Curtain array

[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.

Curtain arrays at Radio Free Europe transmitter site, Biblis, Germany
Curtain array at international shortwave broadcasting station, Moosbrunn, Austria. It consists of 4 columns of horizontal wire dipoles, suspended in front of a wire screen. The vertical parallel wire feedlines to each column of dipoles are visible. The entire antenna is mounted on a rotating truss structure, allowing it to be pointed in different directions.
Antenna G1 at Hörby shortwave station, which was operated by Radio Sweden but was shut down in 2011. It consisted of 16 horizontal wire dipoles in a 4×4 array, suspended in front of a wire screen. Each of the 4 columns of dipoles is fed by a separate open-wire transmission line, which can be seen exiting at an angle from the center of each column. The diagonal wires in the foreground are guy wires. The CCIR designation for this type of antenna (below) is HR 4/4/0.5
Former Radio France Internationale (RFI) Issoudun Relay station feeders and curtain arrays
Simulated radiation pattern of a 15.1 MHz HR 6/4/1 curtain antenna (24 horizontal dipoles organized in 4 rows of 6 elements each, in front of a reflector), driven by a 500 kW transmitter. The transmitter is located in Esquimalt and the pattern covers Central America and parts of South America, showing the long distances achieved with this antenna. The main lobe of the pattern is flanked by two sidelobes , which appear curved due to the global map projection.
Example of a simulated HRS antenna radiation pattern from a shortwave relay station in Canada. It consists of a main lobe with two major sidelobes . The sidelobes look curved because of the map projection .
ALLISS antenna as viewed underneath
55Zh6M Nebo-M mobile multiband radar system, developed by NNIIRT