A Full-Height Waveguide to Thin-Film Microstrip Transition with Exceptional RF Bandwidth and Coupling Efficiency

Excellent progress in the development of Submillimeter-wave SIS and HEBmixers has been demonstrated in recent years. At frequencies below 800 GHz these mixers are typically implemented using waveguide techniques, while above 800 GHz quasi-optical (open structure) methods are often used. The choice of waveguide offers several advantages over quasi-optical techniques, such as the use of broadband corrugated feed-horns with well defined on axis (Gaussian) beam patterns. However, it has in practice been a problem to efficiently couple energy to the mixer (DUT) over a large fractional RF bandwidth. Many waveguide probe transitions have been proposed over the years, most of which with RF bandwidth less than 35%. Some of these probe designs require significant reductions in waveguide height (lower impedance). Unfortunately, reducing the height makes machining of mixer components at terahertz frequencies rather difficult. It also increases RF loss as the current density in the waveguide goes up, and surface quality is degraded. We propose a variation on the “single sided ” rectangular microstrip probe that is found in common use at the lower microwave and millimeter wave frequency bands. The proposed constant radius thinfilm microstrip transition couples with better than 99 % efficiency (VSWR ≤ 1.20) to an entire waveguide band. Extensive HFSS simulations, backed by scale model measurements are presented in this paper. The radial probe is seen to give vastly improved performance over existing designs discussed in literature. It is found that by selecting the appropriate substrate and radius any real impedance between 10-60 Ω can be achieved. Keywords Radial probe, full-height waveguide to thinfilm microstrip transition, capacitive waveguide tuning step, split-block, hot electron bolometer (HEB), superconducting-insulating-superconducting (SIS) tunnel junc-tion. I