RTL-SDR /LNA gain dropping randomly

Hello everyone,

I’m seeing some strange behavior on my ground station (SatNOGS Network - Ground Station VZLU-PRG). It’s using RTL-SDRv3 on RPi 3B+, with Kuhne MKU LNA 432 A. This setup seemed to be working well at first (i.e. for a couple of months), however, recently I noticed large gaps in the waterfalls - see for example SatNOGS Network - Observation 5457127.

It seems like for some reason, gain drops off randomly, so I’m suspecting issue either in the RTL-SDR, or the LNA. A broken coax from the antenna could be another possible cause (the station has a rotator), but I wouldn’t expect it could randomly recover from that. As far as I can tell just by inspecting the cable, there should be no issues there.

In case it’s helpful, here’s the relevant portion of the config file. Any ideas what could be wrong are appreciated!

satnogs_antenna: RX
satnogs_rf_gain: '28'
satnogs_rot_model: ROT_MODEL_NETROTCTL
satnogs_rot_port: localhost:4533
satnogs_rx_samp_rate: 2.048e6
satnogs_soapy_rx_device: driver=rtlsdr

Check you antenna connections, cable, connectors etc.

I had similar happen.
Ended up being USB to rtlsdr cable not correctly plugged in.

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@vk4smc @PE0SAT Thanks for both suggestions, I’ll check it out as soon as I can - disadvantage of having the station in a different city I guess :slight_smile:

I observed this behaviour when enabling the AGC function(s). Nearby strong signals will overload the RTL-SDR stick. I have two local repeaters not far away and whenever they open, I have observed this.
Switch of AGC and adjust manual gain to a minimum value where you are just nit limited. y the noise figure if your RTL-SDR (in my case aroubd 25-30 db for 2m/70 cm bith woth extenal LNAs and 38 db for L-Band).

As far as I can tell, AGC should be disabled. I still did the excercise with manual gain/noise floor checking and ended up lowering gain to 11 dB (instead of the 28 I was using previously). This is the result: SatNOGS Network - Observation 5470534

Still it’s cutting out, so now it’s starting to look more like a cable issue.

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