- Nov 15, 2011Hi 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!

73

Paul

PD0PSB

--- 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

> - << Previous post in topic