A systematic study is made of the coupling of RF energy to a plasma column by means of an external set of current sources in the toroidal cavity constituted by the machine. The coil loading due to the absorption of RF power by the plasma and by the wall is discussed inside various frequency domains of interest for RF heating. The plasma is described by hot-fluid theory with effective collision frequencies; the latter are either particularized to describe analytically linear collision-less damping or used as parameters to simulate other loss mechanisms like the non-linear ones. Boundary conditions consistent with the magnetic confinement are taken at the plasma-vacuum interface.Special emphasis is placed on the bounded-plasm a effects in the entire frequency range (from ω≪ωciup to ω>ωLH). In particular, magnetoacoustic resonances may strongly increase the efficiency of RF energy transfer when the physical absorption mechanisms are electron TTMP or ion cyclotron resonance. At higher frequency, resonance modes of the coaxial type are found in which the plasma plays the role of the central conductor of a coaxial toroidal cavity.