Hi Uwe,
Unlike other hams, I do my own research: 1500 hours of study and field tests around this topic.
I have over 1000 data files from my measurements. About half are available online for you to view.
You will need the freeware software ANTscope (from Rig Expert) to view many of the files.
I suggest you visit my web site and study the information on these two sections:
*Password protected. Contact me direct to get the password.
AND, the symmetrical twin-wire choke is a Maxwell. It has at best a CMI of about 1500 Ohms.
The 1:1 Guanella I recommend for OCFD antennas has 8k Ohms. In most cases 6k Ohms would work, but 1500 Ohms is much too small.
BTW, my daughter lives in Giessen. If you see her . . . RUN!
73 - Rick, DJ0IP