A flexible bandpass filter based on Liquid Crystal Polymer substrate

In this paper, the coupling coefficient value of the quarter-wavelength impedance transformers in bending case is obtained. Then a two-stage CPW bandpass filter with simple λ/4 impedance transformers as coupling element is proposed. The proposed bandpass filter is fabricated on LCP substrate. Liquid Crystal Polymer is a flexible material and used as substrate for the filter. The bandpass filter is constructed and comparing the filtering performance under the flat and bending state. The filter shows an insertion loss of 1 dB at the center frequency of 5GHz and the return loss of better than 15 dB. In addition, the filter has a 3-dB bandwidth of 25% at 5.0GHz. There is a small offset in center frequency of filter when substrate is bending and insertion loss of filter is about 0.2dB offset in the center frequency.