Gain Setting

This is my feeder, a full kit 1090/978 bought from ADSB - X. 15' of LMR-400 to the 5.5 dBi outdoor antenna. I read the threads and guides on gain setting and after a couple of weeks, edited the default config file to reduce the gain from 49.5 to 6 dB less. I've read the guides and forum posts on gain setting but have been experimenting by editing the config file and looking at the graphs. I'm in a fairly low RF environment but do have a lot of helo traffic flying above.
See below for the effect of the gain change:



Now compare this to my new feeder on the test bench (Pi4B, Radarbox Green stick, 15' LMR-400, 5.5 dBi ADSB-A dual band antenna, 1090 feeder only) which will be deployed in a much worse RF environment in a high rise urban HNL location. Note that the current image file had a default gain of about 40 dB, with auto gain on, which I changed to auto gain off and bumped the gain up by 3 dB.:



I think that I still have clipping in my original feeder and the noise floor looks high.
The feeder on the test bench has a lower noise floor. Note that the graph above for the test feeder shows the reboots with the gain change, but not with enough graph resolution to see the reboot at about 3:02 PM HST.

Any opinions and advice before I deploy the new feeder and adjust my home feeder on gain setting?


Sorry for the long post, and Mahaloto Mr. W for getting the card burning issue resolved earlier today. Stay away from Etcher. Use The Raspberry Pi burner instead.

 

Understood. What would be really useful would be the levels at the SDR in absolute terms rather than dBFS. That way you could get apples to apples comparisons of gain settings, antennas, etc. anr really optimize for all 1090 dongles and set the gain to the center of the effective demodulator efficiency range.
Aircraft range is not an effective measurement for me because the current feeder is in a valley, with limited LOS, so I can't use that as a proxy.
When I get around to moving the feeder up on top of the ridge behind me where LOS will be less an issue, then the game will change again. Many Maholos for the tip on using the Pi burner on the other thread. STAY AWAY from Etcher! Spurious Burn Fail messages that undermine confidence.

 

Mahalo Mr. W.
My comparisons to date on the SDR dongles to date seem specification similar, yet I cannot convince myself why the gain settings seem to result in different noise floors:
1. Existing online feeder with outdoor 5.5 dBi antenna - ADSB-X dongle with 20 dB preamp. filter and ESD protection, metal case.
2. Test Bench feeder (on-line, indoor 5.5 dBi antenna)- Rader Box dongle with 20 dB preamp, filter and ESD protection, plastic case.
3. #3 test bench feeder (coming next month) will be with another ADSB-X dongle with preamp. filter and ESD protection, metal case, just ordered today.

Anyhow, I'm going to attempt to do real performance comparisons between the dongles. I have the outdoor antenna and real splitters I can use between test beds, a spectrum analyzer good to 4 GHz and other tools for testing. I plan on using the SA to look at the raw antenna output, then look at levels into both dongles that result in similar reported ADS-B levels into dual receivers, then tweak gain on both receivers to see what differences manifest in performance, especially noise floors and clipping.

Any and all opinions from the community are welcome. I'm doing these experiments because I have another project (other than contributing to this community) that will need some calibrated data, not to mention that the feeders I am building will be deployed to better locations to greatly improve coverage of interesting areas.

 

Mahalo,
I there any documentation on the various SDR filter> amP> etc topolidies that you know of, besides me googling and experimenting with? A directpath to knpwledge woulb be appteciated.