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 a microstrip configuration. Two filter designs, based on the forward-coupled structure, were fabricated using YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) high-temperature superconducting (HTS) thin films on LaAlO/sub 3/ 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-/spl Omega/ 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.<<ETX>>