Efficient Feed Port Modeling of Patch or Microstrip Antennas in a Finite-Element Method

The modeling of antennas driven by a microstrip line requires a source or feed model. The most accurate one is the coaxial cable but it increases the burden of mesh generation. Simplified feed models have been proposed for the method of moments (MoM). Regarding the finite-element method (FEM) that, unlike the MoM, easily handles an anisotropic and/or highly inhomogeneous environment, a wire feed line can be modeled by a voltage source in the finite-width feed-gap. The input impedance is then obtained from the circulation of the magnetic field on a line around this gap. We show that this method cannot be used for a strip feed line and propose a novel and simple technique to compute the antenna impedance that can be straightforwardly implemented in an FEM numerical code. This technique can also be used for a wire feed line modeled as a strip provided that the wire radius is small enough. The pertinence and efficiency of this model is numerically evaluated on three types of antenna: dipole, patch, and microstrip.