so we’re basically saying that unless an aircraft is sitting on top of your antenna then any overload is happening via the gain in the dongle being set too high?
and even then if -1dB signals can be decoded than we should be looking at -1dB rather than -3dB for a threshold???
by the way, this is a sincere question, i’m trying to understand…
lawrence hill, can you comment re this posting in the RTL-SDR. blog (although this is relative to their 1090MHz LNA i hope it is relevelant)
March 21, 2018 - 4:11 am admin
Max RF input allowed by the LNA chip is 20dBm, but the limiting factor is the max allowed by the dongle which is 10 dBm. So taking into the account the 27dB gain of the LNA, aim for -17 dbm max input to the LNA to avoid damaging the dongle.
So worst case with perfect rad pattern alignment, a powerful 10dBi gain antenna, and a large aircraft transmitting ADS-B at max 500W power, as long as you’re more than ~350m away from the ADS-B transmitter you won’t damage the dongle.
The figures in the blog look correct when considering another preamp in front of the dongle.
With another 27dB of gain in front of the dongle then yes, you will need to be more than 350 metres away from the aircraft in order to keep the level into the dongle below the max rating of +10dBm.
Compression of the dongle’s first preamp would start to occur with the aircraft at approx 700 metres away with the RTL blog preamp, rather than 30 metres away without it.
Usual caveats apply but pretty much.
Yes 
another preamp
They are not referring to Flightaware ProStick. They are referring to their own dongle.
Does RTL-SDR Blog dongle has a built-in preamp?
Can you please give calculations as to how you arrived at this figure?
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No, the RTL-SDR Blog dongle does not have a built in preamp.
However the post I was asked to comment on was related to the dongle’s stated maximum input rating of +10dBm. Those figures in the blog post seem to be correct.
My last sentence relating to compression does however refer to the situation where the preamp is placed in front of a ProStick Plus. I probably should have made that clearer.
Those figures are for damaging the receiver. They say nothing about the overload and actual usable signal dynamic range.
Sure:
Transmit output of aircraft is +57dBm (500W)
1dB compression point of the preamp device in the dongle is -10dBm. Assuming 6dB antenna gain this is -16dBm at the antenna.
So compression will start to occur when the path loss is less than 73dB.
At 1090MHz 73dB path loss is a distance of 100 metres.
Please note the corrected distance of 100 metres not 30 metres as I originally stated. I have corrected the original post.
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Correct. The original post is referring to the damage level for the RTL dongle. The compression figures come from the data sheet of the preamp device used in the ProStick plus.
The Quick Reference to Specs of Dongle’s Tuner Chip R820T says:
- Max input power = +10 dBm
- [dBm]=[dBµV on 75Ω] -108.75 DB
Please note that tuner chip’s input impedance is 75Ω, and NOT 50Ω
Reason: The DVB-T is designed for TV reception, which uses 75Ω system (DVB-T = Digial Video Broadcast Terrestrial)
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@LawrenceHill
Thanks for the calculations and clarification. It makes things clear.
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Hmm, time to rethink my plans… Currently using a Prostick but want to add a Uputronics for filtering and by using build in the amp also lowering (total) noise figure. However, closest planes (military helicopters) are infrequently less than the mentioned 100m and not always just quickly passing by …
Here is download link for full Specs (pdf) of Dongle’s Tuner Chip R820T. Currently dongles use R820T2, which is a new improved replacement for the R820T. They are functionally identical. The T2 offers better performance above 1.0 Ghz and has lower noise figure.
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I am trying to optimize my gain today after receiving the FA Antenna. I have run the optimize python script a few times, and it seemed like 36.4 was the most optimized for me. I installed performance graphs as well, but I am not sure what to look for. I want to continue playing around with the gain to see if I can find that “sweet” spot. Here are some screenshots at 36.4 gain. Any tips on what to look for?
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Have a look at this post, your gain looks like it’s in the ballpark now.
keep one gain setting for at least a full day. Then change it to the neighbour value the next day.
This gives you a reliable idea which one fits best
That’s what I am doing! Dropping down to 33.8 today at 4 and will compare the two tomorrow at 4.
Appears this post on receiver Gain has been quiet for quite sometime. I have a system that I have had up since March of this year with a config gain setting of “Max”. I see a lot of different numbers mentioned in the early post remarks with “Wiedehopf” suggesting a gain setting of 35 as a good starting point. Was hoping someone could tell me what the “Max” setting equates to in the receiver config? Thanks for any feedback. I am using a FA 1090 antenna with the blue FA pro stick feeding a RPi zero.
Max gain for a prostick is 49dB
Piaware 8.2 has the autogain feature available.
You could use that as well.
Optimal gain for your system is totally depending on your environment so it is hard to advise number for you without knowing the local environment.