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Re: 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?

That's a pretty broad statement to make about the average reciever having
a minimum discernible of -140dbm.  Bandwidth and the noise factor of the
first amplifier in the chain have alot to do with it.  Narrowing the
bandwith
from .5 to .1 kc with a narrow CW filter can improve the MDS by 7 db.

>  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? 

MDS is the point where the desired signal is equal to the noise, not
necessarily the point where information can be recovered.  The 
human ear can pick out tones below the noise as with CW.  SSB
phone transmissions typically have to be above the noise a few dBs
before we can make any sense out the them.  Sensitivity measurements
are the measurement of the magnitude of signal needed to recover
the desired information.  Mike's desired information was detection
of the signal through fourier analysis, not picking out a voice in the
noise with his ears.  The -140 dbm you quoted above was probably a
10db s+n/n measurement which is the point where the desired signal is
10 db above the noise, a fairly strong signal to his fftdsp program.

>  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 ?????

No, he's already established his detection level to be -179 dbm. The
signal minus path losses plus the antenna gain has to be at least
-179 dbm to be detected by his equipment. A 25 db preamp does not
equal 25 db of antenna gain. If it did you could add one to an omni
and do moonbounce. hi hi.  The calculated path loss for the MGS
was -189 dbm.  The 18 dbi array gain brought the signal up to 
-171 dbm, above the dection level for his equipment.

>  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)

The .2 uv is into a 50 ohm load.  Solve for power and then 10 times
the log of the power is the dbm.  The answer is -151 dbm. (I think).
Remember this is for a certain bandwidth, narrow the bandwidth and
the figure gets better. 
 
>  Its obvious that I'm not sure of my math skills, but I had to
> ask...

That's what this forum is for.  If you don't ask you'll never know.

>  Thanks , AB2CJ

No problem, Mike, KB8YHV/KH2




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