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A few days ago I made a trip to the VOR-DME near my local airport LOWG, because I always wanted to see one. It's GRZ on 116.2.

When I was there, I wondered why there is a giant metal grid under the actual antennas of the VOR.

VOR GRZ

Source: Klaus Krasser

Close picture:

VOR Close up

Source: Klaus Krasser

The grids squares have a size of estimated 10x10 centimeters.

What is this grid for? Is it maybe used as a shield or reflector? 116.2 Mhz has a wavelength of ~2.6 meters, so the grid would reflect the radiation. If thats the use of the grid, why is it needed?

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I would imagine the main reason is to make the performance of the antenna system more predictable and consistent.

Many antennas systems need a ground reference. All antenna systems are affected by nearby conductors. Using the grounds surface as your ground reference has a few issues.

  1. It means your antenna system is at ground level. That means any nearby structures can potentially interfere with your signals.
  2. The conductivity of the ground is weather-dependent. This would make the performance of the antenna system weather-dependent.
  3. Speaking of obstructions you have to put all the electronics somewhere. You don't want a cabin sitting right next to your ground level atenna but you also don't want long feed cables between your antennas and your electronics.

By lifting the system up off the ground and using a metal mesh as the "ground" reference issues with obstructions and varying ground conductivity are avoided.

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I think this is an example of a counterpoise:

[It] is a network of suspended horizontal wires or cables (or a metal screen), used as a substitute for an earth (ground) connection in a radio antenna system (...) when a normal earth ground cannot be used because of high soil resistance or when an antenna is mounted above ground level.

Generally used for LF and VLF antennas as that is where they are most efficient.

An example from that Wikipedia page of an AM radio antenna counterpoise:

enter image description here

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    $\begingroup$ I believe it's also the same principle that lets the antennas on the airplanes themselves, or on your car work. Though when it's a solid piece it's referred to as a "ground plane". $\endgroup$ – Perkins Sep 1 '17 at 20:30
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    $\begingroup$ oh, and thanks for the edit @ymb1 - much better. I should have done that myself :-) $\endgroup$ – Rory Alsop Mar 22 at 15:38

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