  We present simulations and analytic calculations of the electromagnetic
microwave fields of coplanar waveguide (CPW) resonators in the vicinity of
highly conducting metallic samples. The CPW structures are designed with the
aim of investigating electron spin resonance (ESR) in metallic heavy-fermion
systems, in particular YbRh$_2$Si$_2$, close to the quantum critical point.
Utilizing CPW resonators for ESR experiments allows for studies at mK
temperatures and a wide range of freely selectable frequencies. It is therefore
of great interest to evaluate the performance of resonant CPW structures with
nearby metallic samples. Here we study the microwave fields at the sample
surface as a function of sample distance from the waveguide structure and
analyze the implications of the sample on the performance of the resonator. The
simulation results reveal an optimum sample distance for which the microwave
magnetic fields at the sample are maximized and thus best suited for ESR
studies.
