Low frequencies ( < 2 GHz) are
crowded:
Very powerful transmitters (TV and FM) (emitters ~100 kW)
Low power transmissions and GSM (emitters of 10 - 100 W)
Satellites
Protected (for radioastronomy) bands are very few and difficult to keep free of interferences:
151, 327, 408, 610, 1400 MHz
with bandwidth from 3 to a few ten MHz, in principle .
Satellites emissions may occur very close (149.9 MHz) to astronomy band
Antennas provide usually a poor rejection ( 10 db)