FlightAware Discussions

Gain & "Strong Messages" with a LNA

Installed the RTL-SDR 1090 LNA this morning to see how far out the antenna can receive (195 miles post-install). My setup uses the blue pro stick with the FA 1090 antenna up in the attic. LNA is up at the antenna. Prior to the LNA install, strong messages approximated 20% with auto gain. Post LNA install, it’s running 67-70%. Also, almost no close-in GA traffic is showing despite a good day to fly. Is there a correlation with the strong message count and lack of close-in traffic?

I set the gain (currently 39) based on a rough aircraft/distance count. Not sure how to proceed other than playing with the gain to see if the GA flights show up again. Advice appreciated on what else to tweak.

Yes there is a correlation. Using an LNA at the antenna with the FA pro-stick, or any other dongle with an amplifier built in means you will have to reduce the gain considerably. The reason you are not seeing close by aircraft is that they are overloading the dongle so it can’t decode them. A setting of 39 is almost definitely too high.

Auto gain does not work with ads-b since the signals are intermittent and from multiple sources, and results in a setting higher than maximum gain (a quirk of how the dongles work). A good rule of thumb for strong signals is to have them averaging below 5% - you will likely need to have the gain set quite low as you have a lot of amplification in your receive chain.

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Thoughts on optimizing gain

My guess would be that a gain of 8.7 or 12.5 or something along those lines would be appropriate.
1 to 10 percent strong messages is what i would recommend to aim for.
If you have more strong messages, reduce the gain.

Indeed gain is still too high. I did run a similar setup with gain levels of 8…12.

Thanks to all for the replies. I turned down the gain to 15 and strong messages dropped to around 4%. The lower gain increased close-in traffic count but surprisingly didn’t really affect the range.

As for the so-called strong messages, do those factor into the receiver stats (e.g. ADS-B, MLAT, Other) in any way?

The strong signals don’t affect your stats directly, it’s just a count of how many of them you get. If they make up a high proportion of the signals you receive however, it’s an indication that the gain is too high and that you are likely losing messages which might affect stats a bit. You want to aim to have the gain high enough that you receive long range signals well, but not so high that closer range signals are overloading the receiver and getting lost.

With as much amplification as you have, signal strength it probably not going to be the limiting factor in determining the range you can see.