  A key challenge in developing terahertz front-ends is achieving high coupling
efficiency between the waveguide feed horn and the optical beam. In this paper,
we have quantified the alignment requirements for the widely used E-plane split
diagonal horn antenna through theoretical analysis, electromagnetic simulation,
and experimental validation within the 325-500 GHz frequency range. The results
from our analytical models, simulations, and measurements are consistent and
shows good agreement. They reveal that even minor geometric asymmetries can
cause significant increases in fractional power radiated to the cross-polar
component due to amplitude and phase imbalances in the TE10 and TE01 modes.
Furthermore, a misalignment of approximately 8% of the wavelength was observed
to result in a 3-dB degradation in the optical coupling to a Gaussian beam
(Gaussicity) in middle of the waveguide band. These findings highlight the
critical importance of precise alignment and feed horn machining for the
successful implementation of terahertz front-end systems.
