Please consider that transmission lines theory leading to Zo formula is based on the assumption that no radiation occurs: that condition is only true when the distance between conductors is very smaller than the signal wavelength. Besides Zo function versus height is a logarithmic law, so increasing weakly at height increasing. For example, having 0.04 inch conductor diameter and εr=1 it is necessary a height of about 5.5 inch to have Zo = 377 Ohm…

Yes,it can be. Consider the reported Zo formula: increasing the distance from the ground increases the inductance per meter and reduces the capacitance per meter of the transmission line, differently than in free space where they are fixed by physics (space permeability and space permittivity ).So Zo may reach values even higher than 377 Ohm.