As has previously been stated it all depends on the radar, buildins, topography of the land etc.
For example the Tiree radar, situated fairly high on the island of Tiree, looks out over the Atlantic and quite often picked up ships at some distance. The Aberdeen radar is tilted slightly to view down onto the North Sea giving better coverage for the helicopters, but over the mountains inland its poor.
So to 'see' your mythical chopper depends on many factors.