Curtain array

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 radio 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.[2][3]

Other early curtain arrays included the Bruce array patented by Edmond Bruce in 1927,[4] and the Sterba curtain, patented by Ernest J. Sterba in 1929.[5] 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[5] 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.

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

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.[1] 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.[1] 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.[1] 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.[1] A curtain array may have a gain of 20 dB greater than a simple dipole antenna.[1] 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.[1][6]

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.

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. See ALLISS-Antenna. Alternatively, some modern versions are constructed as phased arrays in which the beam can be slewed electronically, without moving the antenna. Each dipole or group of dipoles is fed through an electronically adjustable phase shifter, implemented either by passive networks of capacitors and inductors which can be switched in and out, or by separate output RF amplifiers. 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.[7]

Former Radio France Internationale (RFI) Issoudun Relay station feeders and curtain arrays

Since 1984 the CCIR has created a standardised nomenclature for describing curtain antennas, CCIR HF Transmitting Antennas consisting of 1 to 4 letters followed by three numbers:

First letter

Indicates the orientation of the dipoles in the array.

• "H" indicates the dipoles are oriented horizontally, so the antenna radiates horizontally polarized radio waves.
• "V" indicates the dipoles are oriented vertically, so the antenna radiates vertically polarized radio waves.

Second letter (if present)

Indicates whether the antenna has a reflector.

• "R" indicates that there is a simple (passive) reflector on one side of the array, so the antenna radiates a single beam.
• "RR" indicates that the array has some kind of "reversible reflector", so the direction of the beam can be switched 180°. Very few of this type have ever been built. RCI Sackville in Canada may have 2 HRRS type antennas—perhaps the only ones in North America.
• If "R" and "RR" are missing, the antenna has no reflector, so the dipole array will radiate its energy in two beams in both directions perpendicular to its plane, 180° apart.

Third letter (if present)

• "S" indicates that the array is steerable.

Numbers

Following the letters are three numbers: "x/y/z". "x" and "y" specifies the dimensions of the rectangular array of dipoles, while "z" gives the height above the ground of the bottom of the array:

• "x" (an integer) is the number of collinear dipoles in each horizontal row. (The number of columns in the array)
• "y" (an integer) is the number of vertically arranged dipoles in each column. (The number of rows in the array)
• "z" (a decimal fraction) is the height above ground in wavelengths of the lowest row of dipoles in the array.

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.

For example, a "HRS 4/5/0.5" curtain antenna has a rectangular array of 20 dipoles, 4 dipoles wide and 5 dipoles high, with the lowest row being half a wavelength off the ground, and a flat reflector behind it, and the direction of the beam can be slewed. 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.

Notes on HRS nomenclature

• HRS antennas of type HRS 1/1/z are undefined as such (such a thing would consist of just a single dipole).
• HRS antennas of type HRS 1/2/z and 2/1/z exist, but see little practical use in shortwave broadcasting. An antenna with the designation of "H 1/2/z" is commonly referred to as a "Lazy-H" antenna. VHF and UHF repeaters for FM radio or television in the UK quite often employ a pair of horizontal dipoles (or short yagis) one above the other (i.e. HRS 1/2/z) to concentrate transmission power in the vertical plane.
• The Russian Duga Over The Horizon Radar may have used an antenna of type HRS 32/16/0.75 (estimated – not verified), with potential directional ERP in the gigawatt range.

The HRS type antenna is one of the most common types of curtain array. The name comes from the above CCIR nomenclature: it consists of an array of Horizontal dipoles with a Reflector behind them, and the beam is Steerable. These antennas are also known as "HRRS" (for a Reversible Reflector), but the extra R is seldom used.

However, as far back as the mid-1930s, Radio Netherlands was using a rotatable HRS antenna for global coverage. Since the 1950s the HRS design has become more or less the standard for long distance (> 1000 km) high power shortwave broadcasting.

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

If there is an "S" in the antenna's designation, it is a steerable design. Following the ITU-recommendation, it might be called 'slewable design'.[7] This might be achieved electronically by adjustment of the electrical wave phases of the signals fed to the columns of dipole antenna elements, or physically by mounting the antenna array on a large rotating mechanism. 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 curtain antenna array is suspended between the towers and rotates with them as the towers go around the circular railway. Another physical rotation technique is employed by the ALLISS system where the entire array is built around a central rotatable tower of great strength.

Electrically slewed antenna arrays can usually be aimed in the range of ±30° from the antenna's physical direction while mechanically rotated arrays can accommodate a full 360°. Electrical slewing is typically done in the horizontal plane, with some adjustment being possible in the vertical plane.

• For a 2-wide dipole array, the beamwidth is around 50°
• For a 3-wide dipole array, the beamwidth is around 40°
• For a 4-wide dipole array, the beamwidth is around 30°

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.

• A 2-row high array has a typical takeoff angle of 20°

is most commonly used for medium range communications.

• A 4-row high array has a typical takeoff angle of 10°

is most commonly used for long range communications.

• A 6-row array is similar to a 4-row, but can achieve 5° to 10° takeoff angles.

can be used in shortwave communications circuits of 12000 km, and is highly directional.[7]

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.

This is an example of theoretical HRS design shortwave relay stations. This may help one better understand HRS antenna directivity.

• 
  Targeting Australasia
• 
  Targeting Indonesia
• 
  Targeting Latin America
• 
  Nebo-M Tactical Radar
• 
  Nebo-M (closeup)
• 
  Nebo-M layout

This is an incomplete list of stations using only HRS antennas, sorted by country name.

RADAR Systems using HR Type Antennas

55Zh6M Nebo-M mobile multiband radar system, developed by NNIIRT

Some portable tactical antenna systems still use HR type antennas, mostly not HRS as the antennas are rotatable.

ALLISS Technology portals

• http://HireMe.geek.nz/ALLISS.html
• http://www.w8ji.com/curtain%20sterba%20USIA%20array.htm