Design and Fabrication of 3-Pole BPF Configured by Hairpin Resonators and Different Types of Coupling and Feed Types at 20 GHz

We configure and fabricate three types of BPFs using microstrip type $\lambda_{g}/\pmb{2}$ open-ended hairpin resonators at 20 GHz. The structures of input/output feed lines and coupling structures between the resonators of BPFs are different each other. The surface finish on copper foil is assumed electroless nickel immersion gold (ENIG) to prevent corrosion. The electrical influences of ENIG excluding insertion loss against BPFs are not found. It is confirmed by comparing with center frequency $\pmb{(f_{0})}$, fractional bandwidth (FBW) and insertion loss from simulation and measurement results. As the results, it is found that the BPF with gap type of input/output feed structure is better than one with tap type feed at 20 GHz.