Re: Offset dish geometry

Neil Smith G4DBN

I think it is mainly about convenience of transport and packaging.  The shape of the outside is a projection of a cone with its vertex at the dish focus. It is usually a circular cone, but can be an ellipse, triangle, square or maple leaf or any cross-section you like, so long as the features at the edge are large compared to a wavelength to avoid diffraction and other weirdness. In general the projection of the cone needs to be ovoid if you want to have a flat rim and best stiffness.  Again, for convenience, the centre of the parabola usually coincides with the edge of the ovoid surface, but it doesn't need to. If the feedhorn is large, it is sometimes better if the cone is offset a bit more to reduce feed blockage.  One or two segments of an 8-segment PF dish work fine when used as an offset, so long as you can illuminate the resulting Dairy Lea Slice effectively

Neil G4DBN

On 30/04/2021 15:27, David Law wrote:
My 10gHz 1m Gibertini dish sits flat on the floor, im guessing most do.

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