Because I want to reduce the noise from stuff in the bulding compared to
whatever hits the whip antenna. Sure, attenuation might reduce SNR in a noisy
resistance (or subsequent gain stage) but NOT due to due to simple shrinkage
of scale (R = Ratio...), but that's why I want to get the SNR higher to start
with. It's the separation of internal noise signals from external wanted
signals that matters, same as for anyone using coax. Surely it's not suddenly
wrong because I'm doing it? If so, this isn't about science anymore.



Well firstly, adding an attenuator WILL degrade the Noise Figure of your
receiver.

Now whether this will have any significance depends on the original NF
of the receiver, and the level of External noise (atmospheric and man made).

It we take your 'ideal' set-up where you have eliminated the pick-up in
the building; you SNR will depend on the NF of the rx and the levels of
signal and received noise at the antenna.

So if we have to put an attenuator in line to make the rx happy so it
does not overload, then we will have degraded the rx's NF by the value
of the attenuator. So say a 10db attenuator will have degraded you NF by
10dB. It will also have degraded the level of input signal by 10dB, so
you have a double wammy.

Now the effect on your SNR will depend on which band you are listening
on ant the level of atmospheric noise compared to the NF of the rx. At
lower frequencies the effect will be less because the atmospheric noise
is greater and will swamp the rx NF to some degree. As you go up in
ferquency the NF will start to become dominant.


If we take the case of leaky coax and pick-up in the building, sure the
picked up noise will degrade the SNR, but the leakage is likely to be
very small in absolute level and adding any attenuation will take it
below the noise floor anyway. The differences in leakage between the
types of coax under discussion will be insignificant compared to the
attenuation values you will have to add to make the rx happy. That is
why I questioned the choice or rx, having to add attenuation negates the
need to have the ultimate in screening unless the pick-up is a very
high level (in which case you are stumped anyway).

Also if your building is that noisy then there will be significant noise
pick-up directly into the antenna which you can do nothing about. There
is a significant possibility that this will be much greater than any
direct leakage into the cable making such pick-up insignificant.

Perhaps you should tell us why you think there will be this pick-up and
what you think the source is in the building. That will help you get a
sensible answer.

One final point, you say "It's the separation of internal noise signals
from external wanted signals that matters, same as for anyone using
coax." I suspect that is about the last thing that most people think
about when choosing coax, they normally think about the impedance first,
then the transmission loss, size and cost. I doubt if the "the
separation of internal noise signals from external wanted signals"
crosses their minds at all!!

Jeff