X-band flexible bandpass filter based on ultra-thin liquid crystal polymer substrate

The performance of an X-band flexible band-filter on ultra-thin liquid crystal polymer (LCP) substrate is investigated. The 50 μm-thick LCP substrate is adopted to enhance the flexibility of circuits. For the purpose of fabrication by the conventional printed circuit board process, copper-clad laminate with 18 μm thickness is selected. Excellent microwave transmission characterisation is demonstrated by measuring the microstrip line at flat and bending conditions. An X-band interdigital bandpass filter based on a half-wavelength microstrip line resonator is designed and fabricated for purpose of demonstration. The measured results at the flat condition show that the insertion loss is 2.1 dB at centre frequency of 9.4 GHz and the return loss is better than 20 dB at the designed band. The performance at the bending condition shows that the bending effect causes little deterioration to the performance at a different bending radius.