Abstract
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.
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