After 178 x 2 minute WSPR transmissions I have a result and can share the outcome of this experiment.
I have been transmitting with a homemade magnetic loop with a 92% efficiency installed inside the garage on the floor! Transmitter was the W5OLF WSPR VCXO-AXE 30m transmitter with 1W. This transmitter has been built last January. I always wanted to have a independent beacon transmitter which doesn't use the mains net and doesn't use my home base antennas. So I powered the transmitter with solar power. The result is amazing considering the setup. Best DX was ZL1RS with a distance of 17750km.
I really didn't expect it would work this well with a antenna inside a building and on the floor.