receiving specturm range

Hi. I just use the the RTL2832U and Rt820t dongle to scan the wireless and draw the specturm of what i get. But i find the data i calculate is never bigger than -46dbm also never smaller than -63dbm.Is that the dongle's receiving ability is just like that? By the way, here is how i test the dongle: 1、i turn on a basestaion 2、i used the gsm module to detect the power of basestaion nearby,i get about -20dbm as the biggest 3、i used the dongle which i set its working frequency to the frequency that basestaion used 4、i calculate the spectrum using the data return by dongle(honestly, i used the rtl_power to calculate) the result i calculate is never big than -46dbm,what a difference from the result from gsm module. So i wonder whether the dongle's receiving abllity is just ordinary or the way i calculate is wrong?