If the cross-section is a parabola then the equation is of the form y-k=a(x-h)^2. The constant a controls the spread of the parabolic arms, but for now we'll put a=1. If 0<a<1 the arms are wider apart than if a≥1. We'll use a=1. If the end of the feedhorn forms the vertex of the parabola at the origin, h=k=0 so y=x^2 for a U-shaped parabola. If the length of the feedhorn is 9" then y=x^2 and -3≤x≤3 so that the largest value for y=3^2=9. Variations are possible: x=y^2; -3≤y≤3; y=-x^2 -3≤x≤3; x=-y^2 -3≤y≤3. The parabola is the same shape in each case with its vertex at the origin, but there are four different orientations of the parabola in all (north, east, south, west).