[1] They constitute a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vertical plane, often positioned in front of a "curtain" reflector made of a flat vertical screen of many long parallel wires.
[1] These are suspended by support wires strung between pairs of tall steel towers, reaching heights of up to 90 m (300 feet) high.
Curtain arrays are extensively used by international short-wave radio stations for broadcasting to large areas at transcontinental distances.
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.
In order to allow the beam to be steered, sometimes the entire array is suspended by cantilever arms from a single large tower which can be rotated.
Alternatively, some modern versions are constructed as phased arrays in which the beam can be slewed electronically, without moving the antenna.
Adding a constant phase shift between adjacent horizontal dipoles allows the direction of the beam to be slewed in azimuth up to ±30° without losing its radiation pattern.
Geopolitical necessity leads some international broadcasters to occasionally use three separate antenna arrays: highband and midband, as well as lowband HRS curtains.
An HRS 4/4/0.5 slewable antenna with 16 dipoles is one of the standard types of array seen at shortwave broadcast stations worldwide.
An example of this can be seen at NRK Kvitsøy, where a circular railway carries a pair of wheeled platforms, each of which supports a tower at opposite ends of a diameter-arm.
The number of dipole rows and the height of the lowest element above ground determine the elevation angle and consequently the distance to the service area.
Note that it is possible for details of the antenna site to wreak havoc with the designers plans such that takeoff angle and matching may be adversely affected.