This paper presents the modeling of coaxial probe used to excite an electromagnetically coupled patch antenna. The modeling is based on the sub-cell modeling of the inner conductor of the coaxial probe and considering the extension of the coaxial probe below the ground plane of the patch antenna. The problem that is tackled here is the method of exciting and truncating the coaxial probe in order to avoid the unstable and unreliable behavior of finite-difference time-domain (FDTD) solution. Three different structures and excitations are considered. First, the antenna is excited using a hard source placed along the inner conductor of the coaxial probe. In the second model, the hard source excitation is replaced by a resistive voltage source. In both of the noted modeling cases, the inner conductor is truncated two cells above the lower absorbing UPML boundary. In the third modeling technique, we consider the inner conductor extended to the boundary layer. The results show progressive refinement in the simulated solution of the input impedance, also compared with measured data in all cases.
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