[gorlilla]

The ground plane is typically orthogonal to the antenna. Both TX and Rx antennae will have their own 'effevtive' ground plane relative to each other at any give point in time that will effect their Tx/Rx capabilities. Add into that, parking lots are full of metal objects and typically nearby large buildings. Those, also, will change the propagation characteristics of the general environment.

There's a lot at play in open-air environments. This does seem quite easy for DIY experiments using RTL-SDRs in varying environments.