S meter noise goes down with ANT preamp increase

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I’m trying to understand why my 6700 S-meter noise readings go down from let’s say S3 with ANT preamp set to 0 dB and then reads S1 or S0 with ANT preamp set to 20 dB.

I’m assuming it’s resulting in an improved signal to noise ratio for some reason but it’s counterintuitive.
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Rick - W5FCX

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Posted 2 months ago

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Ken - NM9P, Elmer

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Essentially Yes.  ... at least until you get to the point that the internal receiver/preamp noise is lower than the antenna/band noise.  Then any additional preamp gain will not be productive.  You can tell by two means:

1) if your background noise on a band rises when you connect the antenna, then you don't need any more gain.

or

2) if your noise floor doesn't go down a little when you bump the preamp gain another notch, then you don't need the additional gain.

Having too much preamp gain gets you nothing except reduced dynamic range.

I don't need any except for 10 dB on 10 meters, and 20 dB on 6 meters.  Sometimes, on a really quiet 12 Meters, I may add 10 dB.  and if I am using a low gain receive loop on 80/160, then I may add some more gain to raise signal levels.

Ken - NM9P
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Bill W2PKY

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Believe Flex said if you observe 10 dbm of additional noise when attaching the antenna then no more pre-amplification is necessary. 
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Ken - NM9P, Elmer

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I believe you are correct, I was just trying to simplify my answer.
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HCampbell WB4IVF

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Paul

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Hi Rick, it might help you to look up Frii's rule for cascaded noisy stages eg. section 2.3.4 here,
which might augment Tim's description:

http://cas.ee.ic.ac.uk/people/dario/f...

Now just apply it to a two stage system where stage 1 is the pre-amp (With gain = G1, noise figure = F1) and stage 2 is the rest of the radio (with gain= G2, noise figure=F2):

Total noise Figure Ft= F1+(F2-1)/G1

You can see that with high G1, the contribution to the total noise from the rest of the radio (F2-1) is reduced by the factor G1. So, preamp in reduces noise contributed from the rest of the radio. Switch out the preamp the G1 is unity (G1=1) , hence overall noise goes up.

Caveat: a full calculation would include sky noise, feeder etc. Hope this helps. 73, Paul.
(Edited)
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HCampbell WB4IVF

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“So, preamp in reduces noise contributed from the rest of the radio.”

That’s my understanding also.  At “low band” frequencies, atmospheric (or “antenna”) noise is predominant.  At higher frequencies, the radio’s internally generated noise is of greater importance compared to external noise.  Each radio stage passes along the previous stages’ noise and adds its own.  Therefore, if the first stage is a “low noise” amplifier stage, it amplifies the signal with less of its own noise for succeeding stages to amplify and pass along in turn.  As frequency is increased, “antenna noise” decreases, and radio noise (and therefore the preamp’s utility) assumes more importance. 

Howard


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Rick - W5FCX

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That matches what I see. Improved S/N on 20-10m on my 5 band spiderbam at 20dB preamp setting, which reduces noise floor and brings in weaker signals. Only negative side effect is very strong signals IMD interfere more.

On 40m and lower bands, atmospheric and other noise from wire antennas dominates.
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Craig Williams

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I must use every button. Resistance is futile.