Novel microfluidically tunable bandpass filter with precisely‐controlled passband frequency

A novel microfluidically tunable bandpass filter (BPF) is proposed and it consists of a second‐order microstrip BPF with five plastic tubes placed above the coupled quarter‐wavelength resonators. The tubes can be filled with high‐permittivity water. By properly choosing the locations of the tubes and the water‐filling sequence, the electrical length of the resonator can be tuned, and thus the passband centre frequency of the filter can be precisely adjusted. A demonstration example is designed, and the simulated and measured results of the filter are given for verifying the theoretical prediction.