(1) A parabolic antenna is a huge radar target for anyone hunting submarines.
(2) A parabolic antenna, or any other very directional antenna, requires a stable platform. Either it's on a huge inflatable raft, or the sub is rigidly attached. Either way, the operation will be noticed.
(3) Sacrificial systems in submarine warfare are rare. They exist but, like aircraft launching flares, they draw attention and are therefor only used once the sub has been somehow detected. Subs don't like anything that draws attention unless things are already hitting the fan.
(2) A parabolic antenna, or any other very directional antenna, requires a stable platform. Either it's on a huge inflatable raft, or the sub is rigidly attached. Either way, the operation will be noticed.
(3) Sacrificial systems in submarine warfare are rare. They exist but, like aircraft launching flares, they draw attention and are therefor only used once the sub has been somehow detected. Subs don't like anything that draws attention unless things are already hitting the fan.