Determining microstrip antenna input impedance using standard circuit CAD programs

Microstrip antennas have been widely used during the past four decades due to mechanical and economical advantages. The rigorous analysis of the microstrip antenna involves the solution of an electric field integral equation (EFIE) whose kernel must account for the effect of the dielectric slab. In this paper a simple approach for determining input impedance and resonant frequencies of microstrip antennas is given. Arbitrary shaped microstrip antenna can be divided using TLM (Transmission Line Modelling) approach into an integer number of square and half square (triangle) subsections. Every subsection can be replaced with crossed transmission lines, i.e. with their lumped element model. There are four types of subsections: inner, outer, corner, and diagonal [1]. Inner subsection shows nondispersive parallel plate behaviour. The outer subsection model requires effective dielectric calculation and is frequency dependent. Models for corner and diagonal subsections are obtained using inner and outer section models. For microstrip antennas analysis model given in [1] have been improved concerning conductor loses, and substrate loses are involved, too. In this way it is possible to consider simple electrical circuit instead of considering complex electromagnetic structure. Such circuit can be clumsy a lot, but it can be solved using standard CAD programs. A few years ago there were problems with regard to number of nodes and the largeness of the circuit file. Nowadays, those limitations are practically declined and this admission is actual. The appropriate interface program is realised for automated circuit file generation.