A Low-Reflectivity Vacuum Window for Rectangular Hollow Waveguides

The design and performance of a broadband and low-reflectivity millimeter-wave vacuum window for rectangular hollow waveguides are presented. A reflectivity below -15 dB is achieved over almost the full W-band. The vacuum window is sufficiently compact, so that two separate channels fit into a single ConFlat CF-35 vacuum flange. The proposed vacuum window utilizes a well-known circuit concept based on two-quadrature hybrid couplers together with two identical vacuum barriers (similar to a balanced amplifier). Due to the dielectric discontinuity at the vacuum barrier, a part of the incident wave is transmitted and another part is reflected. A hybrid lattice network recombines the transmitted signals and reroutes all reflected power to absorbers. The proposed low-reflectivity vacuum window has been fabricated in the WR10 waveguide technology. Broadband branch-guide couplers enable an operational frequency range of 75-105 GHz. Ultrahigh vacuum compatibility is verified by a helium leak rate test, and electrical measurements show good agreement with full-wave simulations.