Location of focus of feed horn?
 

On Fri, 3 Jun 2005 09:03:29 -0700, DaveC wrote:

What point is considered the focus point of the horn. The feed point for the
dish is calculated and known, but I'm not sure what point of the feed horn to
considered when aligning the horn to the dish.

Hi Dave,

A horn is an impedance matching device, not a lens. It is generally
pointed in the direction of desired maximum radiation. If you want to
employ what appears to be the equivalent to your application of focus,
then its "throat" is the term you should be interested in (at least
from the common literature on this subject - a subtle hint, in spite
of your protestations, on how to google this).

The plane of the throat is usually placed at the focus of the
truncated parabaloid (otherwise known as the dish) and orthogonal to
the parabaloid's major axis (the line that points in the direction of
the opening's maximum radiation). Hence both are collinear (and of
necessity, the horn blocks some of the available radiation), unless
this is an off-center feed. In such a case, the math becomes a little
more involved (but all structures conform to these usages).

73's
Richard Clark, KB7QHC