COMPACT FORWARD-COUPLED SUPERCONDUCTING MICROSTRIP FILTERS FOR CELLULAR COMMUNICATION

A compact forward-coupled microstrip filter structure is presented for narrow-band applications in cellular communications. The structure consists of a parallel array of fully aligned half-wavelength resonators, Coupling is controlled by the spacing of the adjacent resonators, and is based on the even- and odd-mode phase velocity difference of coupled lines in microstrip configuration. Two filter designs, based on the forward-coupled structure, were fabricated using YBa2Cu3O7(YBCO) high-temperature superconducting (HTS) thin films on LaAlO3 substrates. One filter, a 9-pole 2.8% fractional bandwidth YBCO filter at 900 MHz, has less than 0.27 dB insertion loss and better than 16 dB return loss, The other filter, a 5-pole 1.2% fractional bandwidth YBCO filter with 10-Ohm internal impedance at 2 GHz, has less than 0.35 dB insertion loss and better than 12 dB return loss. All designs showed excellent measured frequency response.