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Top1. Introduction
A channel filter, also known as bandpass filter (BPF) passes frequencies within a single band and rejects all other frequencies outside the band (Nwajana, Dainkah, & Yeo, 2017). This type of filter is widely used as the building block in the design of complex and multi-port circuits and systems. Some of the more complex devices that can be formed from BPFs include filtering antennas (Liu, Leung, & Yang, 2020; Wei, Zhao, & Shi, 2019), multi-band filters (Nwajana, 2020; Yeo & Nwajana, 2013; Hou, Liu, B. Zhang, Song, Wu, J. Zhang, & He, 2020), filtering power dividers (Nwajana, Otuka, Ebenuwa, Ihianle, Aneke, & Edoh, 2020; Dainkeh, Nwajana, & Yeo, 2016), diplexers (Nwajana &Yeo, 2016; Nwajana, Dainkeh, & Yeo, 2018), etc. Figure 1 shows the response from a bandpass filter that passes all signal components between a lower frequency limit, fL and an upper frequency limit, fH, while attenuating and rejecting all other signal components that fall outside the fL and fH band. A bandpass filter can be formed by combining a lowpass filter with a highpass filter. Bandpass filters are widely used in radio frequency (RF) front end of cellular radio base station transceivers. Its main function in the transmitter is to limit the bandwidth of the output signal to the band assigned for the transmission. By this, the transmitter is prevented from interfering with other stations. In the receiver, a bandpass filter permits signals within a certain band of frequencies to be received and decoded, while stopping signals at undesirable frequencies from getting through.
Many authors have reported BPFs designed and implemented using various transmission line technologies including waveguides (Shi, Zhang, Zhou, Feng, Cao, & Che, 2019; Dahle, Laforge, & Kuhling, 2017; AbuHussain & Hasar, 2020), microstrip (Huang, Wang, & Zhu, 2016; Nwajana & Yeo, 2016; Cai, Wang, Zhu, & Wu, 2016; Zhang, Liu, Chen, Weng, & Yang, 2020), and substrate integrated waveguide (Qui, Wu, Xie, Yin, & Mao, 2018; Azad & Mohan, 2018; S. Hu, Y. Hu, Zheng, Zhu, Gao, & Zhang, 2020). The BPF presented in this paper is based on the microstrip technology. The filter relies on the microstrip hairpin resonator to achieve compact size. It is also of high selectivity and sharp roll-off. Some filter design characteristics such as selectivity, cost, size, sensitivity to environmental effects, power handling capacity, in-band and out-of-band performance metrics, are critical specifications in the development of RF and microwave communication front end devices. Filter developers are often required to make compromise between several conflicting requirements as it is rather difficult or even physically and/or electrically impossible to simultaneously achieve all design criteria or specifications. For instance, achieving higher channel selectivity usually requires the use of more resonators, which will result in higher insertion loss along the transmission path since insertion loss is approximately proportional to the number of resonators used in the construction of a filter (Nwajana & Yeo, 2020). Hence, care must be taken when selecting design specifications to meet the most critical design targets.
Some popular manufacturing techniques that have been employed in fabricating filters include printed circuit board (PCB) (Nwajana, Yeo, & Dainkeh, 2016), low temperature co-fired ceramic (LTCC) (Wong, Wang, Chen, & Chu, 2014) and liquid crystal polymer (LCP) (Dalmia, White, Sundaram, & Swaminathan, 2004). In terms of low cost and commercial availability, the PCB wins and hence, has been utilized in the fabrication of the BPF reported in this paper.
Figure 1. Channel/bandpass filter characteristics