Antennas 6.2.10

10.3. Gregorian Reflector

This option allows the user to create a gregorian reflector system. When this option is selected, or the "gregorianReflector" command is called without parameters, the following panel will be shown:

"Add Gregorian Reflector" panel

"Add Gregorian Reflector" panel

Additionally, a preview of the antenna (using the current parameters of the panel) is shown in the geometry panel. This preview will be shown in green and will be updated everytime a parameter is modified in the panel.

Preview of the gregorian reflector system in the geometry

Preview of the gregorian reflector system in the geometry

A gregorian reflector system is composed of a main (paraboloidal) reflector and a secondary (ellipsoidal) reflector. The geometry of the main reflector is a portion taken from a paraboloid with its axis pointing in the same direction as the Z axis. The portion is determined by the offset: the main reflector of a front-fed reflector system (no offset) will be centered at the vertex of the paraboloid. In an offset-fed reflector system, the main reflector will be a portion of the paraboloid separated by a distance D' from the vertex of the paraboloid.

On the other hand, the geometry of the secondary reflector is defined by the geometry of the main reflector and the eccentricity of the ellipsoid the secondary reflector is based on.

The following parameters define a gregorian reflector system:

  • Position: This point will be the vertex point of the paraboloid that defines the main reflector. Note that the vertex point does only match the geometric center of the main reflector if the gregorian reflector is front-fed (i.e.: no offset).
  • Focal length: Defines the distance, along the positive Z axis, from the vertex point to the focal point of the paraboloid. This point will also be one of the foci of the ellipsoid.
  • Feed: Defines the distance, along the positive Z axis, from the vertex point to the point where the antenna feed should be located. This distance should be smaller than the focal length. The point defined by this parameter will be the second focus of the ellipsoid.
  • Diameter: Defines the diameter of the main reflector. Note that this is the diameter of the circular projection of the main reflector on the XY plane (projected along the Z axis).
  • Eccentricity: Defines the eccentricity of the ellipsoid used to create the secondary reflector. Therefore, this parameter will affect the curvature of the secondary reflector (values closer to 0 will cause the secondary reflector to be more spherical-shaped, while values closer to 1 will cause the secondary reflector to be more ellipsoidal-shaped).

Additionally, the user is able to select if they want to create a front-fed gregorian reflector or an offset-fed one. By default, a front-fed gregorian reflector system will be created, and by checking the "Offset-fed" option the user can create an offset-fed gregorian reflector:

"Add Gregorian Reflector" panel when the "Offset-fed" option is selected

"Add Gregorian Reflector" panel when the "Offset-fed" option is selected

There are two ways of creating an offset-fed gregorian reflector:

  • Circular shape: Selecting this option will cause the subreflector to have a circular projection on the XZ plane. This mode of creating the reflector requires an additional parameter: the separation (also called D') along the X axis between the left border of the main reflector and the vertex of the paraboloid it is extracted from.

Offset-fed gregorian reflector with circular shape

Offset-fed gregorian reflector with circular shape

  • Custom shape: This option allows the user to customize the shape of the main reflector (and consecuently, the shape of the secondary reflector) by selecting a closed set of curves that define the projection of the main reflector on the XZ plane. To do this, the user needs to select the "Custom shape" option, select the set of curves and press the "Select curves" button (the preview will be updated if the selected curves are adequate).

Offset-fed gregorian reflector with custom shape, and the curves used to define the shape

Offset-fed gregorian reflector with custom shape, and the curves used to define the shape

In order to actually create the geometry of the gregorian reflector system, the user needs to press the "Save" button.

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