HRS antenna

Curtain arrays are a class of large multielement directional wire radio transmitting antennas, used in the shortwave radio bands. They are a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vertical plane, often in front of a 'curtain' reflector made of a flat vertical screen of many long parallel wires. These are suspended by support wires strung between pairs of tall steel towers, up to 300 ft (90 m) high. They are used for long-distance skywave (or skip) transmission; they transmit a beam of radio waves at a shallow angle into the sky just above the horizon, which is reflected by the ionosphere back to Earth beyond the horizon. Curtain antennas are mostly used by international short wave radio stations to broadcast to large areas at transcontinental distances.Targeting AustralasiaTargeting IndonesiaTargeting Latin AmericaNebo-M Tactical RadarNebo-M (closeup)Nebo-M layout Curtain arrays are a class of large multielement directional wire radio transmitting antennas, used in the shortwave radio bands. They are a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vertical plane, often in front of a 'curtain' reflector made of a flat vertical screen of many long parallel wires. These are suspended by support wires strung between pairs of tall steel towers, up to 300 ft (90 m) high. They are used for long-distance skywave (or skip) transmission; they transmit a beam of radio waves at a shallow angle into the sky just above the horizon, which is reflected by the ionosphere back to Earth beyond the horizon. Curtain antennas are mostly used by international short wave radio stations to broadcast to large areas at transcontinental distances. Because of their powerful directional characteristics, curtain arrays are often used in government propaganda radio stations to beam propaganda broadcasts over national borders into other nations. For example, curtain arrays were used by Radio Free Europe and Radio Liberty to broadcast into Eastern Europe. Curtain arrays were originally developed during the 1920s and 1930s when there was a lot of experimentation with long distance shortwave broadcasting. The underlying concept was to achieve improvements in gain and/or directionality over the simple dipole antenna, possibly by folding one or more dipoles into a smaller physical space, or to arrange multiple dipoles such that their radiation patterns reinforce each other, thus concentrating more signal into a given target area. In the early 1920s, Guglielmo Marconi, pioneer of radio, commissioned his assistant Charles Samuel Franklin to carry out a large scale study into the transmission characteristics of short wavelength waves and to determine their suitability for long distance transmissions. Franklin invented the first curtain array aerial system in 1924, known as the 'Franklin' or 'English' system. Other early curtain arrays included the Bruce array patented by Edmond Bruce in 1927, and the Sterba curtain, patented by Ernest J. Sterba in 1929. The Bruce array produces a vertically-polarised signal; Sterba arrays (and the later HRS antennas) produce a horizontally-polarised signal. The first curtain array to achieve popularity was the Sterba curtain, patented by Ernest J. Sterba in 1929 and this was used by Bell Labs and others during the 1930s and 1940s. The Sterba curtain is however a narrowband design and is only steerable by mechanical means. Curtain arrays were used in some of the first radar systems, such as Britain's Chain Home network. During the Cold War, large curtain arrays were used by the Voice of America, Radio Free Europe, and Radio Liberty, and analogous Western European organizations, to beam propaganda broadcasts into communist countries, which censored Western media. The driven elements are usually half-wave dipoles, fed in phase, mounted in a plane ​1⁄4 wavelength in front of the reflector plane. The reflector wires are oriented parallel to the dipoles. The dipoles may be vertical, radiating in vertical polarization, but are most often horizontal, because horizontally polarized waves are less absorbed by earth reflections. The lowest row of dipoles are mounted more than ​1⁄2 wavelength above the ground, to prevent ground reflections from interfering with the radiation pattern. This allows most of the radiation to be concentrated in a narrow main lobe aimed a few degrees above the horizon, which is ideal for skywave transmission. A curtain array may have a gain of 20 dB greater than a simple dipole antenna. Because of the strict phase requirements, earlier curtain arrays had a narrow bandwidth, but modern curtain arrays can be built with a bandwidth of up to 2:1, allowing them to cover several shortwave bands. Rather than feeding each dipole at its center, which requires a 'tree' transmission line structure with complicated impedance matching, multiple dipoles are often connected in series to make an elaborate folded dipole structure which can be fed at a single point.