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

Abstract

The authors present a 10 dB 90° branch line coupler operating at 1.8 GHz 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 Ω of characteristic impedance with 1 mm of conductor width is realised by adding the DGS, while 1 mm corresponds to 82 Ω 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° branch-line 10 dB coupler has been fabricated using the 150 Ω line with DGS. Its measured performances are in good agreement with the predicted results.