Cylindrical coils near self-resonance.

We present a model of birdcage resonator operation when the size of the resonator approaches a quarter wave length resulting in significant phase shifts along the current conducting elements. In the model resonators are treated as generalized transmission lines. Line termination determines whether the resonator is of the low pass or high pass type. The model correctly describes the dependence of resonant frequency on resonant mode number as well as the distribution of current along the length of the resonator.