I bought a 23cm transverter from SG-Lab and Hristiyan (LZ5HP) added a small PCB antenna.
Time for a LiteVNA sweep!
Almost perfect! VSWR of 1.1 at 1296 MHz! It should easy to fix it by shortening it to reach a VSWR of 1.
For further Investigation you can have a look at the S1P files:
This files can be investigated using NanoVNASaver.