Jim-NN7K . wrote in
:



So-- Which is the most relevant noise measurement? Noise Figure-
or Noise Temperature? If one is better than another at a given

As both Ian and I mentioned, Noise Figure is based on the degradation in
S/N ratio assuming that the source contributes 290K thermal or Johnson
noise (KTB noise) from the equivalent source resistance.

This if fine for describing the operation of a receiver when driven by a
standard signal generator.

The radiation resistance component of the equivalent source impedance of
an antenna is not a source of KTB noise, but is a source of received
noise power from various sources, and the level varies with many factors
including frequency and time.

Expressing a receive system performance as a Noise Figure assumes an
external or 'ambient' noise component that is of little application
relevance.

Expressing a receive system performance as an equivalent Noise
Temperature expresses only the receiver's internal noise, which is a
limited perspective from an application point of view. However,
comparison of the system's internal noise with the external noise gives
insight into the S/N degradation due to the system.

Both measures contain sufficient information, just that you have to
transform NF to obtain Teq which is the more direct input to calculation
of system S/N, or exploration of cascaded stages for example. Because of
this, NF is sometimes misinterpreted as to its direct signifcance.

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?

The first stage is very important in determining system noise
temperature, but in high performance stations, so are the losses in the
feed system, switching etc. The contribution of later stages should not
be considered insignificant until calculated.

Often, the LNA runs with so much gain that the transceiver AGC reduces
gain sufficiently to degrade transceiver noise temperature to perhaps
30,000K (NF=20dB). Consider a 0.5dB NF 35dB gain LNA (T=35K, Gain=3,000),
then it rolls 30,000/3000=10K into the system noise temperature which may
be significant depending on the external noise level.

Even worse is the scenario where an OM installs a 20dB attenuator between
LNA and transceiver to 'correct' S meter readings. In that case, a 5dB NF
receiver with 20dB attenuator has NF=25dB, T=90,000K, so it rolls
90,000/3000=30K into the otherwise same system... but this is done!

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!

IIRC the coldest part of the sky in the 5 - 10GHz region is around 4K. As
I mentioned in an earlier post, practical antennas capture significant
energy in their sidelobes, so the total noise input power might be well
in excess of 4K.

The more interesting question is the background when pointing in the
desired direction (eg the moon for EME), and how much sidelobe noise is
received.

Owen