I've been testing the new KC901V for a couple of weeks now, and find that in most respects this model is a significant upgrade to the KC901S, which I also have. Kudos to the engineers who designed it.
One problem that I noticed though is that the unit needs 10-15 minutes of warm-up time to generate accurate measurements. Otherwise S11 values for a calibrated open or short can be off by 0.5 - 1.0 dB across the entire frequency range. This was not the case with KC901S.
Please see the image below. The unit was SOL calibrated and shut down for more than an hour. On power up, I attach the same short that I used for calibration, expecting an S11 reading of 0 dB. Instead I get the curve shown in blue (by pressing the hold button). 10-15 minutes later the curve shifts to the yellow line, as expected.
I performed the same test with 901S and only see a 0.05 - 0.1 dB shift there. The large drift can of course be a problem if the unit is designed to be operated in the field, where waiting for 10 minutes for the unit to warm up may be impractical.
Is this a known issue for the beta version of 901V? Is there a possibility of correcting this issue with a firmware update?