I love it when the maximum range plot is the same as predicted.
Me too. I thougt already about changing antenna, but it might not get a better result. The only thing which can be improved is the message rate. I do not get above 1300, this might be the limitation of the stick. To improve this, i would need an Airspy, where i need to get the approval of my financial minister at home 
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Ever take a VNA reading through a long cable? It can be quite nasty. That’s one good reason. Although there are arguments for both sides and I tend to agree with both as it depends on one’s situation. That said, if you want to compare side by side, the rig with the long cable run will almost always perform less admirably. Don’t peg hopes on Flightaware’s global ratings either - it’s useless for comparison purposes.
It doesn’t really matter - many will be moving to Pi4 while keeping Wifi and HDMI running full-tilt creating all sorts of extra noise anyhow (haven’t touched on the extra noise generated by the USB3 chipset yet). It really doesn’t matter in the end to have a few less or more messages or planes a day - not like the providers are handing out checks. That said, some (like me) like the challenge of learning and squeezing out every last drop - just because, so it keeps it interesting. 
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Don’t forget that this is after all a digital encoded signal. They are pluses of energy, with variable position.
Is not like the AM modulation, the exact amplitude of the signal does not matter, as long is just a little bit left, data can still be recovered.
Cable runs are prior to signal hitting the ADC (disregard amplification for a minute). Longer runs introduce varying amounts of noise depending on location, quality and length that the decoder ultimately needs to chow through - resulting in higher CPU use as an end result depending on the SNR. In some cases it can be a dramatic difference since cable acts like an antenna on it’s own regardless of how well insulated or thick. For this reason, results tend to be better when eliminating if/when feasible.
Another solution is to hang the ADC (radio) at the antenna and run an extended USB cable, but high bandwidth devices such as Airspy tend to choke on them depending on the length, so some will just run their entire rig at the antenna - which brings up some of the concerns you brought up about moisture, heat, you name it. It’s all a compromise based on location and simplicity/availability.
Actually, I believe the ADSB error correction algo is based solely on amplitude/SNR scoring - I could be mistaken of course,
finding the right USB cable can also be challenging.
I have one with 3 Meter that works fine and a 1 Meter which reduces the performance of messages, aircraft and range by 40%
Cable introduces only attenuation, not noise.
In order to compensate for that attenuation at the receiver, the gain need to be raised with that amount. That is what adds noise.
But if you raise the level before the cable attenuation, the main noise component is the LNC noise one. It’s basic electronics theory…
That’s why all the antenna sites say to put the amplifier as close as possible to the antenna.
We need to let everyone know so they can stop wasting time and effort with shielding.
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Because we are talking about car jumper cables…?
When I said “Cable” I replied to your statement and I assumed that you were talking about the RF cables suitable for ADS-B. Coaxial that is.
But hey, if you fell like contradicting, more power to you.