This figure illustrates the parameters of the pyramidal horn with generally unequal slant radii in the principal plane. The slant radius is used to compute the quadratic phase factor S in each plane given the aperture length and wavelength of operation. 
RHOPTG (Linux version) - Pyramidal horn designed to gain
REBWDN (Linux version) – Pyramidal horn designed for given beamwidths
HORNPY (Linux version) – Pyramidal horn parameters from dimensions