Influence of Displaced Waveguide Feed on Terahertz Power Measurements

Calorimetric sensors that incorporate a rectangular waveguide interface facilitate absolute and precise power measurements. Moreover, by using a waveguide taper that transitions to a smaller, single-mode waveguide feed, the bandwidth limitation of a standard waveguide band can be extended to higher frequencies, enabling accurate power measurements across a wide portion of the electromagnetic spectrum. For aligned waveguide apertures without discontinuities, the incident field remains in the fundamental mode, thereby enabling broadband power measurements. In this work, we present a systematic study of intentionally misaligned waveguide junctions, specifically symmetrical H-plane or E-plane displacements at the WR-10 input flange of a PM5B power sensor, and their overall impact on the mismatch. The return loss was measured and simulated over a frequency span of 75 GHz to 1100 GHz, exceeding a decade. Electromagnetic simulations agree well with the experimental results, showing an excellent return loss for the aligned junction and a degradation attributed to the onset of higher-order parasitic modes in the misaligned case. In particular, it is found that the onset of a propagating TM<tex-math notation="LaTeX">₁₁</tex-math> mode due to E-plane displacement is the primary concern, while H-plane displacements have a negligible effect. However, a return loss better than 20 dB can be maintained if this offset is kept below 4% of the waveguide height. Hence, broadband, precise, and absolute power measurements are feasible provided that waveguide feed junctions are aligned and fabricated with high tolerances.