Dielectric spiral resonators

Abstract

The possibility of constructing a resonator with a dielectric spiral inner is discussed. The modelling conducted shows that the mode related to the ceramic quarter-wave resonators also exists within a dielectric spiral, the properties of which are strongly dependent on the thickness and the number of turns of the spiral. The low filling factors associated with the resonant mode explains the high quality factors possessed by some of the spirals studied.Introduction: Loaded cavity resonators are often used at microwave frequencies in communication filters. They can be realised in a variety of ways but for applications at several hundred megahertz; for applications where a high quality factor of resonance is required a dielectric resonator is used. For some applications the use of dielectric resonators is prohibited at low frequencies ( 900 MHz) by volume constraints. For these applications quarter-wave resonators are used, and at still lower frequencies metal helical resonators are used. However, the loss owing to the inner structure dominates the loss of the overall resonators and hence the filter loss. An innovative solution to this problem is to replace the inner metal with a dielectric. This concept was proved to work for dielectric combline resonators (DCR) by Wang et al. [1].The dielectric spiral resonator is based on the same principle as that of a DCR [1], where the metal inner is replaced by a high permittivity dielectric rod. It is based on the concept that a ceramic material, with sufficiently high permittivity, can exclude the electric field from within the ceramic and hence behave rather like a metal [2].The key advantage is that conductor losses on the surface of the inner metal finger are eliminated with a corresponding increase in Q. This work investigates whether a dielectric spiral can act as a resonant structure and what effect the change has in the various dimensions of the spiral on the resonator frequency and losses. The Letter, also serves as a validation for the modelling software for this type of problem. A ceramic spiral was made and the method of fabrication is discussed. The frequency and Q of the spiral have been measured and are compared to the modelling results of the structure.