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16806Re: Perseus sensitivity

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  • pd0psb@hotmail.com
    Nov 15, 2011
      Hi Leif,

      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.
      > 73
      > Leif / SM5BSZ
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