Electromagnetic resonators are used in many fields of science and technology, from magnetic resonance to radar, and many resonator designs are known. For instance, W. N. Hardy et al. Review of Scientific Instruments, Vol. 52 (2), p. 213 (1981), disclose a "split-ring" resonator that can be used in the frequency region 200-2000 MHz. See also M. Mehdizadeh et al., IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-31 (12), p. 1059 (1983). Other exemplary prior art embodiments of split-ring resonators are shown in P. Jezek et al., Microwaves & RF, p. 132, June 1984, and J. R. Delayen et al., IEEE Transactions on Magnetics, Vol. MAG-17(1), p. 939 (1981).
A particular and commercially important use of resonator elements is in RF or microwave filters. For the sake of concreteness, the discussion below generally will be limited to elements used in filters, although the instant invention is not necessarily so limited. More specifically still, for the same reason the discussion below will be substantially in terms of such filters used in wireless communication systems. Examples of wireless communication systems are cellular telephone systems, personal communication systems and mobile radio systems.
Because of rapid growth of demand for wireless communication services, it is expected that the spectrum currently allocated and/or planned for, e.g., cellular communications in the US, will be saturated before the end of this decade. The growth in cellular communication has placed enormous pressure on communications technology to increase system capacity, i.e., to increase virtual channel density and enhance data transfer rates within the current frequency spectrum allocations. However, currently available filtering techniques typically will not be able to provide the level of filtering required for the desired closer channel spacings, due for instance to the relatively low quality factor (Q) of typical prior art resonators. Thus, filters with rapid roll off and low insertion loss are needed. Furthermore, filters that are more compact than analogous prior art devices would be highly desirable, due for instance to the typical requirement to provide equipment for many communications channels at a given installation, with the equipment for each channel typically comprising a filter both at the transmitter and at the receiver. A typical prior art 6-stage cavity RF filter for cellular radio can be as long as 20 inches, with a diameter of 11 inches, frequently making it difficult to accommodate many such filters at a given installation.
In view of the above recited facts, it is submitted that a resonator that can yield a compact filter that can have low insertion loss and rapid roll-off at frequencies relevant for wireless communications would be highly desirable. This application discloses such a resonator, and filters that utilize the resonator.