S meter noise goes down with ANT preamp increase

Rick - W5FCX

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.

Ken - NM9P

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

Bill W2PKY

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

HCampbell WB4IVF

Here is Tim's explanation:

https://helpdesk.flexradio.com/hc/en-us/articles/204923669-How-to-determine-the-amount-of-RF-Preamp-...

Howard

Ken - NM9P

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

HCampbell WB4IVF

“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

Rick - W5FCX

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.

Craig Williams

I must use every button. Resistance is futile.