DESIGN OF MICROSTRIP LINE COUPLER WITH IMPROVED DIRECTIVITY

Abstract

In this project, a symmetrical microstrip line directional coupler has been designed using the synthesis technique. The introduced design procedure does not require the prior knowledge of the physical geometry of the coupler and requires only the information of the port impedances, coupling level, and operational frequency. The width of coupled microstrip lines, spacing between them and length of the coupler is then determined. The directivity of the conventional coupler designed is improved by bringing the changes in the physical geometry of the coupler by phase velocity compensation, which utilizes the coupled microstrip with square wiggles on its inner edges. In this project, a directional coupler has to be designed at the operational frequency of 1.5GHz with the coupling of -15dB. The width, length and spacing of coupled microstrip line are calculated with the help of Agilent ADS linecalc and then S-parameters of this coupler are calculated. The layout is plotted down in the layout window and is simulated. Then, the changes in the geometry of the coupler are made in order to improve the isolation and thus the directivity. The change in geometry raises the odd mode inductance more strongly than that of even mode and this results in phase velocity compensation between even- and odd-modes. The coupler is designed on Rogers-R03210 substrate with dielectric constant of 10.2 and thickness of 0.5mm. The simulations are carried out with the help of software Agilent Advanced Design System 2008.