Elaborating: The "measurement range" quoted by the manufacturer is the range between the displayed carrier level and the displayed noise level. That range deteriorates from 300 - 600 MHz because the lower energy second harmonic is used for those ranges. It further deteriorates for the upper ranges because the third harmonic is used there.
The manufacturer does not specify accuracy in measurement and Alan Victor's mathematical observation is borne out by the variance of observed return loss measurements at values lower than -30 dB.
Not only are RL measurements below -30 fairly inaccurate, they are also nearly irrelevant for the sort of work this unit was designed for. If you truly need RL measurements below -30 dB then you need to be prepared to spend more than $50.
Warren Allgyer
WA8TOD
Warren Allgyer