some good reading here.
https://www.boonton.com/forms/principles-of-power-measurement-form
Easier to search for "boonton principles of power measurement"
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=2ahUKEwisu_-Fm-LeAhWKmuAKHbrfAuAQFjABegQIAxAC&url=http%3A%2F%2Fwww.boonton.com%2F~%2Fmedia%2FBoonton%2FReference%2520Guides%2FWTG_RefGuide_F1128_sm_web.ashx&usg=AOvVaw3CUwYZFunzbmZd4JDw4qhH
RFI-EMI-GUY