Many thanks for all the contributions.
Here are the results from my investigations:
Yes, it is the 5th harmonic problem. I could tune my LNB to that frequency on 12.038 MHz using SDR-Console and could see some TV transponder signals at -115 dBm. But when I transmit at lower power I get the 5th harmonic of my transmission at -80 dBm. When I increase the power I get -45 dBm. So it seems that the activation of the doherty path of the amplifier is making this problem.
I inserted now a 200W circulator and the situation improved a little bit. I can transmit now 333 kS at beacon level without increasing the noise level.
I am using bandpass filters for 10W between driver and final amplifier, built for Wifi on 2410 MHz, 27 MHz bandwith, but 10W CW only.
High power low loss low pass filters could be built according this: https://www.changpuak.ch/elect…mpedance_Lowpass_Coax.php. But I don't have the ability for that kind of high precession work.
Alternativly those could be built from low loss PCBs with increased copper layer thickness.
I also do have a WLAN duplexer, but only rated for 20W and I don't want to risc this eather.
So next will work on optimizing the doherty amplifier to reduce the 5th harmonic.
73 de Gerhard OE3GBB