How to configure gain - satnogs not nearly as good as gqrx

I’ve built a ~140 MHz VHF turnstile. When I connected it to my RTL-SDR and started GQRX, I had a great signal from e.g. NOAA-19, which completely suppressed noise. I then shortened the coax from 12 meters to 2 meters, and connected it to my satnogs station. Now, my satellite signals are hiding in the noise. I first tried with the default gain - I think that’s zero. I then increased to 25.4 dB - exactly same result. Now, I’m trying 49.6 dB gain, to see if that helps.

In GQRX, I can choose LNA gain (I don’t have an LNA, though), OR click “Hardware Gain”. Can I enable hardware gain in satnogs?

1 Like

The RF Gain option in the config is the amplifier built into the RTLSDR. (The gain setting in GQRX / SDR#)

Not sure what Hardware Gain is on GQRX

Okay. I’ve seen that an older version of GQRX also has something called MIX gain and BB gain, but it isn’t documented anywhere.

The funny thing is: Disabling HW gain in GQRX lets me play with the slider LNA gain, and it has a huge effect, even though I don’t have an LNA. It must be a built-in LNA in the RTL-SDR. I guess the HW gain button is some sort of AGC, which satnogs apparently does not utilize.

Hardware AGC (automatic gain control) is an option that rtlsdr exposes and changes dynamically it’s gain depending on the noise it receives, however it doesn’t work fine all of the times.

SatNOGS Gnuradio scripts don’t enable this option. So you will need to set the gain that rtlsdr will use (LNA Gain in Gqrx).

You will need to experiment to find the right one for your station, you can start by finding out in gqrx near what value you get the best results and then try the specific gains, around the gqrx value, in SatNOGS.

Note 1: About the specific gains, rtlsdr can get specific values as gain, you can find them either by running rtl_test in command line or in this wiki page.
Note 2: In SatNOGS for rtlsdr you will need to change only RF Gain, IF and BB gains are not used by this device.

3 Likes

Thanks for the clarification! I’ve now made a new observation of NOAA-19 with my gain maxed out at 49.6 dB, and it clearly makes no difference at all:
https://network.satnogs.org/observations/598892/

The signal is just as weak as with a gain of 0 dB. What can be the cause of this?

Could be your antenna is not well suited for NOAA observations.

But when I connect the antenna to my rtl-sdr, put my rtl-sdr in my laptop and use gqrx, I get a great signal, free from noise. That’s what puzzles me.

How is your waterfall adjusted in GQRX? It could very well be that the S/N ratio is identical between both software packages, but the waterfall in GQRX is adjusted a little bit differently and thus yields a “nicer” look? Just an idea.

–Roy
K3RLD

1 Like

I seem to have a similar problem with my station. My results with GQRX with RTL-SDR v3 are considerably better than when using SatNOGS. I tried some different gain values without obvious improvements.

1 Like

What ever came of this?

Same problem here…

Some more information about the OP’s station setup would be useful here.

I then shortened the coax from 12 meters to 2 meters, and connected it to my satnogs station. Now, my satellite signals are hiding in the noise.

This makes me wonder if there is an issue with noise radiated from the station itself, i.e. from a noisy switch-mode supply or similar. I have seen similar on stations powered by PoE injectors. Turn the PoE injector off and run the RTLSDR from a laptop, and the noise magically disappears…