How do I design a feedhorn antenna for terahertz frequency operation?

Designing a feedhorn antenna for terahertz operation follows the same electromagnetic principles as microwave horn design but requires extreme precision in manufacturing because the wavelength at 1 THz is only 300 micrometers. Corrugated conical horns are the standard choice for terahertz systems due to their symmetric Gaussian beam pattern, low sidelobes, and low cross-polarization, which are essential for coupling to quasi-optical systems and cold loads in radiometer calibration. The horn aperture diameter is typically 5-15 wavelengths (1.5-4.5 mm at 1 THz), producing beam widths of 5-15 degrees. The corrugation depth must be approximately one quarter wavelength (75 micrometers at 1 THz), with corrugation periods of one-tenth wavelength or less. Manufacturing approaches include direct CNC machining of split-block horns (practical to about 1 THz), electroforming on a sacrificial mandrel, and deep reactive ion etching (DRIE) of silicon for frequencies above 1 THz where conventional machining cannot achieve the required tolerances.