"Joel Koltner" wrote in
:

"Owen Duffy" wrote in message
...
"Joel Koltner" wrote in
:
But here's an interesting paradox: If I take that output with 57dB
SNR and feed it to another, identical amplifier, shouldn't the SNR
at its output now drop to 54dB?
Appealing, but wrong.

Well, correct *under a certain set of assumptions*. As with, e.g.,
manufacturer's data sheets and quiz/exam problems done in school,
often these assumptions are unstated.

In other words, I'm purposely not stating my assumptions to
demonstrate how to get yourself into trouble more readily. :-)

An real antenna might supply much less through to much much more
noise.

How does an antenna at 290K supply less? I mean, ignoring how well it
works as an antenna, shouldn't it still have kTB worth of noise
generated just from the resistance in its conductors?

An antenna's feedpoint impedance comprises radiation resistance and loss
resistance.

Radiation resistance is a virtual resistance and does not contribute
thermal or Johnson noise.

It is a common mistake to consider that an antenna always includes 290K
due to kTB in its radiation resistance. If that were the case, we would
never have need for receivers with Teq much less than 290K! Mind you, if
a directive antenna points at hot earth, then external noise will never
be much less than 290K, so the requirements for terrestrial shots will be
different to space shots.

Attenuation gives rise to noise, and feed system loss is no exception.

An antenna does receive noise power from its environment, lets call it
external noise, and that needs to be factored into a receive system for
an overall figure of merit.

The ratio Gain/Temperature is antenna gain divided by total equivalent
noise temperature (internal and external) all referred to a common
reference point (usually the antenna connector or w/g flange). It is an
overall figure of merit, and if the power flux density (or field
strength) at the receive antenna is known, then S/N can be calculated
from that and G/T.

Hams tend to not use G/T.


I think that's rounding differences and my using T0=290K rather than
289K as a reference.

The 289 was not the reference, it was the result of using 3.00000dB NF (I
know you stated Noise Factor =2, but I used your rounded NF=3dB value).
....
I find noise temperatures just as if not more intuitive than noise
figures, and (to me) it's more obvious what's going on when you have a
string of amplifiers.

That was my point. Dealing with K is like dealing with power (P=kTB).


You might find this little calculator interesting / helpful:
http://www.vk1od.net/calc/RxSensitivityCalc.htm .

Looks nice, thanks!

Thanks.

There is a related calculator for deterimining the level of ambient noise
when receiver noise figure is known, see
http://www.vk1od.net/calc/anc.htm .

By and large, although lots of hams express an interest in weak signal
working, they aren't very interested in noise... which is a key parameter
determining whether a signal can be copied. I have asked scores of weak
signal enthusiasts their ambient noise level, and to date, only one has
answered (though not in absolute terms, but nevertheless had an
appreciation of the issue).

Owen