce the power radiated into the sky or down toward the earth. Theoretically, when stacking idealised lossless dipole antennas in such a fashion, doubling their number will produce double the gain, with an increase of 3.01 dB. In practice, the gain realised will be below this due to losses.
Collinear dipole arrays are often used as the antennas for base stations for land mobile radio systems that communicate with mobile two-way radios in vehicles, such as police, fire, ambulance, and taxi dispatchers.
Multiple directional antennas mounted parallel to each other are referred to as "stacked"
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