A 65-115 GHz VNA Extension Frontend Based on a High-Directivity Misaligned Waveguide Branch Coupler

The vector network analyzer (VNA) is the core instrument for S-parameter measurements in microwave and millimeter-wave bands. Frequency extension modules are indispensable for high-frequency testing, in which the directivity of the directional coupler directly determines the reflection measurement accuracy. To address the limited directivity and fabrication difficulty of conventional branch couplers in loose-coupling (10-dB) designs, this paper proposes a novel 12 dB waveguide branch coupler based on a misaligned structure and applies it to a 65-115 GHz VNA extension frontend. By introducing an offset in the branch waveguide, the coupler excites the TE₂₀₁ mode and utilizes the resulting transmission zero to improve bandwidth and amplitude flatness. Meanwhile, the branch-height compensation mechanism introduced by the misaligned structure effectively alleviates the fabrication difficulty in loose-coupling designs and enhances directivity. Based on this coupler, a miniaturized extension frontend of only 60 × 60 × 30 mm³ is designed and implemented. The indirectly estimated directivity of the six-branch coupler is better than 16 dB over the 65-115 GHz band, and the module exhibits a dynamic range exceeding 100 dB at a 20 Hz IF bandwidth with an effective directivity better than 38 dB. Compared with mainstream commercial modules, the proposed design achieves comparable electrical performance while offering smaller size and lower fabrication complexity, providing an effective technical pathway for ultra-wideband, high-performance, low-cost, and integrable VNA frequency extension frontends in the millimeter-wave regime.