Result of today, successfully calibrated range (1000V AC) but not finished it completely. Otherwise, let me say that the ut181a is calibrated at several different frequencies and goes up to 100KHz.
Just to add that you will need a function generator to calibrate the AC range, preferably with the highest possible output voltage, in my case it is 20V.
It is possible that you will need a function generator amplifier for this job since the sine signal drop quickly above 4Khz and that is only if you decide to go on this adventure.
What is important is that for everyday work it is possible to calibrate the AC voltage with the help of a small ordinary transformer using it as an amplifier of the AC signal.
To be honest, I doubt anyone will measure 1000V at 100KHz.
That's it for now, I'll look at making a little tutorial with pictures after the job is done to make it easier to set up.