i use an antenna indoor an amplify using s9018 transistor but i normally get 50 hz consistently...not sure why is that...then i play another antenna next to use and the frequency oscillate around 10 hz to 20 hz only...
why is the frequency of the amplification output so small. how much is the frequency from the antenna without amplification normally?
Assuming you live in Europe or at least not in US. Power lines are 50 Hz AC (if you were in US you would see 60 Hz). You probably have the antenna near some power lines either a power cord or the power lines running through the wall and are picking up AC power line noise.