New to SDR adjustments and slowly learning RF basics through experimenting with my setup.
I started out with a 1090/978mhz Coco Hi Gain 22dbi antenna at around 35ft AGL routed through a ADSBexchange.com Blue R820T2 RTL2832U, 0.5 PPM TCXO ADS-B SDR w/Amp and 1090 Mhz Filter receiver. I’m running on a RPI4 with Raspbian/Debian, dump1090-fa, and PiAware 9.0.1.
Fantastic results. Great position & aircraft reports. I then wanted to add 978mhz UART with an ADSBexchange.com Orange R860 RTL2832U, 0.5 PPM TCXO SDR w/Amp. Loaded dump978-fa, configured to see each SDR through their serial numbers.
I initially used a simple SMA Y-splitter from my single antenna to the two receivers and my 1090 results took a dump. 20-30% less… OK, Y-Splitter is likely creating huge line losses…?
Setup my 978 receiver on a second lower gain poorly placed antenna and my 1090 receiver got great results again! Though, due to the crappy 978 antenna, I didn’t receive the range that my 1090 antenna was.
So, I got a better low-loss splitter (-3db) and a LNA for the higher gain antenna. 22dbi Antenna → NooElec Wideband LNA → Wide Band -3dba 2-way splitter → 1090 & 978 receivers. I didn’t use cables, I used female to female SMA connectors to reduce line loss.
Results took a dump again. >25% loss of position reports.
Where do I start to troubleshoot? I definitely suspect gain settings to be an issue, but perhaps my hardware setup?
I see that dump1090-fa uses an automatic dynamic range gain setting and it sets itself up at about a tuner gain of 28.0db for 360nm. I suspect I’m in an over-gain setting but don’t know how to see this or evaluate it.
A splitter unavoidably splits the received power, so each device is only seeing half the received power (that’s where the -3dB comes from, -3dB is ~50% loss). The total insertion loss will actually be more than 3dB, perhaps 3.3dB - 4.0dB.
rtlsdr dongles are relatively noisy and feed some noise back towards the antenna. If your splitter does not have good isolation between the output ports, then that noise may feed sideways through the splitter and back towards the other device, worsening the effective noise floor and making it harder to receive weak signals. Look for >20dB isolation here.
Dumb y-splitters with no internal circuitry are not great for this, you need something more like this: https://www.minicircuits.com/pdfs/ZAPD-2-272+.pdf (just to give a general idea, not necessarily exactly that, and you may want to look at the second-hand market as they’re not cheap new). What’s the splitter you’re currently using?
I would expect that you would need to increase the gain on your 1090 setup slightly – in theory, by however much the insertion loss of the splitter is, but the rtlsdr gain steps are not very accurate so do experiment!
Coax splitters are never a good solution in this application and will almost always create more problems than they solve. They are electrically dirty and cause signal reflections to just name a few issues. As your results have demonstrated you are best served by separate antenna and low loss 50 ohm cable feeds that are never passed through splitters. You should consider loading the “graphs1090” software that is available through links in this forum. It will give you great graphical feedback on the performance of your system.
Thanks for the information! I understood the halving at the splitter which is why I installed the wideband low noise amplifier before the splitter. I was hoping to boost the signal prior to halving it. My surprise was that this setup was no better than using a simple SMA wye cable…
Regarding dump1090-fa, the documentation on GitHub is light so do you have any suggestions as to which parameters I should experiment with? I did adjust the --adaptive-range-target up to 35dbi with some success.
Nice, but almost 6 dB loss is a lot at the antenna if you are interested in maximum range. It would be neat is someone created an integrated amplifier and filtered version of this.
The highpass filter knocks out strong AM/FM broadcast signals and has minimal insertion loss. The amp has moderate gain of 15 dB. The roofing filter passes 978 and 1090 MHz. Possibly add some gain in the 978 and 1090 MHz ports also.
