A general analysis of a microstrip-to-vertically mounted slotline(VMS) coupler is presented with a view to developing an equivalent circuit, and the efficient evaluation of the related circuit element values. Based on this theory, the effects of frequency and structure parameters such as aperture length and VMS width on the characteristics of the coupler are studied. In order to check the validity of the proposed analysis and design theory, a C-band linearly tapered slot antenna fed by an aperture-coupled back-to-back microstripline-to- VMS coupling structure is optimally designed using a hybrid genetic algorithm. Moreover, the computed characteristics from the network analysis is compared to the measurement and simulation results. The obtained results fully validate the efficiency and accuracy of the proposed network model.