Talking about RF when sticking a big custom sawed heatsink on an FA stick and finding out the impact of that. Niche in practice.
It’s full with specs. Really nice for an electro noob.
In that case I would say, that nothing is working at 5V on the board, but I cannot be 100% sure, that’s why I am asking.
This seems to suggest something. So 4,75V might be the minimum input to get 3,3V output.
Yesterday I measured the stick on the computer, today on the Pi.
Pi throws out a higher voltage than the computer: 5,18V on the 3 meter cable without load. 4,85V on a load of 130 msg/s in Dump1090 and 4,82V with 250 msg/s. I cannot measure with a higher rate because the stick has to be in an inaccessible position then.
I also measured the LDO output… 3,32V. Further along the board…two or three times 3,3-3,32V. So it sounds unreasonable to get a performance penalty from a long USB cable because the LDO still puts out 3,32V…unless some other component is using 5V.
But now practice again… And a big surprise: Taking out the cable…boosted the msg/sec rate massively. So @wiedehopf is right once again.
(Yes gain is too high, it will be tuned when antenna is in final position.)
In theory it needs a 3 cm free radius? Damn.
All in all, once more the plans are ruined. 
At this moment I am strongly considering to replace the 3 meter USB cable with 3 meters of high quality coaxial cable. With the stick inside, active cooling would be possible then.