  Transformation optics is a powerful tool to design various novel devices,
such as invisibility cloak. Fantastic effects from this technique are usually
accompanied with singular mappings, resulting in challenging implementations
and narrow bands of working frequencies. Here in this article, we find that
Fabry-P\'erot resonances can be used to design various transformation optical
devices that are not only easy to realize but also can work well for a set of
resonant frequencies (multiple frequencies). As an example, we fabricate a
prototype for a cylindrical concentrator for microwaves.
