  The loop-gap resonator (LGR) was originally developed to provide a uniform
microwave magnetic field on a sample for electron spin resonance (ESR)
experiments. The LGR is composed of one or more loops and gaps acting as
inductances and capacitances respectively. Typical LGR designs produce a
uniform field on a sample at a single resonant frequency, but for certain
experiments it is necessary to study the response of a material to uniform
fields at multiple frequencies applied simultaneously. In this work we develop
an empirical design procedure using finite element method calculations to
design an asymmetric loop-gap resonator with uniform fields at two frequencies
in the same sample volume and analyze the field uniformity, frequency
tunability and filling factors, providing comparison to a manufactured device.
