well, measuring only the antenna reflection is not the total picture.
the cable plays an important role to, so yes I’ve tested cable+antenna with at least 6 db because I noticed that lower generator power gave wrong results.
sorry I didn’t see your scanner program (but I will)
I test a F range from 100-200 mhz in steps of 1 mhz normally, that’s fine enough (and fast 
)
I never meassured this (did you maybe ?) but the RF output of that rf95w when set to a certain level… how consisted is that level over the whole range ?
so you set it to 2 db but , is it really 2 db on 820 and 920 ?