## 2 Design and Simulation  
One method to design a filter bank is utilising lumped element components. However, millimeter waves can be distorted by the lumped components due to parasitic effects. A more favorable solution is to use surface-mounted devices (SMDs), as they lack lead wires and their compact size makes them easier to handle. Another approach involves the use of microstrip-based filters. There are various techniques for designing filters with microstrip technology, such as stepped impedance resonators and coupled resonators, but this specific design employs the hairpin technique. The proposed design includes four bandpass filters integrated with high-switching-speed SP4T MEMS switches. The switches are connected to an external control circuitry that includes Arduino UNO that has microcontroller Atmega 328p installed in it. Low-loss SMA connectors are used to pass signals through the filter bank. The proposed filter bank consists of four filters where the centre frequencies of these bandpass filters are 2.1 GHz, 2.4 GHz, 3.2 GHz, and 4.0 GHz with bandwidths of 9%, 8%, 6%, and 5%, respectively, as illustrated in Figure 3.  
These frequency bands include the ISM band and other sub-6 GHz frequencies, which are widely used in both commercial and defence areas. Figure 3: Frequency bands of hairpin-based bandpass filter bank. Figure 2: Design methodology for hairpin filter bank. Hairpin Filter Design and Fabrication  
The design of hairpin filters involves the use of microstrip technology and the unique \"hairpin\" shape. The basic structure of a hairpin filter consists of a microstrip transmission line that is folded back on itself to form a U-shape. The input port is connected to one end of the transmission line, while the output port is connected to the other end. To tune the filter to the desired frequency response, resonators in the form of stubs are added to the transmission line. The length and impedance of these stubs are calculated precisely to resonate at the target frequency. Multiple stubs can be utilised to achieve a more intricate response if needed. The selectivity and stopband rejection of the filter are determined by its quality factor, which is influenced by the design of the stubs and the microstrip transmission line. Careful attention must be paid to the dimensions and materials used in the design to ensure optimal performance. By combining the unique hairpin shape with microstrip technology and precise resonator design, hairpin filters provide a cost-effective solution for many RF and microwave applications. The size of the microstrip coupled line BPF is large compared to a hairpin filter due to the use of normal \(\frac{\lambda}{2}\) resonators. The design principle of the hairpin filter is same as that of half-wave parallel coupled resonators. The only difference is it takes less space, as the resonators are folded in the design. However, this folding leads to shorter coupled line lengths, diminishing the coupling between resonators. The resonator folding introduces coupling capacitance, thereby altering the scattering parameters of a filter and presenting a considerable optimisation challenge. The attenuation rate is contingent on the quantity of hairpin resonators within the filter. The dimensions of transmission lines or microstrip lines in a hairpin filter are dictated by the signal wavelength, and their relative electrical size is determined through a comparison with the wavelength. For hairpin filters, two types of input structure can be used, namely, coupled line and tapped line. Due to the low spacing at the end of resonators in coupled line input, large coupling appears, which degrades the insertion loss (IL) and the return loss (RL). In contrast, tapped input resonators give flexibility in the tapped input location along with better insertion and return loss. The quality factor and mutual coupling between resonators can be calculated using following equations.