Measuring S21 of FET using diode mixers and external generator. Feasible?

Setup:

1) PCB board with two parallel microstrip lines connected to power divider from one side and to two identical diode mixers from other side. One of lines have a gap for common source FET placement. Before placement gap is soldered with copper foil piece for calibration purposes.

2) Frequency generator for frequency F goes to power divider. One of the outputs of power divider modulated with frequency of ~1kHz making equal to Fm=F+1kHz. Unmodulated frequency acted as LO for two mixers. Modulated frequency used as input for parallel microstrip lines board.

Signal from generator F goes through power divider and modulated with frequency 1kHz. Modulated signal Fm goes to PCB board with parallel microstrip lines. As microstrip lines have identical lengths, relative phase shift must be equal to zero 0. IF would be 1kHz with equal phase for both mixers.

Then we open gap soldered with copper foil and replace it with FET transistor. Now parallel lines have non-zero phase shift because FET provides additional S21 angle phase shift. IF would be still 1kHz for both mixers, but with phase difference equal to S21 angle of a transistor.

Is it feasible to produce at X,Ku bands? Any flaws in this design?

Measuring S21 of any device requires a signal source (generator), and a pair of detectors, plus a power splitter, to get the test line and reference line.

How you design the above test devices is on you. You can use mixers, multipliers and more but you must calibrate all like in a "standard" configuration. With mixers you must use good filters to avoid unwanted signals from interfering.

VNAs utilize frequency synthesizers and converters to get up to >100 GHz, to measure all S parameters with a high accuracy.