We will prove an analogue of Landau's necessary conditions [Necessary density
conditions for sampling and interpolation of certain entire functions, Acta
Math. 117 (1967).] for spaces of functions whose Hankel transform is supported
in a measurable subset S of the positive semi-axis. As a special case,
necessary density conditions for the existence of Fourier-Bessel frames are
obtained.Comment: To appear on J. Funct. Analysi