# Talk:Noise figure

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## Noise figure in attenuators

"Attenuators have a noise factor F equal to their attenuation ratio L" I just can't see how this can be correct, for example an attenuator made of capacitors (voltage divider) will not introduce any new noise so the SNRin and SNRout will be the same, therfore the noise figure of such an attenuator will be zero, I am sure that there are many other examples where the NF is zero or irrelevant. A resistive attenuator's NF would be a function of the resistors used not the level of attenuation. — Preceding unsigned comment added by 114.34.217.17 (talk) 04:31, 27 March 2014 (UTC)

The signal gets attenuated by L but the thermal noise stays at the same level; consequently, the NF gets worse by the attenuation factor. Glrx (talk) 03:35, 22 December 2015 (UTC)

## Satellite communication systems

The article currently says:

...in the case of satellite communications systems, where the antenna is pointed out into cold space, the antenna effective temperature is often colder than 290 K. In these cases a 2 dB improvement in receiver noise figure will result in more than a 2 dB improvement in the output signal to noise ratio.

From the definition of NF, I don't see how this can be true:

${\displaystyle \mathrm {SNR_{out,dB}} =\mathrm {SNR_{in,dB}} -NF}$

The receiver doesn't know anything about the signal source. What am I missing? GyroMagician (talk) 10:29, 19 March 2010 (UTC)