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.