  We analyse the spatial diversity of a multipath fading process for a finite
region or curve in the plane. By means of the Karhunen-Lo\`eve (KL) expansion,
this diversity can be characterised by the eigenvalue spectrum of the spatial
autocorrelation kernel. This justifies to use the term diversity spectrum for
it. We show how the diversity spectrum can be calculated for any such
geometrical object and any fading statistics represented by the power azimuth
spectrum (PAS). We give rigorous estimates for the accuracy of the numerically
calculated eigenvalues. The numerically calculated diversity spectra provide
useful hints for the optimisation of the geometry of an antenna array.
Furthermore, for a channel coded system, they allow to evaluate the time
interleaving depth that is necessary to exploit the diversity gain of the code.
