Master's thesis presentation Andre George Koj

Abstract: Horn antennas are commonly integrated with waveguide components for terahertz applications. Typically, the antenna structure is milled with the waveguide feed in two metal halves, split along the E-plane, and clamped to form the horn antenna. However, misalignment between two split-blocks due to machining tolerances will degrade the coupling to a free-space fundamental Gaussian mode (Gaussicity). Especially at supra-terahertz frequencies (>3 THz), the tolerances can be a few per cent relative to the wavelength and radically degrade the optical coupling. This master thesis theoretically investigates the split-block misalignment's effect on a diagonal horn antenna, validated using a scaled experiment at 360 GHz. Moreover, the effect of misalignment is investigated for diagonal-spline and pyramidal horns to find a more robust alternative.

The analytical investigation found that an E-plane misalignment leads to a phase and amplitude shift between the TE10 and TE01 modes at the aperture of a diagonal horn, decreasing the Gaussicity. To verify the theoretical investigation, a diagonal horn is machined in split blocks with three different alignment cases and characterised using a WM-570 near-field measurement system. Finally, full-wave electromagnetic simulations of a diagonal, diagonal-spline and pyramidal horn at 360, 415 and 470 GHz are performed and compared for different alignment cases.

The measured radiation pattern of a diagonal horn agrees with the analytical and simulated results. A 2-dB decrease in Gaussicity is observed for a misalignment of 6% relative to the wavelength. Furthermore, based on simulated results, a diagonal-spline has a Gaussicity of 96% and deteriorates less with misalignment than a diagonal. In addition, a simple pyramidal horn, with a Gaussicity of 88%, is the least sensitive to misalignment due to the single-mode operation.

A diagonal horn is susceptible to alignment errors and, therefore, is concluded to be less suitable as an E-plane split horn for supra-THz applications. In contrast, the pyramidal or diagonal-spline horns are more robust against misalignment.