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Wideband millimeter-wave branch-line couplers have been demonstrated using micromachined air-filled rectangular-coaxial lines supported by quarter-wavelength stubs. The coaxial lines are constructed by bonding five layers of gold-plated silicon or SU8 slices. The measured coupler shows an insertion loss of 3.2-4.0 dB in the coupled and thru ports between the frequencies 31.3-47.6 GHz. The isolation is better than 12.8 dB, and the return loss less than -10 dB. In addition to providing support, the stubs enhance bandwidth when properly placed.