[Legacy Report] Composite Dielectric-loaded Resonators forBandstop and Bandpass High power filters

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The temperature stability of narrow-band filters that are based on coaxial resonators is a serious problem. The passband of a bandpass filter or the notch band for a band reject filter are usually specified with performance requirements that do not allow for much drift in the frequency domain. There have been many solutions to this problem over the years, but all have had shortcomings or have traded size and weight against stability. For example, the filters can be constructed using metal enclosures with very low temperature coefficients of expansion, such as Invar. This is a nickel-steel that has about six times the weight per volume of aluminum based structures, but about one-sixth the thermal coefficient of expansion (TCE), and thus essentially solves drift with a significant penalty of weight. Many filters employ resonators supported with dielectric material, either to provide some loading capacitance or simply to support the resonator such that vibration and shock are not a problem. However, the dielectric material also has a significant TCE and frequently contributes to the filter drift problem. It is possible to construct filters using dielectric materials that have very low TCE and avoid the drift. The cost in these cases is typically limited bandwidth performance, either narrow passband or narrow stopband, because of higher order modes that are more readily supported by the dielectric material. In these cases, the enclosures can be lightweight aluminum, so no weight penalty is imposed…but the limited frequency domain performance is not always acceptable.
RS Microwave has recently developed a new approach to resonator design, called “Composite Resonators” (Patent Applied For). Coaxial resonators consist of an inner conductor and an outer conductor, separated by a composite dielectric structure consisting of three layers: an inner layer of soft dielectric, acting as a cushion; a second layer of higher dielectric constant, low-loss material such as alumina, acting to provide the main dielectric support; and finally, an outer layer of soft dielectric, again acting as a cushion. The coaxial line resonator is either shorted, open circuited, capacitively coupled to the next resonator, or some combination, based on the particular filter design intended. The effective dielectric constant (er) between the inner and outer conductor determines the electrical length of the resonator, and with proper selection of layer materials and thicknesses, is typically in the range of er=5 to 7. The temperature stability of the resonator is primarily determined by the relatively thick second layer of high dielectric constant material,
and is very close to what might be obtained using the heavy steel (Invar) for the entire structure. These resonator designs have recently been applied to the design of very wide passband filters. The physical structure will be shown to be essentially the same, but with directly connected resonators rather than capacitively coupled resonators as used in bandstop designs. It has been found easy to incorporate both connection types in one structure, with the result that it is possible to build wide-band high power bandpass filters, with notches “built-in” for rejection of specific

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