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Path loss and signal calc...



Hi all,
 
  AF9Y calculated the path loss for the Mars probe at -179dbm. 
If the average receiver has a minimum disernable signal of -140dbm.
How can one hear a signal thats -39db below the receivers noise floor?

 I would assume that  antenna gain and any pre-amplification
is taken into account, and the fact that Mike,(AF9Y) used his DSP
program to graphically display this signal that could not be heard
by the human ear , but how can one receive a signal that is far
below that of the receivers noise floor? 

 Is the calculation for this problem as simple as taking the 
preamp gain and antenna gain and adding them to the path loss
of the signal? If so, assuming a total gain in the system of 
50db then.... 

 -179dbm + 25db(pre-amp) + 25db(antenna gain) = -129dbm ?????

 Also, if the sensititvity of a receiver is said to be .2 micro
volts at (10db s+n/n), that would mean that the receiver would 
produce a signal of 10db above the noise for an input of .2 uv.
 How can one relate this mesurment with  signal strengths
that are expressed in dbm or decibles as they relate to microvolts?
That is, how does a signal that is said to be -140dbm relate to
the measument of .2uv at (10db s+n/n)


 Its obvious that I'm not sure of my math skills, but I had to
ask...

 Thanks , AB2CJ




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