Collinear antenna array

Both dipoles and quarter-wavelength monopoles have an omnidirectional radiation pattern in free space when oriented vertically; they radiate equal radio power in all azimuthal directions perpendicular to the antenna, with the signal strength dropping to zero on the antenna axis.

In practice, the gain realized will be below this due to imperfect radiation spread and losses.

This style of collinear antenna is usually housed in a fiberglass radome, to provide both support and environmental protection to the relatively fragile coaxial elements.

This style tends to be less efficient due to coil losses, but has the advantage that it can be constructed with the elements supporting themselves, doing away with the need for a protective radome.

Collinear 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.