Hi Bernd, good achivements in such combination of foil balloon and RS41 board. Congrats!
Do you use original RS41 board without any hardware elements prunning?
With 2 x AA Energizer Ultimate Lithium batteries weight of it is around 50g so pretty heavy for such small balloon. Do you power it this way?

I know this 4FSK software. Works very well.
We also use it in our flights.