## 1 Introduction  
Microstrip technology has been widely used in modern communication systems since it is a mature and highly reliable technology for microwave- and millimeter-wave applications [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. In particular, multiband bandpass filters (BPFs) have been utilized in wireless communication transceivers to separate frequency spectrum in different bands. The C-band covers 4 GHz to 8 GHz in the microwave range used for weather radar systems and mobile and satellite communications (Satcom). In relation to satellite communication systems, the design of microwave filters has challenges associated to several factors, which include the spectrum scarcity and host environment. In order to satisfy individual filters' requirements, conflicting solutions or custom technologies might be necessary, which result in a trade-off at component and system level [12]. Although distributed filters composed of many resonators can be a desired solution to satisfy the satellite communication requirements, such as high selectivity and low dissipation loss, such configurations lead to a circuit size with dimensions of several wavelengths, making the microwave front-end filters one of the bulkiest elements in the RF payload. As the mass and size of the on-board devices have a significant effect on the cost, reducing the footprint without compromising the filter performance becomes an important priority. Hence, the satellite communications industry has created a demand for low-mass narrowband low-loss filters with high selectivity [13; 14; 15]. As a result, affordability and compactness have become two critical requirements of microwave filters and other electromagnetic components used in the upcoming new generation of low-cost Satcom systems, where satellite communication-on-the-move will become a part of everyday life [12; 13; 14; 15; 16; 17; 18; 19]. Hence, high-performance filters with a compact size and low-cost manufacturing are in high demand for such applications. Multiband BPFs have a particular place in communication receivers due to their properties of high insertion loss (IL) in rejection area, tunable response and sharp transition band. According to the recent literature, different techniques were suggested for designing BPFs, such as balanced BPFs [20; 21; 22; 23; 24], multilayer structures [25; 26], balun BPFs [27], substrate integrated waveguide (SIW) technology [28; 29], fractal configurations [30; 31] and coupled resonator circuits (CRCs) [32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45]. In [35], a BPF was designed using coupled feed lines and compact main resonators. It had a short transition band, but the stopband was narrow. A similar coupling system was used along with some bended stubs to create a bandpass response through a compact filter design with 10 dB attenuation at the stopband in [36]. In [37], a dual-band BPF was developed using an improved stepped-impedance resonator, which showed a good suppression level due to several transmission zeros in the stopbands at a price of larger size. A simple configuration of BPF was implemented based on the coupled geometry in [38]. Although the intruder signals were deactivated under a desirable level, the circuit size was relatively large. Another BPF was realized based on a coupled topology in [39], in which the isolation between the passbands was only 17 dB. A more complex coupling system was presented in [41] using ring resonators that created high insertion losses in passbands with weak isolation between the bands. Modal analysis is another approach used to predict the location of transmission zeros, which can be adopted to realize a dual-band bandpass filter in [42]. The BPFs reported in [43] were implemented using the open/short stubs coupled with quarter-wave lines. These BPFs provided an acceptable attenuation level between the passbands; however, they were not size-efficient. Using the interdigital geometries, dual-band BPFs were fabricated in [44].