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Hi.  The big ships typically have 2 sizes of radar domes: the big one is 4 feet in diameter, I think this is for internet.  What is the purpose of the smaller one - 22 inches or so in diameter?  Eg. is this for satellite phone, or is this just an alternate frequency for internet?

Radar doesn't sit in a dome, its a long horizontal array antenna thats size depends on the band they're using.

 

The largest dome, is the vsat for data communications. The next size is typically the satellite tv, and the small domes are typically telephone.