Thank you very much for your elaborate response.
It is exactly the info I was looking for and quite a bit more as a bonus :-)
As a rule of thumb I always try to achieve a rise in noise of around 6dB when plugging in an antenna for HF DX use, so that roughly corresponds to your presented calculations.
Very clear explanation and very much appreciated!
--- In perseus_SDR@yahoogroups.com, Leif Asbrink <leif@...> wrote:
> Hello Paul,
> > Regarding this subject I have a question for you which has been on my mind:
> > Suppose.
> > At the antenna I find a noisefloor of -115dBm and a weak signal
> > of 3dB SNR just above the noisefloor.
> > How far does the noisefloor+signal need to be lifted above
> > the RX selfnoise so the original SNR at arrival does not deteriorate?
> > IOW How far apart does the bandnoise at antenna and RX selfnoise
> > need to be to make (partly) masking of the weak signal negligable?
> Negligible depends on who you are. For casual DXing you might
> say that 1 dB is negligible. In the EME community a signle dB is
> a significant loss. When listening for weak signals bounced off
> the moon one would not want to loose even 0.1 dB.
> > I have always assumed that if antenna noise and RX selfnoise
> > are equal, a weak signal still deteriorates by a certain amount
> > by the adding of those "2 noise layers" , is this correct?
> > and how much?
> Yes. Absolutely.
> The noise from the antenna and the RX selfnoise is uncorrelated.
> They add linearly by power. When the noise floor of your antenna
> equals the self noise of your RX the sum of both of them is
> twice the power compared to the selfnoise alone. Twice the
> power means 3 dB. That means that if your noise rises by 3 dB only
> when the antenna is connected, 50% of the noise is the selfnoise
> and your S/N is half as good as it whould have been with an appropriate
> preamplifier. You would have a 3dB S/N loss.
> The key factor is by what amount your noise floor rises when the
> antenna is connected. Here are some examples where R=selfnoise and
> A=antenna noise.
> Rat=(R+A)/R is the amount by which the noise increases when the
> antenna is connected. It is a power ratio and it can be expressed
> in dB or as a factor in linear power scale.
> S/(R+A) is the S/N ratio of the desired (weak) signal.
> S/A is the S/N ratio we would have in a noise-free receiver.
> Sens=[S/(R+A)]/[S/A] is the sensitivity we have vith respect to
> the sensitivituy we would have had without selfnoise. It is a
> power ratio. We can express it linearly or in dB.
> Sens simplifies to A/(R+A) =1-R/(R+A)
> The following table gives some numbers:
> Rat Rat Sens Sens
> (dB) (lin) (lin) (dB)
> 3 2 1-0.5=0.5 -3.01
> 6 4 1-0.25=0.75 -1.24
> 10 10 1-0.1=0.9 -0.45
> 16 40 1-0.025=0.975 -0.11
> You should measure Rat with the best noise blanker enabled.
> Pulses can be eliminated and should therefore not be included
> in the noise floor summations.
> A DXer should have at least 6 dB noise increase when
> connecting the antenna while a moonbouncer should have a
> noise increase of at least 15 dB when connecting the
> antenna-mounted preamplifier.
> With too much gain the receiver might saturate. There is no
> reason to go above 18 dB or so. If you have saturation problems
> it is reasonable to go as low as 6 dB. If it is necessary
> to attenuate more one should use filters to filter out
> the frequency of the local station(s) that cause saturation.
> Leif / SM5BSZ