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I was reading about the radio telescope - Murchison Widefield Array (MWA) situated in Western Australia. Antennas of this telescope are quite unique and different from the usual dish radio telescope. In MWA a four by four regular grid of dual-polarisation dipole elements are arranged on a 4m x 4m steel mesh.

So, I was wondering why these dipole elements would have been mounted over the mesh? On searching over internet I found out that this mesh is called a counterpoise (though I am not sure if I am correct). I read more about it and I partially understand that the mesh and ground (Earth) act as two plates of a capacitor but I didn't get why it was done and what was the use of it? In this image mesh can be seen on which dipole are mounted - Image courtesy: SKA

Single 4x4 tile for the Murchison Widefield Array Source click for full size

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  • $\begingroup$ +1 Speculation; for a simple dipole a good ground plane spaced at the right distance will increase the gain by 3 dB. The Earth here looks dry and the wavelength is short so it will not be a proper ground plane. However a ground plane will modify both the directionality and the frequency response as well, so I think a good answer will address the optimization of this 70 to 300 MHz design, including why each polarization has three inclined dipoles. (I'm guessing that makes it less directional so a wider field in the sky is received). $\endgroup$
    – uhoh
    May 25, 2020 at 1:42
  • $\begingroup$ Also note that array will be made of many of these "tiles" spaced out in a complex pattern over large distances. $\endgroup$
    – uhoh
    May 25, 2020 at 1:42
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    $\begingroup$ counterpoise vs ground: 1, 2, 3 and The Murchison Widefield Array: Design Overview which mentions the wide angular acceptance: "covering that area of ground with antennas sensitive to the entire 2-pi steradians of the sky" and calls it a "ground plane". $\endgroup$
    – uhoh
    May 25, 2020 at 1:58

1 Answer 1

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The mesh acts as a ground plane. An antenna design project can be divided into two theoretical regimes: free space, and ground plane. A free space antenna is an antenna that is located far enough away from anything, that nothing will alter its electrical properties. This is the simplest way of doing a design, but not applicable to this antenna

This antenna is mounted on the Earth. The soil under the antenna has electrical properties that can not be precisely known. The properties may change with the amount of vegetation or rainfall through the year. To avoid having to deal with the exact properties of the soil, we place a metal shield between the antenna and the soil. The shield is call the ground plane. Its purpose is to present known electrical properties to the antenna. Ideally the ground plane is always at zero Volts relative to the antenna.

The reason for using a mesh as opposed to a solid sheet of metal due to economy of materials facilitated by a neat property of electromagnetic waves: a wave cannot pass through an opening that is smaller than its wavelength. Therefore instead of spending all the money, and gaining all the weight of a solid sheet, we can adjust the width of the mesh to block (that is, reflect) the frequencies we are interested in.

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