Can't get below -46 dB

Hi I have an Elonics 4000 USB receiver, and I-m trying to do some measurements on the digital television received power in my area. Everything works fine, except that the received power is never lower than -46 dB, even on channel which are not occupied. I was expecting to get values like -120 dB or something similar, instead in my measurements the lowest point in any portion of the spectrum is circa -46 dB. Am I missing something? Thank you Best