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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 usual dish radio telescope. In MWA a four by four regular grid of dual-polarisation dipole elements arranged on a 4m x 4m steel mesh. So, I was wondering why these dipole elements would have been placed over the mesh, on searching over internet I found out that this mesh is called as counterpoise (though I am not sure if I am correct or not) further I read about it and partially understand that these mesh and ground (Earth) act as two plates of capacitor but I didn't get why it is done and what's use of it?


(source: skatelescope.org)

MWA antenna - image courtesy: SKA

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A grounded surface forces the potential to be zero everywhere on this surface and has the same effect as if the antenna arrangement were mirrored on the earth's surface. If the distance is correct, the fields of the original antenna arrangement and the mirrored arrangement superimpose constructively in the upward direction, i.e. they add up.

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Each dipole by itself has a toroidal radiation pattern. When combined into an array, the resulting radiation pattern still has two symmetrical lobes. A conductive surface reflects the unwanted lobe back toward the direction of interest.

The metal mesh is much more effective at this than the dry soil underneath it.

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In electromagnetics and antenna theory, image theory is an important tool for the analysis of many problems. It turns out that, whenever you have a structure over an perfect conductive plane with infinite size, it's often mathematically equivalent to having the structure itself and its mirror image. The classic result of image theory is the analysis of a monopole antenna - a monopole antenna over a conductive plane is equivalent to a dipole antenna in free space. The first half is the antenna body, the second half is the image antenna.

A dipole over a conductive plane

For monopole antennas and other antennas designed with a conductive plane in mind, the conductive plane under the antenna is as important as the antenna body above it. If the antenna installation does not include a conductive plane, the antenna no longer functions as designed, since the assumptions of its analysis no longer holds. Worse, the antenna may start interacting with the circuit or surrounding metal objects in an unpredictable manner, creating strange radiation pattern or electromagnetic interference.

This conductive reference plane is known as the "ground plane" or the "counterpoise". Historically, the first monopole antennas for longwave radio transmitters used the Earth ground itself for this purpose, thus the conductive plane became known as the ground plane. When an antenna is traditionally used with Earth ground, but the Earth ground is unsuitable (e.g. poor soil conductivity) or unavailable (e.g. above ground installations) for this purpose, metal rods or meshes can be placed at the bottom to achieve the same effect. In this case, the conductive object that serves as the substitute of the Earth ground is known as the counterpoise.

Using the actual Earth ground is not required or may not even be desirable - as long as the radio transmitter is connected across the antenna body and the reference plane (electrically large compared to the wavelength of the radio signal), many kinds of conductors can be used - such as a copper plane on the bottom side of a circuit board, the metal body of a car or an aircraft.

Sometimes the use of an actual Earth ground may not even desirable. At VHF and microwave frequencies, the impedance of the Earth soil is large but the wavelength is short, it's common to use metal rods and metal mesh under the antenna body as its "counterpoise" - although an actual Earth ground connection may still be present, but mainly as lightning protection.

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