3-D Thin-Wire FDTD Analysis of Coaxial Probe Fed in Asymmetric Microwave Components

For the coaxial probe fed in asymmetric microwave components, an improved scheme on the 3-D thin-wire (TW) finite-difference time-domain (FDTD) is implemented without additional grid refinements and/or auxiliary update terms. The fine geometrical discontinuity such as the conductive arm, feed aperture, and finite end of the coaxial probe is approximated to the quasi-static models. The near fields in the vicinity of the probe end are also theoretically calculated from the uniformly charged disk model. It is shown that the spatial dependency of the near fields around the finite end of its probe agrees well with the direct solutions of the fine-grid (FG) FDTD simulation. The dominant functions of the near-field behaviors in the vicinity of the probe are easily incorporated into the correction factors of the coefficients for the 3-D Cartesian FDTD update equations. For the choice of the cell size in the proposed TW FDTD, the input admittances of a coaxial monopole probe in air are calculated and compared with the FG FDTD and the measured data. To evaluate the effects of the asymmetric geometry in the vicinity of the coaxial probe, coaxial-probe fed waveguide launchers are numerically analyzed as a function of the excitation frequency, the eccentric position, and the axial height of the coaxial probe. In comparison with the standard TW FDTD, the proposed TW FDTD provides a very close agreement with the reference data.