What can be a good modelisation for an open wire?
It depends on length and frequency. For instance, a coax cable of 25 cm length fed with 300 MHz (wavelength 1 metre) will look like a short circuit to ground. It's called an impedance transformer: -



The relationship between the characteristic impedance, Z0, input impedance, Zin and load impedance, ZL is: -



If you just used a wire it would radiate and look like a few tens of ohms. It can be quite complex when the wavelength of the signal approaches that of the wire/cable length.

Here's a good source for learning. It deals with quarter, half and 3/4 wave transformers and is pictorially very good.