Calculating power emitted from electronics
You can measure the cosmic background radiation using relatively simple equipment.

The guys in this report did it, and I've read reports from others who did it as well.

I have a setup that could be used for those measurements. I haven't gotten the software finished to actually do it, though.

The usual way to account for thermal noise is to calibrate it out.

You take measurements with the dish pointed at objects with a known temperature, and measure the intensity of microwaves that they put out.  From that, you can figure the measured intensity for any temperature.  If you extend the line down to absolute zero, then you'll find a microwave intensity that is non-zero.  That's the thermal noise contribution of the whole system.  Ideally, there would be no radiation at absolute zero so any value above zero is the error (thermal noise) of your equipment.

The linked report describes using liquid nitrogen and ambient temperature as two calibration points.

It also describes using ambient temperature and heated water as calibration points.

Both experiments delivered results that agree with the commonly accepted value for the CMB.

If you search on the names (Piat alone or Stein and Förster together) then you can probably locate their own reports of their work.



The difficulty with trying to do anything with a signal to noise ratio here is that the signal you are trying to pick up is noise.  It is simply wide band noise, and a typical receiver will pick up a portion of it because the receiver has a limited bandwidth.

Whatever you do, you'll have to calibrate your equipment to read as a temperature.  You just do the calibration so that you can pick out the total system noise as well.



Stein and Förster were high school kids in Germany when they made their measurements of the CMB.