#### Offset dish geometry

John Lemay

Hello all

Hope this is not a silly question; With an offset dish, is the rim "flat"?
In other words if the dish is placed with its rim on a flat surface, will it
be in contact all around the rim ?

Can't get my head around the geometry !

Thanks

John G4ZTR

KENT BRITAIN

Can be, but doesn't have to be.

Kent WA5VJB

On Thursday, April 29, 2021, 01:22:41 PM CDT, John Lemay <john@...> wrote:

Hello all

Hope this is not a silly question; With an offset dish, is the rim "flat"?
In other words if the dish is placed with its rim on a flat surface, will it
be in contact all around the rim ?

Can't get my head around the geometry !

Thanks

John G4ZTR

Pete G4HQX

A standard offset (non Sky) dish is basically a plane cut through a parabolid volume of revolution so the edge would be flat and in fact would describe an ellipse. With Sky and some Vsat dishes the edges are then removed which stops it being flat!

Pete G4HQX

David Law

My 10gHz 1m Gibertini dish sits flat on the floor, im guessing most do.

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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