A Novel Technique for the External Coupling of Coaxial Dielectric Microwave Filters

Abstract

Coaxial dielectric resonators made from materials with high relative permittivity, including ceramics, have good temperature stability and quality factors of the order of 1000 in the UHF band. At these frequencies, large wavelengths are disadvantageous from the point of view of size, but high dielectric constants, up to 90, can be used to minimize them. However, coupling the input/output of filters, i.e., their excitation, remains a critical issue, especially when wide passbands are required. The objective of this paper is to present a novel technique to make this coupling, to solve problems that arise when using classic techniques. The approach consists of nesting a conducting rod in each central core of the input/output resonators, which creates another coaxial TEM mode that will couple with the fundamental TEM mode of the input/output resonator. The length and/or position of the rod provide the opportunity to easily and accurately tune input/output couplings over a wide dynamic range. In this paper, we show the effectiveness of the solution, presenting the experimental results of sixth-order filters centered at 435 MHz, one made on the basis of a standard input/output coupling and the other on the basis of the new solution proposed here.