The set-up and capabilities of a VNA Wiltron 360 extension to the frequency range 70.5 - 82.5 GHz are reported. The swept signal (1.5 - 13.5 GHz) emitted by the test-set is up-converted (see Fig. 1) in a broadband mixer by a fixed local oscillator (BWO at 69 GHz). A 70 GHz cut-off filter (suppression of the carrier better than 80 dB causes an amplitude error less than 0.001 dB and a phase error less than 0.006 degrees /1/) behind the mixer allows only the upper sideband to pass, whose amplitude and phase information is known because the baseband serves as a reference (automatically in the test-set). After passing the device-under-test in transmission the signal in the frequency range 70.5 - 82.5 GHz is down converted to the baseband in a second mixer, which is pumped by the same local oscillator via a flexible phase stable (see Fig. 2) dielectric fiber with corresponding transitions to rectangular waveguide (see Fig. 3). So geometrical lengths of devices up to 25 cm can be compensated and different geometrical positions of the ports, too. This is a significant improvement related to rigid rectangular waveguide for compensation /2/ and also related to a proposed quasi- optical line stretcher /3/. After amplifying this signal is fed in the test-set of the VNA. Because the up and down conversion is done by the same source, no problems (e.g. source locking, measurement of high Q-factors) occur in spite of a FM deviation of nearly 1 MHz of the local oscillator. A noise floor of -80 dB relative to a thru calibration (at minimum bandwidth and 512 points averaging) is achieved, although not optimized single ended harmonic mixers, which require directional couplers to separate and combine the signals, respectively, are used in the fundamental up and down mixing operation. A second problem caused by using single ended mixers in reflection is the cross-talk via the dielectric fiber. So three isolators (isolation better than 75 dB) are provided to suppress the cross-talk. Furthermore the amplitude and phase reproducibility is better than 0.1 dB and two degrees, respectively (see Fig. 4).