Hi all,
What Sinisa is describing is the noise that will come out the audio
output of the receiver when a termination is connected to the antenna
jack. When we're really using the receiver, in addition to all of that
is the noise picked up by the antenna. On the lower bands the noise
picked up by the antenna will almost always be the dominant noise, being
way, way above the thermal noise and the excess noise of the receiver.
Only in a really poor receiver (with a very high noise figure) or a very
inefficient antenna system would the receiver noise be anywhere near the
antenna noise. As we move up to higher frequencies terrestrial noise
gets lower and the receiver thermal noise and excess noise become a
greater proportion of the total noise.
With an antenna connected, output noise can be represented as:
output noise = gain * (noise from antenna + thermal noise + excess noise)
and on the lower bands with a fairly efficient antenna and any decent receiver
we can almost ignore the thermal noise + excess noise and be pretty accurate
saying:
output noise = gain * noise from antenna
One could argue that everything coming from the antenna is a signal. It is NOT
receiver noise, so it is not part of the receiver noise figure.
As we try to improve the desired signal to undesired signal ratio we sometimes
use very inefficient receiving antennas, and the contribution from thermal
noise and excess noise may again become significant.
Ken N6KB
Sinisa Hristov wrote:
Hi Ken
Quoting from http://en.wikipedia.org/wiki/Noise_figure
"...noise figure (NF) is the ratio of the output noise power of a device
to the portion thereof attributable to thermal noise..."
The output noise can be represented as:
output noise = gain * (thermal noise + excess noise)
where
thermal noise is a physical constant constant (for a given temperature and BW)
excess noise = added noise of an "imperfect receiver",
referenced to the input
Now
noise figure = (thermal noise + excess noise) / thermal noise
(usually expressed in dB)
Therefore, the audible noise is affected by two variables:
* the first is "excess noise" and the receiver is
clearly better if it has less of it;
* the second is mere gain and it cannot change the signal to noise ratio.
Now consider the following two receivers:
* RX A having a lot of excess noise and a very low gain;
the output noise will be low, but the signal to noise ratio
will be degraded seriously due to the large excess noise;
* RX B having almost no excess noise and a very high gain;
the output noise will be high, but the signal to noise ratio
will be degraded very little.
73,
Sinisa
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