Investigation of Transformer Turns Ratio Between Feed and Slot of Aperture Coupled Slot Antenna by Using $S_{11}$ Parameter

This communication presents a simplified design approach to obtain the turns ratio nf which models the coupling between the microstrip feed and slot line of an aperture coupled slot antenna by using the S₁₁ parameter. Equations are derived using S₁₁ parameter to determine nf and simulations are carried out at 2 GHz using substrates of PCB FR4, Duroid 5870, and Duroid 6010. Results indicate nf is proportional to the slot length from 0 to 20 mm increasing rapidly with length. Over 20 mm slot length, the trend reduces and the value of nf tends towards 1. In addition, nf varies with the height of substrate but is little affected by dielectric constant of substrate which could therefore be ignored to simplify the design procedure. Measured results show good agreement with the theoretical model.