Dave C wrote:
"What point is considered the focuspoint of the horn?"

The equation for the parabolic reflector`s surface contour is:
R=2L/1+cos theta
R=distance to a parabooid`s surface point from focal point
L=vertex to focal point distance
theta=angle made by a direct ray to a surface point with the axis of the
parabola

A feeedhorn radiates into the reflector`s focal point, usually on the
parabola`s axis. All rays perpendicular to the plane across the mouth of
the paraboloid converge at the focal point due to reflection.

At the plane across the mouth of the paraboloid, the total distance from
the mouth to the reflector and from the reflector to the focal point is
the same for every point on the plane. Rays come and go in parallel
routes to its axis, in and out of the parabola, and all of these pass
through its focal point.

A parabolic reflector tends to collimate radiation from its focal point
into a beam parallel with its azis. Reciprocity rules so that incoming
radiation has the same directional characteristic.

Best regards, Richard Harrison, KB5WZI