## 1 Introduction  
Radar technology holds significance, as it enables the detection and monitoring of objects and substances from a distance, even in challenging visibility conditions. Its applications span various fields, such as aviation, military operations, meteorology, and law enforcement. For instance, in aviation, radar plays a crucial role in aircraft detection, tracking, and air traffic control. Additionally, in meteorology, radar is employed to identify and monitor precipitation, aiding meteorologists in predicting storms and issuing severe-weather warnings [1; 2; 3]. Its operation involves the exclusion of unwanted frequencies and the execution of a post-processing sequence on the corresponding signal. Filters play a vital role in carrying out the essential task of eliminating unnecessary harmonics [4; 5; 6]. The acquired wideband signals contain an inflated signal-to-noise ratio (SNR), which in return impacts the harmonics and data rate for channel capacity, diminishing the signal's effectiveness. Devices that can extract narrow-band signals and improve the SNR and information data rate in addition to a white Gaussian noise channel are becoming a necessity for the system. In recent RF and microwave communications systems, narrow-band technology is widely used as the consequence of narrow-band potential [7]. Narrow-band systems have a very wide range of applications in sub-6 GHz space and in numerous devices, including filters, antennas, and other RF circuits. At microwave frequencies, microstrip technology is optimal for designing filters and antennas [8; 9; 10]. Designing a narrow-band filter for microwave frequencies is challenging, as it must filter out signals with low bandwidth and attain a sharp attenuation rate at the frequency band edges. In a narrow-band frequency signal, harmonic reduction is also challenging. Through the use of a microstrip-based narrow-band filter, these counterfeit effects can be diminished [11; 12; 13]. A low insertion loss, high return loss, and sharp roll-off rate are required from these microstrip-based filters. Various techniques are used to implement microstrip filters. A vast body of literature explains the process of reducing harmonics in narrow-band signals using devices like filters. In the realm of microwave frequencies, microstrip filters prove to be optimal, offering maximum efficiency and high performance while maintaining low cost and compact size. Effective mechanisms within narrow-band filters are essential for blocking any interference in the signal. These filters serve as fundamental components in major wireless communication systems, particularly in the context of radar technology [14]. A narrow-band channel is one in which the signal bandwidth does not crucially exceed the coherence bandwidth of the channel. Narrow-band signals for bandpass filters have a small fractional bandwidth, which is given below:  
\[\Delta=\frac{f_{H}-f_{L}}{f_{C}} \tag{1}\]  
where \(f_{H}\), \(f_{L}\), and \(f_{C}\) are the upper cutoff frequency, lower cutoff frequency, and center frequency, respectively. The low-frequency spectrum of narrow-band signals also assists in minimising the power consumption required to transmit and process the signal. In radar systems, employing multiple filters for various frequencies incurs space, time, and financial expenses. A preferred solution to address this drawback is the use of a switchable filter bank [15; 16; 17]. This configuration involves placing two or more filters in close proximity, ensuring high isolation and improved performance. Furthermore, these filters can be tuned to their respective frequencies, offering a more efficient and cost-effective solution [18]. A narrow-band filter was designed at the Ku band with a 0.05% bandwidth and sharp rejection of \(>\)48  dB. Technology based on dielectric resonators was utilised in the development of the filter [19]. A highly sensitive and high-rejection narrow bandwidth of 0.5% bandpass filter was designed using stripline technology for nonlinear junction detectors.