A resonator is provided comprising a conductive element, and a closed-slot in the conductive element, where the length of the closed slot dictates the resonating properties of the resonator. The closed slot generally confines the electromagnetic field resonated by the resonator in a direction perpendicular to a plane of the resonator, providing greater flexibility in the placement of the closed-slot resonator within the filter housing. Further, where three-dimensional resonators are used, compact resonator configuration such as spiral resonator configurations may be securely mounted within the filter without incurring severe filter losses due to the mounting mechanism, allowing filters designed with closed-slot resonators to be more compact in size than filters designed with prior art three-dimensional resonator configurations.