Reflective array antenna

Lore & Background

Reflective array antennas have been built in various forms, including large VHF versions that resemble highway billboards, sometimes called billboard antennas. Other names include bedspring array and bowtie array, depending on the element type. The curtain array is a larger version used by shortwave broadcasting stations. The individual elements are most commonly half-wave dipoles, though they may include parasitic elements. The reflector can be a solid metal sheet or, more commonly, a wire screen; screens are used to reduce weight and wind loads, provided the holes are smaller than about one-tenth of a wavelength. The screen often consists of a grill of parallel wires or rods oriented parallel to the dipole axes. The driven elements are fed by a transmission-line network that divides power equally, often with a tree-like circuit geometry. Since the 1980s, versions for microwave frequencies have used patch antenna elements mounted in front of a metal surface.

Reader's Guide

Reflective array antennas are significant because they provide a practical means of achieving high gain and directivity in the VHF and UHF bands. By using multiple driven elements in front of a flat reflector, they produce a unidirectional beam, increasing gain and reducing radiation in unwanted directions. The gain increases with the number of elements, while the beamwidth and sidelobes decrease. However, the complexity and inherent losses of the feed network limit practical gain to about 25–30 dB. The reflector improves the front-to-back ratio, making the antenna more directional. The beam can be steered electronically by phase shifting the drive signals to individual elements, a technique used in phased arrays, or mechanically by rotating the entire structure. Notable examples include the Soviet Duga radars, which are hundreds of meters across and contain hundreds of elements. Active array antennas, where groups of elements are driven by separate RF amplifiers, can achieve much higher gain but are prohibitively expensive.

Did You Know?

• The reflector may be a metal sheet or a wire screen, as long as the holes are smaller than about one-tenth of a wavelength.
• The driven elements are most commonly half-wave dipoles, fed in phase by a transmission-line network.
• The Soviet Duga radars are very large reflective arrays, hundreds of meters across, with hundreds of elements.