I repeated the measurements with a short 20cm pigtail from the patch to the miniVNA Tiny and according recalibration. This is what i get now:
This looks much more consistent with what I would have expected. Thanks for the hint DL9SEC. Those larger loops are not observed anymore (although they were to be expected as I've learned here) and the measurements are less noisy. I simply wasn't aware of the impact of the cable length. I put markers 1 and 2 where I would suspect the double dips to be located and marker three around 1.4GHz.
G0MJW Indeed, the real test is what counts. Unfortunately my PA is not ready yet, so I can happily induldge in more or less meaningful measurements.
To me everything looks okay-ish so far and I think I can leave my patch as it is and start doing some careful tests on the air as soon as my PA is finished.
Just for my own understanding: taking the simplified two antenna model into account and assuming that in my measurement above marker 1 corresponds to the inductive and marker 2 to the capacitive antenna: their difference in phase, should be 90° in the optimal case. Is this correct?
Thanks to everyone so far already