Feature: Communications system design


Design of a microstrip


bandpass filter with open-loop resonator for sub-6GHz 5G applications


By Georges Zakka El Nashef from the College of Engineering and Technology at the American University of the Middle East, Kuwait, Ahmet Halit Coşkun of İmaj Otomasyon, İstanbul, Turkey, and Şehabeddin Taha İmeci, also of the American University of the Middle East in Kuwait


T


he rapid growth of wireless technology is calling for reliable and high performing communication systems. Filters play a crucial role here, enabling the selection of desired signals. Te compact filter structures in particular are essential, which oſten require high dielectric constants to reduce their


size. Tus, designing and developing efficient compact RF filters has become a crucial task. From these filters, the microstrip types are preferred due to their


light weight, small size, high efficiency and low cost. Tey provide reliable performance and are easy to make. Tey are designed to


24 December 2025/January 2026 www.electronicsworld.co.uk


Microstrip bandpass filters provide reliable performance: They pass signals within a specific frequency range whilst attenuating those outside the desired bandwidth


pass signals within a specific frequency range whilst attenuating those outside the desired bandwidth. Notably, open-loop resonators offer a simpler fabrication process compared to similar coupling geometries, such as waveguide cavity-coupled filters.


Harnessing the open-loop resonator geometry Here we will discuss the design a microstrip bandpass filter using open-loop resonators, which offer excellent performance in terms of bandwidth, selectivity and stopband attenuation, and reduce the size of the filter. However, a primary challenge lies in the small gaps within the resonator, which could lead to short-circuiting during manufacture. We used inductive and capacitive techniques to achieve a symmetric response across the passband. Te open-loop resonator exhibits unique electrical properties,


making it ideal for bandpass filter applications. Te square- shaped conductor loops are capacitively coupled to the microstrip transmission line, allowing for precise control of the frequency response by adjusting the resonator dimensions. A critical design consideration involves minimising passband loss whilst optimising the skirts of the rejection band to meet the desired design specifications, resulting in a symmetric and well- shaped passband. Beyond their electrical performance, microstrip bandpass


filters with square open-loop resonators offer additional benefits due to their planar structure, which facilitates compatibility with integrated circuit fabrication processes. Teir small size and ease of manufacture make them ideal for applications like wireless sensor networks, mobile communication systems and microwave ICs. Te quality factor of these filters is especially important in high-performance applications, such as satellite and fibre optic communications. In the design of microstrip filters, there are several equations that play a significant role. Tese include:


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