Weak Signal amateur radio is a battle against noise,
if you understand noise and how noise affects your system,
you can do a lot to improve your system
finaly you will arive at the point were you cannot win from thermal noise

What is Thermal Noise

What is –174 dBm/Hz?
This is a convenient number to use, it represents the amount of power in a one hertz bandwidth that a thermal noise source has at the reference temperature of 290°K, which is approximately room temperature. This results from the equation P = kTB where k = Boltzmann’s constant, T is temperature in degrees K, and B is the bandwidth in Hz. For example the available thermal noise power in a resistor in a 1 MHz bandwidth would be –114 dBm, because 10 log (1 MHz), or 60 dB, is added to the –174 dBm/Hz

How do noise powers add?
Noise powers add as incoherent signals which means that their powers must be added. For example if your inject a noise source into a spectrum analyzer and see that the noise floor increases 3 dB, then the actual noise source power is at the original noise floor level. This relationship allows you to calculate the noise power of signals below the measurement noise floor:
10 log [{Inverse log (diff/10)} – 1)]
Where diff is the dB difference in measured powers. Of course, small changes in power occur as the unknown noise is far below the known and this results in increasing inaccuracy as the power goes much lower.