Sensitivity Analysis of Terahertz Horn Antennas

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 horn has a Gaussicity of 96% and deteriorates less with misalignment than a diagonal horn. 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 wavelengths where the alignment tolerance is significant. In contrast, the pyramidal or diagonal-spline horns are more robust against E-plane misalignment errors.