Design of 10dB 90° branch line coupler using microstrip line with defected ground structure

The authors present a 10dB 90 degrees branch line coupler operating at 1.8GHz and having a defected ground structure (DGS), a kind of periodic structure realised by etching on the ground plane under the microstrip line. Owing to the additional effective inductance of the DGS, the characteristic impedance of the microstrip line is increased for the same width of conductor. The microstrip line of 150 Omega of characteristic impedance with 1mm of conductor width is realised by adding the DGS, while 1mm corresponds to 82 Omega of conventional microstrip line on the RT/Duroid 5880 substrate with 2.2 of dielectric constant and 31 mils of thickness. It is shown that a 90 degrees branch-line 10 dB coupler has been fabricated using the 150 Omega line with DGS. Its measured performances are in good agreement with the predicted results.