Abstract: A tiltable upwardly extending supporting arch assembly for supporting a radar antenna and other antenna devices above the roof of a cabin or control bridge superstructure of a boat including a unitary rigid arch member of inverted substantially U-shaped configuration having a generally horizontal transversely elongated top portion and a pair of depending leg portions having feet at their lower ends. The feet are attached to a pair of tilting base mounting assemblies each having a base housing component in the shape of an elongated box shaped housing to be fixed to the roof and having a tiltable top plate member pivoted to the box-shaped housing for movement from a first position disposing the arch member in a raised normal operating position to a second position disposing the arch member in a lower position wherein the leg portions lie substantially parallel to the roof.