Macrostructure and Diamagnetic Response of YBa 2 Cu 3 O 7- x

The experimental low-frequency magnetic response of Y-Ba-Cu-O sintered samples is astonishingly similar to that of model systems made of weakly coupled classical superconducting grains. A quantitative description of this behaviour in the framework of the coherence transition is given. It allows the extraction of important physical information, like the penetration depth λ( T ), and provides explanations for the observed low-field sensitivity and temperature dependence of λ.