Reflective array antenna

rdf:langString Reflective array antenna

rdf:langString left

rdf:langString "Bowtie" UHF TV antenna from 1954

rdf:langString An ALLISS antenna, a modular type of curtain array used by international shortwave stations for broadcasting to distant areas by skywave.

rdf:langString Enormous reflective array antenna of the Duga or "Steel Yard" over-the-horizon radar system, Chernobyl, Ukraine, part of the Soviet early-warning network. It transmits at frequencies between 7 and 19 MHz. The pairs of cylindrical cages at right are the half wave dipole driven elements. Behind them is a reflector screen of horizontal wires, just visible in center.

rdf:langString "Panel array" VHF TV broadcasting antenna. This type is widely used for UHF today

rdf:langString VHF reflective array TV antenna from 1954. The driven elements are folded dipoles, the two long ones cover 54–88 MHz the 4 short ones 174–216 MHz.

rdf:langString A modern form of reflective array is the "bow tie" UHF television antenna. This example has two dipole driven elements in front of a grill reflector. The "bow-tie" dipoles, consisting of two V-shaped elements, have a larger bandwidth than ordinary dipoles, allowing the antenna to cover the wide UHF television band. This example is installed for reception of vertically polarized TV transmissions.

rdf:langString horizontal

rdf:langString Bowtie UHF TV antenna 1954.png

rdf:langString Dipole array TV broadcast antenna.jpg

rdf:langString Reflective array VHF television antenna 1954.jpg

rdf:langString SCR-270 Radar Antenna2.jpg

rdf:langString Woodpecker array.jpg

rdf:langString Reflective array, bow tie, grid, or panel UHF television antenna,.jpg

rdf:langString In telecommunications and radar, a reflective array antenna is a class of directive antennas in which multiple driven elements are mounted in front of a flat surface designed to reflect the radio waves in a desired direction. They are a type of array antenna. They are often used in the VHF and UHF frequency bands. VHF examples are generally large and resemble a highway billboard, so they are sometimes called billboard antennas. Other names are bedspring array and bowtie array depending on the type of elements making up the antenna. The curtain array is a larger version used by shortwave radio broadcasting stations. Reflective array antennas usually have a number of identical driven elements, fed in phase, in front of a flat, electrically large reflecting surface to produce a unidirectional beam of radio waves, increasing antenna gain and reducing radiation in unwanted directions. The larger the number of elements used, the higher the gain; the narrower the beam is and the smaller the sidelobes are. The individual elements are most commonly half wave dipoles, although they sometimes contain parasitic elements as well as driven elements. The reflector may be a metal sheet or more commonly a wire screen. A metal screen reflects radio waves as well as a solid metal sheet as long as the holes in the screen are smaller than about one-tenth of a wavelength, so screens are often used to reduce weight and wind loads on the antenna. They usually consist of a grill of parallel wires or rods, oriented parallel to the axis of the dipole elements. The driven elements are fed by a network of transmission lines, which divide the power from the RF source equally between the elements. This often has the circuit geometry of a tree structure.