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Could one measure S22 of a 10 kW amplifier?

Question asked by drkirkby on Jan 3, 2014
I'm asking this question more out of interest sake than because I have any immediate need for it, though a couple of decades ago when building a high power 144 MHz amplifier for amateur radio use, it might have been useful to me, although I did not own a VNA at that time. 

This follows a discussion that started on a Yahoo group. 

Could one measure S22 of an amplifier rated at say 1.5 kW output, but to be safe one might assume it could generate as much as 10 kW (+70 dBm)?  Now in principle if the amplifier is not oscillating, one could terminate the input in 50 Ohms, and just connect the VNA to the output. But I can't see many people wanting to risk that. So really one would be looking at having something like a 50 dB attenuator between the amplifier and VNA to ensure the VNA can't be damaged. I don't think there is any way a VNA could be calibrated like that. 

Plenty of crazy ideas were suggested by some, but the most promising might be loosely based on an S21 rather than S22 measurement. If one powered the input of the amp, and measure the output voltage at two different load impedances (say 48 and 52 Ohms), one could probably infer the output impedance of the amplifier from how much the output voltage changes for a small change in load impedance. 

I'm guessing doing a response calibration with a 10 kW, 50 dB attenuator would be possible. 

Any thoughts? 

PS, I've had a few beers tonight, so if I sound crazy, put it down to the C2H5OH.