A Design of Microwave Resonator A. A. SULAIMAN 1 , M. F. AIN 1 , S. I. S. HASSAN 1 , A. OTHMAN 1 , M. A. OTHMAN 1 , N. Z. AHMAD 2 , Z. I. KHAN 2 , N. H. BABA 2 , M. H. JUSOH 2 , R. AWANG 2 1 School of Electrical and Electronics Engineering Universiti Sains Malaysia 14300, Pulau Pinang MALAYSIA 2 Faculty of Electrical Engineering, Universiti Teknologi Mara Malaysia, 40450 Shah Alam, Selangor. MALAYSIA Abstract: - A microwave resonator has been designed with the aid of GENESYS software and the circuit was fabricated on Roger microstrip with dielectric constant of 3.48. A combination of two congruent coupled line sections was used to resonate at 1.8 GHz. The discrepancies between measure and simulated result of half-power bandwidth was due to the size of the gap between the two conductors. Such structure has advantage to design ultra-narrowband filter. Some simulation and experimental results have been compared and presented in this paper. Key-Words: - Microwave resonator, ultra-narrowband filter and bandwidth. 1 Introduction Resonator is an important device in designing a filter network. It is also used in controlling or stabilizing the frequency of oscillator, wave meter, antennas and measurement equipment. Each of resonators may resonate at different frequencies. Hence, to achieve an accurate or first-passed filter design it is essential to characterize couplings of coupled resonators whose self- resonant frequencies are different. In general two frequencies in association with the coupling between a pair of coupled resonators can be observed despite whether the coupled resonators are synchronously or asynchronously tuned. If the coupled resonators are synchronously tuned, the coupling coefficient can be extracted from these two frequencies that easily identified either in experiments or in full-wave EM simulations. However, if the coupled resonators are asynchronously tuned, a wrong result will occur if one attempts to extract the coupling coefficient by using the same formula derived for the synchronously tuned resonators. Therefore, other appropriate formulas than those presented in [1-2] should be sought. An open cavity resonator with suspending a spherical reflector above a microstrip line can supports fewer modes than a conventional closed cavity of similar dimensions. The modes sustained within the resonator can be used for reasonable cavity dimensions. When a microstrip line is coupled to such an open resonator mode, the energy stored in the resonator volume becomes accessible to the microstrip circuit, theoretically leads to simple lumped equivalent circuits coupling mechanisms between the open resonator and microstrip. At millimetre wavelengths, the geometric-optics assumption that the wavelength is negligibly small compared to the cavity dimensions is no longer valid and diffraction effects become increasingly significant. Cullen [3] has summarized the best current theoretical models for fields inside a millimetre wave open resonator of practical dimensions. For the purposes of analysis, a scalar-field approach gives satisfactory precision. Some of problems have been solve by the model. Hybrid circuits were another approach that can perform in all microwave applications with resisted efforts at integration of the intrinsic limitations in planar transmission-line media [l]. Although the use of dielectric resonators can alleviate this problem at the millimetre wavelengths, these devices become very small and difficult to mount. The space between two coupled conductors is inversely proportional to the coupling factor of two conductors. The half-power bandwidth (BW) of resonator can be calculated in term of the difference between odd-mode and even-mode characteristic impedances that was inversely proportional to the space Proceedings of the 7th WSEAS International Conference on DATA NETWORKS, COMMUNICATIONS, COMPUTERS (DNCOCO '08) ISSN: 1790-5109 18 ISBN: 978-960-474-020-8