A dielectric spacer for use during microwave ablation of tissue is disclosed. The dielectric spacer includes a housing having a predetermined thickness and a skin-contacting bottom surface. The housing is configured to be filled with a dielectric material having a predetermined dielectric permittivity. The housing is further configured to be placed on the tissue in proximity with at least one microwave antenna assembly, wherein the thickness and the dielectric permittivity are configured to shift a maximum voltage standing wave ratio of the at least one microwave antenna assembly.