Hi Toussaint,
1/ Do you think a permanent and stable 250 km link over the Mediterranean sea is doable ?
There used to be a few (PTT&military) links between Corsica & continental France, some as early as 1947[0,1], although way down around 100MHz[1].
Those areas have a huge advantage of having somewhat high mountains not that far from the sea. Quickly looking at radiomobile, there seems to be around 154dB of free space loss between Grasse (Le Haut Montet, alt 1330m) and Calenzana (Monte Grosso, alt 1900m) for a distance of 210km, with no obstruction from the curvature of earth.
Using commercial ubiquiti 34dB dishes and 30dBm of power, this seems to fit within the receiver sensitivity on airfibers 5x (-90dBm), so depending on fading and tropo ducting conditions, this could work at times.
Bonne chance !
[0] https://www.persee.fr/doc/reso_0984-5372_1996_hos_14_1_3673 (p.190) [1] https://www.hertzien.fr/evolutions_46-60.htm
73 de f4inu