Do not try to measure the source impedance of an active device using a VNA. It is possible but it is not a straight forward measurement and requires additional devices to complement the measurement. This is particularly problematic at large RF power levels. If you wish to measure the COLD non RF output power level that is certainly safe but of course only an estimate as the active device is OFF.
There is a rather simple method for measurement of an active device source impedance and it is defined by the LOAD PULL METHOD. It requires a variable line stretcher, that is a transmission line with adjustable physical length, a known load impedance, for example 100 ohms, a directional coupler and a spectrum analyzer. The heart of the method is based on the principle that if I sample the ratio of the power change between the incident and reflected power from the directional coupler for a known load pull range, I can deduce the magnitude of the source impedance. If this is of interest, I can try to find the details of the measurement system.
Alan
