Collinear antenna array

Lore & Background

Collinear arrays are frequently constructed as a stack of dipoles, but can also be built as a stack of phased quarter-wave antennas. In this configuration, the individual radiators are often made of coaxial feedlines, with the center conductor of one element connected electrically to the shield of the one above, and so on in alternating phase for as many elements as specified by gain or overall length requirements. The top element in the stack is a quarter-wave radiator connected directly to the center conductor of the element below it. This style is usually housed in a fiberglass radome for support and environmental protection.

A third type, rarely seen outside amateur radio VHF/UHF applications, uses half-wavelength monopole elements with phasing coils between each consecutive pair to achieve the necessary phase shift. This style tends to be less efficient due to coil losses, but has the advantage that it can be constructed with self-supporting elements, eliminating the need for a protective radome.

The purpose of stacking multiple antennas in a vertical collinear array is to increase power radiated in horizontal directions and reduce power radiated into the sky or down toward the earth, where it would be wasted. Theoretically, when stacking idealized lossless antennas, doubling their number produces double the gain, an increase of 3.01 dB. In practice, the gain realized is below this due to imperfect radiation spread and losses.

Reader's Guide

Collinear antenna arrays are significant primarily as high gain omnidirectional antennas for land mobile radio base stations, serving police, fire, ambulance, and taxi dispatchers, as well as for broadcasting. Their design allows them to radiate equal radio power in all azimuthal directions perpendicular to the antenna, with signal strength dropping to zero on the antenna axis. By stacking multiple elements vertically, the array concentrates radiated power into the horizontal plane, reducing wasted radiation toward the sky or ground. This makes them practical for communicating with mobile two-way radios in vehicles. The arrays are constructed in several forms: stacked dipoles, phased quarter-wave antennas using coaxial feedlines (often housed in a fiberglass radome), or, in amateur radio VHF/UHF applications, half-wavelength monopoles with phasing coils. The latter is less efficient due to coil losses but does not require a radome. The gain realized in practice is always below the theoretical 3.01 dB increase per doubling of elements due to imperfect radiation spread and losses.

Did You Know?

• Collinear arrays are high gain omnidirectional antennas that radiate vertically polarized radio waves.
• Theoretically, doubling the number of idealized lossless elements increases gain by 3.01 dB.
• One construction method uses coaxial feedlines with alternating phase connections, housed in a fiberglass radome.
• A third type, using half-wavelength monopoles with phasing coils, is rarely seen outside amateur radio VHF/UHF applications.