Hello Pierre,
Yes, what the others have said, your hand changes the loading on half the antenna, both with HT and Nano.
If your goal is max range mountain-topping, you can easily make a much better antenna than a short "rubber duck."
Here is a portable, adjustable VHF/UHF dipole I made years ago with TV rabbit-ears:
You can also easily make 3-5 element yagi wiith PVC pipe and aluminum tape. Model it in MMANA-GAL first.
73, --KV5R
