Jim-NN7K wrote:


So-- Which is the most relevant noise measurement? Noise Figure-
or Noise Temperature? If one is better than another at a given
frequency, than another, and then the other is better at greater freqs,
WHY? (and, keeping in mind the FIRST stage establishes the Noise
figure,IF it's gain is enough to overcome the next stage's noise
figure) , then why is this a consideration?
Finally, as temperature is free space must approach absolute zero,
but, considering space "noise from stars, ect", what is it REAL
absolute Noise Temp of the (cold) sky? Inquiring minds want to know!

Jim NN7K

Depends on the frequency and things like humidity and cloud cover.

At microwave frequencies (say, 10 GHz-ish) 3-4 K is a good starting
point for dry air on a clear night.

If there's any loss in the path (e.g. from watervapor absorption) the
noise temperature comes up.

If there's anything hot in the path (e.g. clouds with liquid water) then
the noise temp comes up.

If there's something in the path (clouds) that reflects the energy from
something hot (earth) then the noise temp comes up.


This kind of thing is used to measure atmospheric moisture (look up
"water vapor radiometer")

I built a precision ground station to record an orbiting radar (on
QuikScat), and you could easily tell when it was humid or there was
cloud cover by just looking at the background noise level.
http://trs-new.jpl.nasa.gov/dspace/handle/2014/18497

Some BYU students made use of it, and have put up a nice website he
http://www.mers.byu.edu/QCGS/cgs_home.htm