The work of this research is the design, analysis and verification of the optical performance of a 4mmreceiver channel
for the 20 m telescope at Onsala Space Observatory, Onsala, Sweden. The 4 mm (75 GHz) receiver is a newly proposed
channel designed to be installed parallel to the existing 3 mm (100 GHz) channel targeting new science at that longer
wavelength. Gaussian beam mode analysis is used to produce the fundamental optical design of the system. The design
is then analysed more accurately with the physical optics approximation. We report on the comparison of simulation
and measurement and verification of the system design.